Polymorphism in Java

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Every object in Java knows its own class and can access this information through method . A. getClass. B. getInformation. C. objectClass. D. objectInformation.

A. getClass.

Polymorphism enables you to: A. program in the general. B. program in the specific. C. absorb attributes and behavior from previous classes. D. hide information from the user.

A. program in the general.

Can we inherit code from more than one superclass?

no

The type of the _____, not the type of the reference, determines which version of a ____ is invoked.

object, method

A ____ reference can refer to different types of ____ over time.

polymorphic , objects

interface

special tool that allows you to represent a common supertype between classes without actually sharing code; consists of a set of method declarations without a method body

downcasting example

((car) v).methodOnCar();

11.30 How do you do the following? a. Create an ArrayList for storing double values? b. Append an object to a list? c. Insert an object at the beginning of a list? d. Find the number of objects in a list? e. Remove a given object from a list? f. Remove the last object from the list? g. Check whether a given object is in a list? h. Retrieve an object at a specified index from a list?

(a) ArrayList<Double> list = new ArrayList<Double>(); (b) list.add(object); (c) list.add(0, object); (d) list.size(); (e) list.remove(object); (f) list.remove(list.size() - 1); (g) list.contains(object); (h) list.get(index);

11.24 Indicate true or false for the following statements: (a) You can always successfully cast an instance of a subclass to a superclass. (b) You can always successfully cast an instance of a superclass to a subclass.

(a) True. You can always successfully cast an instance of a subclass to a superclass. for example, casting apple to fruit is fine. (b) False. You cannot always successfully cast an instance of a superclass to a subclass. For example, casting fruit to apple is not always succeful unless a fruit is an apple.

Defining characteristics of an abstract class

* A class that contains abstract methods must be declared abstract. * An abstract class cannot be instantiated. * A class without any abstract methods can be declared abstract--this is done to prevent it from being instantiated. * Every concrete class that extends an abstract class must provide implementation for all abstract methods.

Queue Methods:

-Enqueue/Add -Dequeue/Remove -IsEmpty -IsFull

Class Variable

-One fixed memory location - Any object can change the value - Can be manipulated without creating instance of a class

The iterator class overloades the...

... ++ and - - and * (dereferencing) operators

Virtual functions allow...

... the most specific version of a member function in an inheritance hierarchy to be selected for execution

Polymorphism occurs when it takes the advantage of the fact that...

...a pointer to a derived class is type-compatible with a pointer to its base class

A Standard Template Library is...

...a set of C++ templates for useful algorithms and data structures

If a class has at least one pure virtual function it is considered...

...an abstract class

Abstract classes and pure virtual functions can be used to define...

...an interface that must be implemented by derived classes

Polymorphism refers to the ability to...

...associate many meanings to one function name by means of a special mechanism known as a virtual function

An abstract class is a class that...

...contains no objects that are not members of derived classes

Base class pointers can point to...

...derived class objects

A piece of code is said to be polymorphic if...

...executing the code with different types of data produces different behavior

Iterators provide...

...member functions begin() and end() that returns the beginning and the end of the container

The compareTo method should return ___ when equals would return true, a neg number when this < obj and a positive number when this> obj

0

What are the two basic rules for type compatibility of inheritance?

1) A derived class pointer can always be assigned to a base class pointer 2) A type cast is required to perform the opposite assignment of a base class pointer to a derived class pointer

Override annotation

@Override before a method in the subclass that is an override.

Single inheritance

A JAVA class may inherit directly from only one superclass.

Virtual methods

A class can declare _____________ and properties

Several

A class can implement __________ interfaces

One

A class can only inherit ________ base class

supertype

A class defines a type. A type defined by a superclass is a supertype.

Child class

A class that extends the base class.

Abstract Class

A class that is used only as the superclass for other classes. You cannot create objects of an abstract class. Such a class must have the keyword "abstract" in its declaration.

What is a container?

A class that stores data and organizes the data

Sorted List

A data collection that maintains its entries in sorted order and retrieves them by their position number within the list.

What is a Queue?

A datastructure of unordered items

Polymorphism

A feature of object-oriented programming whereby the correct version of a method is determined during program execution instead of during compilation.

Dynamic Binding

A method can be implemented in several classes along the inheritance chain. The JVM decides which method is invoked at runtime.

Virtual

A method is said to be a virtual when it is declared

Abstract Method

A method that has no body. The declaration of such a method must include the keyword abstract and end in a semicolon. The class of an abstract method must be abstract.

Super

A reference to the parent. In a method definition, this refers to the the parent

inheritance

A subclass can use iVars, and methods from superclass

polymorphism

A subclass overriding the methods of a superclass is an example of ...

subtype

A type defined by a subclass.

Polymorphic Variable

A variable having a dynamic type.

Declared Type

A variable must be declared a type. A variable of a reference type can hold a null value or a reference to an instance of the declared type. The instance may be created using the constructor of the declared type OR ITS SUBTYPE. Determines which method to match at compile time.

What is an example of an STL?

A vector

Inheritance

A way of building new classes bases on existing ones: creates an "is-a" relationship.

Abstract

Abstract classes are special classes defined with the keyword __________

Abstract/virtual

Abstract operations are declared as

How do we fix the dreaded diamond/multiple inheritance problem of implementation of the same methods in the classes a subclass is multiply inheriting from?

Abstraction - make the conflicting methods in the parent classes that are multiply inherited from abstract; Interfaces in Java

Default Access Modifier

Accessible only within package

Protected access modifier

Accessible within package and outside the package but through inheritance only

Child class

Add new fields and methods

Subclasses

Additional data members

Subclasses

Additional methods

11.32 Suppose the ArrayList list contains {"Dallas", "Dallas", "Houston", "Dallas"}. What is the list after invoking list.remove("Dallas") one time? Does the following code correctly remove all elements with value "Dallas" from the list? If not, correct the code. for (int i = 0; i < list.size(); i++) list.remove("Dallas");

After list.remove("Dallas"), the list becomes {"Dallas", "Houston", "Dallas"}. No. Here is the reason: Suppose the list contains two string elements "red" and "red". You want to remove "red" from the list. After the first "red" is removed, i becomes 1 and the list becomes {"red"}. i < list.size() is false. So the loop ends. The correct code should be : for (int i = 0; i < list.size(); i++) { if (list.remove(element)) i--; }

Polymorphism

Allows a variable of a supertype to refer to a subtype object. (ig An object of a subclass can be used where ever its superclass object is used.)

Polymorphism

Allows abstract operations to be defined and used

Inheritance

Allows child classes to inherit the characteristics of an existing parent class.

Advantages of using dynamic polymorphism

Allows methods to be overriden - allows method to be called on actual type of an object if parent and child both have the same type and we are referring to the object via its parent type.

Interface

An ______________ can implement several interfaces

Difference between an abstract class and an interface

An abstract class can have 0 or more abstract methods as well as defined methods, an interface's methods are all abstract; an abstract class can have member state whereas an interface contains no state (apart from constants); a class can inherit from multiple interfaces but it can't inherit from multiple abstract classes.

If a base class pointer does not actually point to a derived class object, what may happen?

An error

How can polymorphism be accomplished using interfaces?

An interface name can be used as the type of a reference. Such as reference variable can refer to any object of any class that implements that interface. Because all classes implement the same interface, they have methods with common signatures, which can be dynamically bound.

What is an example of an abstract class?

Animal is an abstract class: there are no animals that are not Dogs, or cats, or lions, or tigers, or bears, oh my ......

Abstract Superclass

Another term for abstract class.

Abstract operations

Are defined in the base class and implemented in the child classes

Subclasses

Are more specific and have more functionality

Constructors

Are not inherited

In static polymorphism, when is the decision made about which method (which exists both in parent & child class) to execute for an object that we refer to via its parent type? (i.e. by casting to parent type)

At compile time

11.22 public class Test { 2 public static void main(String[] args) { 3 A a = new A(3); 4 } 5 } 6 7 class A extends B { 8 public A(int t) { 9 System.out.println("A's constructor is invoked"); 10 } 11 } 12 13 class B { 14 public B() { 15 System.out.println("B's constructor is invoked"); 16 } 17 } Is the no-arg constructor of Object invoked when new A(3) is invoked?

B's constructor is invoked A's constructor is invoked The default constructor of Object is invoked, when new A(3) is invoked. The Object's constructor is invoked before any statements in B's constructor are executed.

static

Both Abstract and Interface may have ___________ variables

Class

Can be used through its parent interface

Child class

Can extend the parent class

Class

Can implement an interface by providing implementation for all its methods.

Structures

Cannot be inherited

Java Interface

Cannot contain an implementation of the methods, only the signature (name, parameters and exceptions) of the method.

Disadvantages of using static polymorphism for methods

Cannot override methods - the parent method will be run

Polymorphism

Capability of changing into many forms. It's a feature that allows one method to behave in different ways depending on various factors. Can be implemented through overloading and overriding.

upcasting

Casting an instance of a subclass to a variable of a superclass @An instance of a subclass is ALWAYS an instance of a superclass, so can be implicit or explicit.

Syntax

Class class_name implements interface_name

11.36 Correct errors in the following statements: int[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5}; System.out.println(java.util.Collections.max(array));

Collections only works on ArrayList objects, not on primitive array.

In static polymorphism, what do type errors cause?

Compile errors

Static polymorphism involves making a choice between multiple implementations at?

Compile time

Non-Abstract Class

Concrete Class

Variables and collections

Consequently, we can define (such as arrays) that don't have to know in advance what kind of specific object they will hold, only that they'll hold objects that implement the interface

Class hierarchy

Consists of a superclass and its descendants.

What are the two important data structures in the STL?

Containers Iterators

try block

Contains program statements that we think can raise exceptions

RaceCar mcQueen = new RaceCar();

Create an instance of RaceCar without any parameters named mcQueen (would be in runner class)

Interfaces can have methods. A. 0 B. 1 C. 2 D. any number of

D. any number of

Static polymorphism

Decisions made at compile-time, since we don't know what the true type of an object will be (casted to parent type so can't tell actual sub-type) we run the parent method

String c = "cookie";

Declare a string named "c" that is for "cookie" --- That's good enough for me!

int a = 37;

Declare an integer named a with a value of 37.

New members

Declaring ____________ with the same name or signature hides the inherited ones

11.37 What modifier should you use on a class so that a class in the same package can access it, but a class in a different package cannot access it?

Default.

Interface

Defined as a syntactical contract that all the classes inheriting the interface should follow

Override

Derived classes can __________ the implementation of these members

Polymorphism

Describes how objects in a hierarchy act according to what they are rather than what names they have

How do you indicate an interface in UML

Draw class diagram as normal, put <<interface>> before class name in top section of class diagram, have method names all abstract (in {} ).

Are all methods in Java dynamic polymorphic or static polymorphic?

Dynamic polymorphic

What are the three pillars of object oriented programming?

Encapsulation Inheritance Polymorphism

Encapsulation

Encapsulation in Java is a mechanism of wrapping the data (variables) and code acting on the data (methods) together as a single unit. In encapsulation, the variables of a class will be hidden from other classes.

Polymorphism

Ensures that the appropriate method of the subclass is called through its base class interface

11.31 Identify the errors in the following code. ArrayList<String> list = new ArrayList<>(); list.add("Denver"); list.add("Austin"); list.add(new java.util.Date()); String city = list.get(0); list.set(3, "Dallas"); System.out.println(list.get(3));

Error 1: list.add(new java.util.Date()); is wrong, because it is an array list of strings. You cannot add Date objects to this list. Error 2: list.set(3, "Dallas"); is wrong because there is no element at index 3 in the list. Error 3: list.get(3) is wrong because there is no element at index 3 in the list.

Public and abstract

Every method in an interface is ____ and ___.

Public, static, and final

Every variable in an interface is ____, _____, and ______.

Runtime

Exact method to be called is determined at _________, just before performing the call

Disadvantages of using dynamic polymorphism for methods

Expensive - checks types at runtime

Derived class

Extends its base class

Inheritance

Extensibility, reusability, provides abstraction, eliminates redundant code

True or false: an interface can inherit from more than one class?

False, another class can inherit from more than one interface

11.7 True or false? You can override a private method defined in a superclass.

False. You can only override accessible instance methods.

Overriding

Feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its super-classes or parent classes

Attributes

Fields and Properties

Generalization

General characteristics are specified high up in the hierarchy

Interface

Gives you the ability to specify set of behaviors that all classes that implement the interface will share in common

Interface

Group of variables and methods declarations.

Concrete class

Has implementation of all of its inherited members either from interface or abstract class

Composition

Has-a relationship

Subclasses

Have all of the data membets and methods of the superclass

Object

If a class is not declared to inherit from another then it implicitly inherits from the _______ class

super(name, weight);

If the animal class has the iVars String name and double weight how would you write the "super" line of code to include them in an inherited class Dog?

public Dog(String name, double weight)

If the animal class has the iVars String name and double weight how would you write the constructor of Dog to include name and weight ( { not necessary)

What is the problem with a dreaded diamond?

If the two middle classes independently override a method from its parent class (class at the top), the bottom class (which multiply inherits from the two middle classes) will not know which method to inherit

What problems can occur with multiple inheritance?

If two parent classes have implementations of the same method, which method should get inherited to be implemented?

Derived interface

Implements base interface

Class

Implements interface

Object

In C#, all classes are subclasses of the _____________ class

Multiple

In C#, there is no ________ inheritance

Constructor

In the ______________ of the derived class we use the keyword base to invoke the constructor of the base class

Base

In the constructor of the derived class we use the keyword ________ to invoke the constructor of the parent class

Extends

Indicates inheritance. Java classes can have only one parent.

____ can be applied to interfaces so that one interface can be derived from another.

Inheritance

Inheritance

Inheritance can be defined as the process where one class acquires the properties (methods and fields) of another.

Transitive relation

Inheritance is ______________

Derived class

Inherits base class/parent class

11.35 Correct errors in the following statements: int[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5}; ArrayList<Integer> list = new ArrayList<>(Arrays.asList(array));

Int is a primitive. The correct way is to wrap int to Integer first, and then apply the integer objects to the ArrayList. To use asList(array), array must be an array of objects.

What

Interface defines the ________ part of the syntactical contract

Late binding

Is slower than normal binding

Inheritance

Is-a relationship

11.16 What is the benefit of using the @Override annotation?

It forces the compiler to check the signature of the overridden method to ensure that the method is defined correctly.

Constructor

It is called when an instance of object is created and memory is allocated for the object

Root

It is the __________ of the inheritance hierarchy in C#

super

It is used to invoke the superclass constructor from subclass.

In ad-hoc/sub-type polymorphism (an OOP-specific type of dynamic polymorphism), how does the compiler figure out to run the child method instead of the parent method for an object being accessed via its parent type?

It looks in memory whilst the program is running and finds the object is really of its child type (although being accessed via parent type), and if it finds parent and child implements a particular method, it will run the method in the child (since its figured out the object is actually of the child type not the parent type)

11.14 If a method in a subclass has the same signature as a method in its superclass with a different return type, will this be a problem?

It will be a syntax error.

super

Keyword is used to refer to member of base class from a sub class.

Inheritance

Leads to a hierarchy of classes and/or interfaces in an application

11.20 What is wrong in the following code? 1 public class Test { 2 public static void main(String[] args) { 3 Integer[] list1 = {12, 24, 55, 1}; 4 Double[] list2 = {12.4, 24.0, 55.2, 1.0}; 5 int[] list3 = {1, 2, 3}; 6 printArray(list1); 7 printArray(list2); 8 printArray(list3); 9 } 10 11 public static void printArray(Object[] list) { 12 for (Object o: list) 13 System.out.print(o + " "); 14 System.out.println(); 15 } 16 }

Line 8 causes a compile error, because int[] cannot be passed to Object[]. Int is a primitive and must be wrapped before it can be used as an Object.

11.18 Describe the difference between method matching and method binding.

Matching a method signature and binding a method implementation are two separate issues. The declared type of the reference variable decides which method to match at compile time. The compiler finds a matching method according to parameter type, number of parameters, and order of the parameters at compile time. A method may be implemented in several subclasses. The JVM dynamically binds the implementation of the method at runtime, decided by the actual class of the object referenced by the variable.

Abstract classes

May not be instantiated, and require subclasses to provide implementations for the abstract methods.

Child class

May override some of the behaviors of the parent class

Exception handling

Mechanism to handle the runtime errors so that normal flow of the application can be maintained

11.12 Explain the difference between method overloading and method overriding.

Method overloading defines methods of the same name in a class. Method overriding modifies the methods that are defined in the superclasses.

Virtual method

Method that can be used the same way on instances of base and derived classes but its implementation is different

Dynamic polymorphism

Method with implementation in child and parent is run in the child class, at run-time (so that we know the child's type)

Operations

Methods

Override

Methods that are declared as virtual in a base class can be overridden using the keyword __________ in the derived class

Interfaces are totally abstract, allow for ________ ___________ from them.

Multiple inheritance

Commas

Names of interfaces should be separated by ____.

Can you instantiate an object of an abstract class?

No

Do overridden static fields exhibit dynamic polymorphism?

No

Do overridden static methods exhibit dynamic polymorphism?

No - same implementation for all objects so why would it?

Can you override methods with static, private or final access modifiers?

No - static: all objects have same implementation, private: cannot access method from subclass, final: once method is implemented, cannot be changed

Does casting an object reference generate a new object ?

No. Both variables now reference the same object.

the _____ class is the superclass to all objects

Object

11.27 What is wrong in the following code? 1 public class Test { 2 public static void main(String[] args) { 3 Object fruit = new Fruit(); 4 Object apple = (Apple)fruit; 5 } 6 } 7 8 class Apple extends Fruit { 9 } 10 11 class Fruit { 12 }

Object apple = (Apple)fruit; Causes a runtime ClassCastingException.

java classes are subclasses

Object class

What is the ultimate parent/super Class?

Object. Every class in JAVA is descended from java.lang.Object class. @If no superclass/inheritence is specified on class creation, Object is the default superclass.

Instance Variable

Objects created from the same class have their own distinct copies of instance variables

Type compatibility

Objects of a derived class can be used whenever objects of a base class object are expected

11.15 If a method in a subclass has the same name as a method in its superclass with different parameter types, is the method overridden or overloaded?

Overloaded.

11.13 If a method in a subclass has the same signature as a method in its superclass with the same return type, is the method overridden or overloaded?

Overriden.

Late method binding

Polymorphism is implemented using a technique called

11.17 What is polymorphism? What is dynamic binding?

Polymorphism means that a variable of a supertype can refer to a subtype object. A method may be implemented in several classes along the inheritance chain. The JVM decides which method is invoked at runtime. This is known as dynamic binding.

Multithreading

Process of executing two or more threads simultaneously

11.38 What modifier should you use so that a class in a different package cannot access the class, but its subclasses in any package can access it?

Protected

Child class

Redefine methods (modify existing behavior)

Abstraction

Refers to hiding some non-essential information and only showing parts of it

How does Java fix the problem of conflicting methods in multiple inheritance?

Restricts classes to only having at most one parent, unless the parent class is a a special type of class called an interface

In dynamic polymorphism, what do type errors cause?

Run-time faults (crashes)

Dynamic polymorphism involves making a choice between multiple implementations at?

Runtime

What are the 2 types of containers?

Sequential Associative

11.41 How do you prevent a class from being extended? How do you prevent a method from being overridden?

Set the Class or method to accessibility final.

How might you use static polymorphism when you have a bunch of shapes, each of which have their own draw() method?

Shape doesn't have a draw() method, only the child classes do. Keep a list of Shape references, figure out what each object really is (i.e. square, circle - what child class is it), narrow the reference (downcast to this child class type) and then call draw() on this narrowed reference.

How might you use dynamic polymorphism when you have a bunch of shapes, each of which have their own draw() method?

Shape has a draw() method as do the child classes. Keep a list of Shape references, let the compiler figure out what to do with each Shape reference by calling the draw method on it, figures out which child class it is (i.e. its a Circle object so run draw() in Circle)

Explicit Casting

Similar to casting among primitive types. Allows a subclass to cast it's type to the superclass. EX: Student s = (student) o;

Why are interfaces totally abstract?

So that if there are conflicting methods (same name, arguments, return type but different body), there is no implementation of these methods in parent classes (interfaces) so no conflict caused in subclass, these methods are then implemented in the subclass itself.

Inheritance

Software Reuse -Each class only has one super class - Child class can override parent methods

Abstract classes

Special classes defined with the keyword abstract

What is an interface?

Special classes that are entirely abstract (no state, all methods are abstract), the only classes in Java allowed to do multiple inheritance (i.e. allows classes to multiply inherit from it and more interfaces).

Specialization

Specific characteristics are specified below the hierarchy

Base class

Specify the name of the _________ after the name of the derived

Type of polymorphism with shadowing of fields?

Static polymorphism

What type of polymorphism do we have if we can get different method implementations by casting an object to different types?

Static polymorphism (casting to different types means different parents so different implementation of method from different parents as parent method run in static polymorphism)

Type of polymorphism in ML

Static polymorphism (decision made at compile-time, type errors give compile errors)

Differences between static and dynamic polymorphism (method run, when decision is made, result of type errors)

Static; Dynamic Run parent method; Run child method Decision made at compile; Decision made at run-time Type checked at compile-time, type of child checked at run-time Type errors cause compile errors; type errors cause run-time fault (crashes)

What does a sequential container do?

Stores items in the form of sequences

Superclass

Subclasses can add to the ________

Polymorphism

The ability to take more than one form

Actual Type

The actual class for the object referenced by the variable.

Base class

The class upon which a child class is defined and extended from

Why does abstraction fix the problem of one or more conflicting methods in multiple inheritance?

The conflicting methods have no implementation that can conflict, have the implementation in the child class that multiple inherits instead

Static Type

The data type in a variable's declaration. A static type is fixed and determined during compilation.

Dynamic Type

The data type of the object that a variable references at a point during program execution.

How

The deriving classes define the ________ part of the syntactical contract

11.11 Identify the problems in the following code: 1 public class Circle { 2 private double radius; 3 4 public Circle(double radius) { 5 radius = radius; 6 } 7 8 public double getRadius() { 9 return radius; 10 } 11 12 public double getArea() { 13 return radius * radius * Math.PI; 14 } 15 } 16 17 class B extends Circle { 18 private double length; 19 20 B(double radius, double length) { 21 Circle(radius); 22 length = length; 23 } 24 25 @Override 26 public double getArea() { 27 return getArea() * length; 28 } 29 }

The following lines are erroneous: { radius = radius; // Must use this.radius = radius } class B extends Circle { Circle(radius); // Must use super(radius) length = length; // Must use this.length = length } public double getArea() { return getArea() * length; // super.getArea() }

Dynamic Binding/Late Binding

The mechanism used during program execution to determine which version of a method to call.

11.29 When overriding the equals method, a common mistake is mistyping its signature in the subclass. For example, the equals method is incorrectly written as equals(Circle circle), as shown in (a) in following the code; instead, it should be equals(Object circle), as shown in (b). Show the output of running class Test with the Circle class in (a) and in (b), respectively. public class Test { public static void main(String[] args) { Object circle1 = new Circle(); Object circle2 = new Circle(); System.out.println(circle1.equals(circle2)); } } (a) class Circle { double radius; public boolean equals(Circle circle) { return this.radius == circle.radius; } } (b) class Circle { double radius; public boolean equals(Object o) { return this.radius == ((Circle)o).radius; } } If Object is replaced by Circle in the Test class, what would be the output to run Test using the Circle class in (a) and (b), respectively? Suppose that circle1.equals(circle2) is replaced by circle1.equals("Binding"), what would happen to run Test using the Circle class in (a) and (b), respectively? Reimplement the equals method in (b) to avoid a runtime error when comparing circle with a non-circle object.

The output is false if the Circle class in (a) is used. The Circle class has two overloaded methods: equals(Circle circle) defined in the Circle class and equals(Object o) defined in the Object class, inherited by the Circle class. At compile time, circle1.equals(circle2) is matched to equals(Object o), because the declared type for circle1 and circle2 is Object. (Note that either the declared type for circle1 and circle2 is Object would cause circle1.equals(circle2) to match circle1.equals(Object circle) by the compiler. The output is true if the Circle class in (b) is used. The Circle class overrides the equals(Object o) method defined in the Object class. At compile time, circle1.equals(circle2) is matched to equals(Object o) and at runtime the equals(Object o) method implemented in the Circle class is invoked. In (a), method equals(Object o) is used at both compile time and run time. circle1 and circle2 have different addresses, leading to "false" output. Method overriding follows dynamic binding (determined by the actual type), but method overloading is always determined by the declared type. What would be the output if Object is replaced by Circle in the Test class using the Circle class in (a) and (b), respectively? The output would be true for (a), because circle1.equals(circle2) matches circle1.equals(Circle object) exactly in this case. The output would be true for (b) because equals(Object c) is overridden in the Circle class. With circle1.equals(circle2) replaced by circle1.equals("Binding") and using the the Circle class in (a), the equals method in the Object class is used,circle1.equals(circle2) returns false. With circle1.equals(circle2) replaced by circle1.equals("Binding") and using the the Circle class in (b), the equals method in the Circle class is used,since it casts string "Binding" to Circle, it throws a casting exception. (b) should be reimplemented as follows to to avoid a runtime error when comparing circle with a non-circle object. class Circle { double radius; public boolean equals(Object o) { if (o instanceof Circle) return this.radius == ((Circle)o).radius; else return false; } }

Overloading a method

This means defining multiple methods with the same name, but different signatures. It is a new implementation for a method in the subclass. @Can be either in the same class or different classes related by inheritance @Have the same name, but different parameters as the other method.

final keyword for classes and methods

This prevents the class, method or variable from being extended or overridden or changed.

False

True or False: A class can only implement one interface.

True

True or False: Class makes use of an interface by implementing that interface by name.

Why is there a performance overhead with dynamic polymorphism?

Types are checked at run-time

super

Used by class constructors to invoke constructors of its parent class

Advantages of using static polymorphism for methods

Using it for methods that are upcasted will allow compile errors to be seen by being able to see if the object really is of the upcasted type and calling that method is valid on that type; can overload methods without extra cost of checking at runtime through dynamic polymorphism; types checked at compile time so less expensive

Constant

Variables in an interface should have ____ values.

What are the three types of sequential containers?

Vector List Deque

creating objects: , car is subclass of vehicle

Vehicle v = new Car();

What makes polymorphism possible?

Virtual functions

What problems are there with static polymorphism in OOP?

What if we want to add a new child class? Then we have to go through and add code to check if the object is of this type, and then downcast this object (referred to by its parent class before) to its child type and call the method on this downcasted object.

object

What is the cosmic superclass, included with every class you create.

2323

What is the output? Object x = new String("2323"); out.println(x.toString());

2

What is the output? class A { int i; void display() { System.out.println(i); } } class B extends A { int j; void display() { System.out.println(j); } } class inheritance_demo { public static void main(String args[]) { B obj = new B(); obj.i=1; obj.j=2; obj.display(); } }

2 2

What is the output? class A { public int i; private int j; } class B extends A { void display() { super.j = super.i + 1; System.out.println(super.i + " " + super.j); } } class inheritance { public static void main(String args[]) { B obj = new B(); obj.i=1; obj.j=2; obj.display(); } }

1 2

What is the output? class A { public int i; public int j; A() { i = 1; j = 2; } } class B extends A { int a; B() { super(); } } class super_use { public static void main(String args[]) { B obj = new B(); System.out.println(obj.i + " " + obj.j) } }

17

What is the output? class A{ private int x; public A() { x=17;} public String toString() { return ""+x; } } class B extends A{ } //test code in the main method A one = new A(); out.println(one);

17 17

What is the output? class A{ private int x; public A() { x=17;} public String toString() { return ""+x; } } class B extends A{ } //test code in the main method A one = new A(); out.println(one); one = new B(); out.println(one);

compiler error

What is the output? final class A{ private int x; public A() { x=17;} public String toString() { return ""+x; } } class B extends A{ } //test code in the main method A one = new A(); out.println(one);

What is the dreaded diamond?

When a base class has 2 children who are the parents of another class through multiple inheritance (the same class inherits from both), forming a diamond.

Define polymorphism

When a call to a member function will cause a different function to be executed depending on the type of object that invokes the function

Multilevel inheritance

When a class extends a class, which extends another class

Multiple inheritance

When a class extends a class, which extends another class.

Methods

When a class implements an interface, it implements all the _____ declared in that interface.

Implicit Casting

When a superclass variable wants to be typed as a subclass reference with out explicitly being called. Ex. Object o = New Student(); Object is a superclass of Student. @This only works from super to sub, not the other

downcasting

When casting an instance of a superclass to a variable of its subclass requires explicit casting. Otherwise the complier could give a ClassCastException. @To validate this one can create an if (superclass variable instanceof subclassvariable);

Casting Object

When one object reference is typecast into another object reference.

Multiple inheritance

When we want a class to inherit from more than one parent class

Substitution Principle

Whenever a base class type is expected, a child (or descendant) will suffice.

public class RaceCar extends Car

Write the 1st line class declaration for RaceCar a Subclass of Car

Do overridden methods exhibit dynamic polymorphism?

Yes (providing non-static)

11.28 Does every object have a toString method and an equals method? Where do they come from? How are they used? Is it appropriate to override these methods?

Yes, because these two methods are defined in the Object class; therefore, they are available to all Java classes. The subclasses usually override these methods to provide specific information for these methods. The toString() method returns a string representation of the object; the equals() method compares the contents of two objects to determine whether they are the same.

abstract method

a header without an implementation; the actual body is not specified, to allow/force different classes to implement the behavior in its way

Describe a list

a sequence of items that allows quick additions and removals from any position

non abstract methods are inherited by

a subclass

11.25 For the GeometricObject and Circle classes in Listings 11.1 and 11.2, answer the following questions: a. Assume are circle and object1 created as follows: Circle circle = new Circle(1); GeometricObject object1 = new GeometricObject(); Are the following Boolean expressions true or false? (circle instanceof GeometricObject) (object instanceof GeometricObject) (circle instanceof Circle) (object instanceof Circle) b. Can the following statements be compiled? Circle circle = new Circle(5); GeometricObject object = circle; c. Can the following statements be compiled? GeometricObject object = new GeometricObject(); Circle circle = (Circle)object;

a) (circle instanceof GeometricObject1) => true (object instanceof GeometricObject1) => true (circle instanceof Circle1) => true (object instanceof Circle1) => false (b) Yes, because you can always cast from subclass to superclass. (c) Causing a runtime exception (ClassCastExcpetion)

Which of the following statements is false? a. An advantage of inheritance over interfaces is that only inheritance provides the is-a relationship. b. Objects of any subclass of a class that implements an interface can also be thought of as objects of that interface type. c. When a method parameter is declared with a subclass or interface type, the method processes the object passed as an argument polymorphically. d. All objects have the methods of class Object.

a. An advantage of inheritance over interfaces is that only inheritance provides the is-a relationship.

11.26 Suppose that Fruit, Apple, Orange, GoldenDelicious, and McIntosh are defined in the following inheritance hierarchy: Assume that the following code is given: Fruit fruit = new GoldenDelicious(); Orange orange = new Orange(); Answer the following questions: a. Is fruit instanceof Fruit? b. Is fruit instanceof Orange? 11.26c. Is fruit instanceof Apple? d. Is fruit instanceof GoldenDelicious? e. Is fruit instanceof McIntosh? f. Is orange instanceof Orange? g. Is orange instanceof Fruit? h. Is orange instanceof Apple? i. Suppose the method makeAppleCider is defined in the Apple class. Can fruit invoke this method? Can orange invoke this method? j. Suppose the method makeOrangeJuice is defined in the Orange class. Can orange invoke this method? Can fruit invoke this method? k. Is the statement Orange p = new Apple() legal? l. Is the statement McIntosh p = new Apple() legal? m. Is the statement Apple p = new McIntosh() legal?

a. Is fruit instanceof Fruit true? true b. Is fruit instanceof Orange true? false c. Is fruit instanceof Apple true? true d. Is fruit instanceof GoldDelicious true? true, because new GoldenDelicious() is assigned to fruit. The actual type for fruit at runtime is GoldenDelious. e. Is fruit instanceof Macintosh true? false f. Is orange instanceof Orange true? true g. Is orange instanceof Fruit true? true h. Is orange instanceof Apple true? false i. Suppose the method makeApple is defined in the Apple class. Can fruit invoke this method? No. It will give a compile error. However, you can invoke it using ((Apple)fruit).makeAppleCider(). Can orange invoke this method? No j. Suppose the method makeOrangeJuice is defined in the Orange class. Can orange invoke this method? Yes. Can fruit invoke this method? No. k. Is the statement Orange p = new Apple() legal? No l. Is the statement Macintosh p = new Apple() legal? No m. Is the statement Apple p = new Macintosh() legal? Yes

11.40 In the following code, the classes A and B are in different packages. If the question marks in (a) are replaced by blanks, can class B be compiled? If the question marks are replaced by private, can class B be compiled? If the question marks are replaced by protected, can class B be compiled? (a) package p1; public class A { ? int i; ? void m() { ... } } (b) package p2; public class B extends A { public void m1(String[] args) { System.out.println(i); m(); } }

a. No. Default accessibility does not extend to subclasses in a different package. b. No. Private accessibility does not extend to subclasses in a different package. c. Yes. Protected accessibility does extent to sublclasses in a different package.

11.21 Show the output of the following code: (a) public class Test { public static void main(String[] args) { new Person().printPerson(); new Student().printPerson(); } } class Student extends Person { @Override public String getInfo() { return "Student"; } } class Person { public String getInfo() { return "Person"; } public void printPerson() { System.out.println(getInfo()); } } (b) public class Test { public static void main(String[] args) { new Person().printPerson(); new Student().printPerson(); } } class Student extends Person { private String getInfo() { return "Student"; } } class Person { private String getInfo() { return "Person"; } public void printPerson() { System.out.println(getInfo()); } }

a. Person Student b. Person Person For (a), new Person().printPerson() invokes the printPerson() method definedin the Person class, which then invokes the getInfo() method in the Person class.new Student().printPerson() invokes the printPerson() method definedin the Person class, which then invokes the getInfo() method in the Student class, because the calling object is a Student. For (b), new Student().printPerson() invokes the printPerson() method definedin the Person class, which then invokes the getInfo() method in the Person class. It does not invoke the getInfo() method in the Student class, because it is private and can only be invoked from a method in the Student class. Note that the getInfo() method is not overridden because it is private. You can only override a non-private method such as getInfo() in (a).

Polymorphism enables you to: a. program in the general. b. program in the specific. c. absorb attributes and behavior from previous classes. d. hide information from the user.

a. Program in the general

Which of the following statements is false? a. References to interface types do not have access to method toString. b. Method toString can be invoked implicitly on any object. c. With inheritance, classes and their inherited classes tend to be very similar. d. Dramatically different classes can often meaningfully implement the same interface.

a. References to interface types do not have access to method toString.

11.42 Indicate true or false for the following statements: a. A protected datum or method can be accessed by any class in the same package. b. A protected datum or method can be accessed by any class in different packages. c. A protected datum or method can be accessed by its subclasses in any package. d. A final class can have instances. e. A final class can be extended. f. A final method can be overridden.

a. True. b. False, only subclasses that extend the class in different packages. c. True. d. True. e. False. f. False.

All of the following methods are implicitly final except: a. a method in an abstract class. b. a private method. c. a method declared in a final class. d. static method.

a. a method in an abstract class.

Which of the following statements about abstract superclasses is true? a. abstract superclasses may contain data. b. abstract superclasses may not contain implementations of methods. c. abstract superclasses must declare all methods as abstract. d. abstract superclasses must declare all data members not given values as abstract.

a. abstract superclasses may contain data

Polymorphism allows for specifics to be dealt with during: a. execution. b. compilation. c. programming. d. debugging.

a. execution

Every object in Java knows its own class and can access this information through method . a. getClass. b. getInformation. c. objectClass. d. objectInformation.

a. getClass

11.39 In the following code, the classes A and B are in the same package. If the question marks in (a) are replaced by blanks, can class B be compiled? If the question marks are replaced by private, can class B be compiled? If the question marks are replaced by protected, can class B be compiled? (a) package p1; public class A { ? int i; ? void m() { ... } } (b) package p1; public class B extends A { public void m1(String[] args) { System.out.println(i); m(); } }

a. yes because subclasses can access default methods and variables b. no because if i is private, it cannot be accessed by the subclass. c. yes because subclasses can access protected variables and methods.

polymorphism

ability for the same code to be used with different types of objects and behave differently with each

public members

accessible to all classes

protected members

accessible within all classes in the same package and within subclasses in other packages

polymorphism

achieved by overriding methods

base class

aka superclass aka parent class the class from which the subclass is derived from

if a class implements an interface then ____ methods that appear in the interface _______ with the exact same header

all, must be implemented

how is multiple inheritance is supported through th use of interfaces

allows a class to have more than one super class to have more than one super class and to inherit features from all parent class

A derived class pointer can _________ be assigned to a base class pointer (always, never)

always

any methods that are abstract must be implemented by

an inheriting class

What is an iterator?

an object that behaves like a pointer and is used to access items stored in containers

an interface extends

another interface, rather than implementing

invoking methods

any method declared for type vehicle can be invoked on object v.

derived class

applies only to the immediate parent

Describe a vector

array style of storing items - automatically grows as needed and easy to insert at the end of array - not easy to insert in middle of array

Which of the following is not possible? a. A class that implements two interfaces. b. A class that inherits from two classes. c. A class that inherits from one class, and implements an interface. d. All of the above are possible.

b. a class that inherits from two classes.

Assigning a subclass reference to a superclass variable is safe ________. a. because the subclass object has an object of its superclass. b. because the subclass object is an object of its superclass. c. only when the superclass is abstract. d. only when the superclass is concrete.

b. because the subclass object is an object of its superclass

Which keyword is used to specify that a class will define the methods of an interface? a. uses b. implements c. defines d. extends

b. implements

If the superclass contains only abstract method declarations, the superclass is used for ________. a. implementation inheritance. b. interface inheritance. c. Both. d. Neither.

b. interface inheritance

It is a UML convention to denote the name of an abstract class in ________. a. bold. b. italics. c. a diamond. d. there is no convention of the UML to denote abstract classes—they are listed just as any other class.

b. italics

Which of the following is false? a. You should not call overridable methods from constructors—when creating a subclass object, this could lead to an overridden method being called before the subclass object is fully initialized. b. It's OK to any of a class's methods from its constructors. c. When you construct a subclass object, its constructor first calls one of the direct superclass's constructors. If the superclass constructor calls an overridable method, the subclass's version of that method will be called by the superclass constructor. d. It's acceptable to call a static method from a constructor.

b. its ok to any of a class' methods from its constructors.

For which of the following would polymorphism not provide a clean solution? a. A billing program where there is a variety of client types that are billed with different fee structures. b. A maintenance log program where data for a variety of types of machines is collected and maintenance schedules are produced for each machine based on the data collected. c. A program to compute a 5% savings account interest for a variety of clients. d. An IRS program that maintains information on a variety of taxpayers and determines who to audit based on criteria for classes of taxpayers.

c. A program to compute a 5% savings account interest for a variety of clients.

Which interface is specifically intended to be implemented by classes that can be used with the try-with-resources statement? a. Comparable b. Runnable c. AutoCloseable d. Serializable

c. AutoClosable

Which of the following statements is false? a. In Java SE 8, an interface may declare default methods—that is, public methods with concrete implementations that specify how an operation should be performed. b. When a class implements an interface, the class receives the interface's default concrete implementations if it does not override them. c. When you enhance an existing interface with default methods—any class that implemented the original interface will break. d. With default methods, you can declare common method implementations in interfaces (rather than abstract classes), which gives you more flexibility in designing your classes.

c. When you enhance an existing interface with default methods- any class that implemented the original interface will break.

A class that implements an interface but does not declare all of the interface's methods must be declared ________. a. public. b. interface. c. abstract. d. final.

c. abstract

A(n) class cannot be instantiated. a. final. b. concrete. c. abstract. d. polymorphic.

c. abstract

Classes and methods are declared final for all but the following reasons: a. final methods allow inlining the code. b. final methods and classes prevent further inheritance. c. final methods are static. d. final methods can improve performance.

c. final methods are static

The UML distinguishes an interface from other classes by placing the word "interface" in above the interface name. a. italics. b. carets. c. guillemets. d. bold.

c. guillemets ' '

Which of the following could be used to declare abstract method method1 in abstract class Class1 (method1 returns an int and takes no arguments)? a. public int method1(); b. public int abstract method1(); c. public abstract int method1(); d. public int nonfinal method1();

c. public abstract int method1();

In Java SE 7 and earlier, an interface may contain: a. private static data and public abstract methods. b. only public abstract methods. c. public static final data and public abstract methods. d. private static data and public final methods.

c. public static final data and abstract methods.

public class A extends B {

class A is a subclass of B

public class B extends A {

class B is a subclass of A

keyword abstract needs to be included in

class header, method headers for any methods that are abstract

An abstract class

class with both implemented and abstract methods. can have variables and constructors

Abstract Class

class with one or more abstract methods

Which benefits the class more: interfaces or clients?

clients

Non-abstract classes are called:

concrete

a subclass requires a ____ if the superclass has a constructor

constructor

everything except for _____ is inherited from the superclass including members that are not accessible in the subclass

constructors

Which of the following statements is false? a. As of Java SE 8, any interface containing only one method is known as a functional interface. b. There are many functional interfaces throughout the Java APIs. c. Functional interfaces are used extensively with Java SE 8's new lambda capabilities. d. Anonymous methods provide a shorthand notation for creating lambdas.

d. Anonymous methods provide a shorthand notation for creating lambdas.

Non-abstract classes are called ________. a. real classes. b. instance classes. c. implementable classes. d. concrete classes.

d. Concrete classes.

When a superclass variable refers to a subclass object and a method is called on that object, the proper implementation is determined at execution time. What is the process of determining the correct method to call? a. early binding. b. non-binding. c. on-time binding. d. late binding.

d. Late binding also called dynamic binding.

Which of the following statements is false? a. Prior to Java SE 8, it was common to associate with an interface a class containing static helper methods for working with objects that implemented the interface. b. Class Collections contains many static helper methods for working with objects that implement interfaces Collection, List, Set and more. c. Collections method sort can sort objects of any class that implements interface List. d. With non-static interface methods, helper methods can now be declared directly in interfaces rather than in separate classes.

d. With non-static interface methods, helper methods can now be declared directly in interfaces rather than in separate classes.

Which statement best describes the relationship between superclass and subclass types? a. A subclass reference cannot be assigned to a superclass variable and a superclass reference cannot be assigned to a subclass variable. b. A subclass reference can be assigned to a superclass variable and a superclass reference can be assigned to a subclass variable. c. A superclass reference can be assigned to a subclass variable, but a subclass reference cannot be assigned to a superclass variable. d. A subclass reference can be assigned to a superclass variable, but a superclass reference cannot be assigned to a subclass variable.

d. a subclass reference can be assigned to a superclass variable, but a superclass reference cannot be assigned to a subclass variable.

Interfaces can have methods. a. 0 b. 1 c. 2 d. any number of

d. any number of methods

Consider the abstract superclass below: public abstract class Foo { private int a; public int b; public Foo(int aVal, int bVal) { a = aVal; b = bVal; } public abstract int calculate(); } Any concrete subclass that extends class Foo: a. Must implement a method called calculate. b. Will not be able to access the instance variable a. c. Neither (a) nor (b). d. Both (a) and (b).

d. both a and b

Which of the following does not complete the sentence correctly? An interface . a. forces classes that implement it to declare all the abstract interface methods. b. can be used in place of an abstract class when there is no default implementation to inherit. c. is declared in a file by itself and is saved in a file with the same name as the interface followed by the .java extension. d. can be instantiated.

d. can be instantiated

Declaring a method final means: a. it will prepare the object for garbage collection. b. it cannot be accessed from outside its class. c. it cannot be overloaded. d. it cannot be overridden.

d. it cannot be overriden

Consider classes A, B and C, where A is an abstract superclass, B is a concrete class that inherits from A and C is a concrete class that inherits from B. Class A declares abstract method originalMethod, implemented in class B. Which of the following statements is true of class C? a. Method originalMethod cannot be overridden in class C—once it has been implemented in concrete class B, it is implicitly final. b. Method originalMethod must be overridden in class C, or a compilation error will occur. c. If method originalMethod is not overridden in class C but is called by an object of class C, an error occurs. d. None of the above.

d. none of the above

Which interface is used to identify classes whose objects can be written to or read from some type of storage or transmitted across a network? a. Comparable b. Runnable c. AutoCloseable d. Serializable

d. serializable

local variable

declared inside the method

C++ uses the _________ to determine access to the members of the pointed-to object

declared type of a pointer

member variable

defined outside of any methods but inside the class

The base class pointer must already point to a __________ for rule 2 to work

derived class object

overloading

different header(# of parameters or type of parameters)

keyword for inheriting a class

extends

interfaces can inclue

final static variables with initialization

11.23 Show the output of following program: public class Test { public static void main(String[] args) { new A(); new B(); } } class A { int i = 7; public A() { setI(20); System.out.println("i from A is " + i); } public void setI(int i) { this.i = 2 * i; } } class B extends A { public B() { System.out.println("i from B is " + i); } public void setI(int i) { this.i = 3 * i; } }

i from A is 40 i from A is 60 i from B is 60

dynamic binding

if method is overridden, JVM will determine the correct method to invoke at runtime

What is syntax equivalent of extends for classes in interfaces?

implements

keyword for interface

implements

What do you put after a class that multiply inherits from interfaces?

implements <interface names> e.g. class Car implements Drivable, Identifiable {

use super to

initialize any private superclass variables on the first line of the subclass constructor

an abstract class is not

instantiable, the constructor cannot be invoked unless by an inheriting class using a super keyword

comparable interface requires the method

int compareTo(Object obj);

Extends

is the keyword used to inherit the properties of a class. Following is the syntax of extends keyword.

inheritance=

is-a

Describe a deque

items can be added to the front & back - not efficient when adding in middle

extends

keyword must be used to inherit a class

instanceof

keyword that determines if a superclass variable declared type matches the subclassvariable.

polymorphism is all about

late binding

11.34 Explain why the following code is wrong. ArrayList<Double> list = new ArrayList<>(); list.add(1);

list consists of Double objects. list.add(1) automatically converts 1 into an Integer object. It will work if you change it to list.add(1.0).

interface:

list of method headers, called abstract classes

11.33 Explain why the following code displays [1, 3] rather than [2, 3]. ArrayList<Integer> list = new ArrayList<>(); list.add(1); list.add(2); list.add(3); list.remove(1); System.out.println(list); How do you remove integer value 3 from the list?

list.remove(1) sees this as the index, not the object. so it removes the element at location 1. To remove integer value 3, use list.remove("3");

default accessibility modifier

members can be accessed by same class and from the same package, but no where else @default designation

public keyword

members can be accessed from other classes, packages, subclasses @Intended for users of the class. @accessibility modifier cannot be changed on override.

protected keyword

members can only be accessed by the same class, subclasses, and classes in the same package. @intended for use in extended subclasses, not for users of the class @a subclass can override a protected method and change its visibility to public

private keyword

members can only be accessed from inside the class @Not intended for use outside the class @Cannot be overriden

11.19 Can you assign new int[50], new Integer[50], new String[50], or new Object[50], into a variable of Object[] type?

new int[50] cannot be assigned to into a variable of Object[] type, but new Integer[50], new String[50], or new Object[50] are fine.

why shouldn't an abstract method be declared final

object class cannot be overridden must be if a concrete class ever is to be instantiated

polymorphism means

objects can take many forms and still have the same behaviors

polymorphism allows for

objects that inherit a type or implement an interface to be dealt with in the same way, despite the fact that they may have different methods

protected modifier

only accessible by the classes of the same package and the subclasses present in any package

private members

only accessible within the same class as it is declared

superclass method can be written in two ways:

overriding, overloading

visibility /accessibility modifiers

private, default, protected, public

default access modifier for members in interface

public

a subclass can override any ______ of the super class, but cannot override _____

public instance method, private or static methods

declare an interface Animal with methods eat(String food) and travel()

public interface Animal{ public void eat(String food); public void travel(); }

polymorphic reference

refer to different types if objects over time

A _____ ____ can refer to any object created from any class related to it by inheritance.

reference variable

overriding

same header but different implementation

What are examples of an associate container?

set multiset map multimap

You can typecast the base class to the derived class (given base class Animal, derived class Cat with function setMeow)

static_class<Cat*>(animal) -> setMeow(meow);

everything that is public, protected or default if the subclass is in the same package is accessible from inside the

sub class

Dog

subclass

example of using super on method

super.someMethod();

Animal

superclass

polymorphic variable

take on many different rupees, but it is not known which type it has taken on until the program is executing, the decision is made by the run- time environment

late binding

the same name will be associated with different semantics as the program executes

if a class is declared to be abstract

then some methods in the class may have their bodies omitted

if 2 objects are children of the same parent

then, the equals method return true if they reference the same object

if a programmer wants a class to extended

then, the programmer must make private methods and instance data protected

instanceOF is used when

there is a concern about the actual type of the object

if a method defined on a subclass of vehicle needs to be invoked then

type casting must be used

if the superclass has private instance variables, the subclass constructor must

use a method or constructor from the superclass to initialize it

to access a method from superclass,

use the super keyword

how can one create polymorphic references

using inheritance and using interfaces

instance variable

variables within a class but outside any method

ClassCastException at runtime when

you cast to an incorrect type

Power of Polymorphism

Extensibility

Liskov Substitution Principle

Is-A relationship

Chapter Objectives:

1) Define polymorphism and explore its benefits. 2) Discuss the concept of dynamic binding. 3) Use inheritance to create polymorphic references. 4) Explore the purpose of the syntax of Java interfaces. 5) Use interfaces to create polymorphic references. 6) Discuss object-oriented design in the context of polymorphism.

What is the difference between a class and an interface?

A class can be instantiated; an interface cannot. An interface can contain only abstract methods and constants. A class provides the implementation of r an interface.

interface

A collection of abstract methods, used to define a set of operations that can be used to interact with an object.

Polymorphism allows for specifics to be dealt with during: A. execution. B. compilation. C. programming. D. debugging.

A. execution.

Private Access Modifier

Accessible only within the class

Protected Access Modifier

Accessible within package and outside the package but only through inheritance

Which of the following is not possible? A. A class that implements two interfaces. B. A class that inherits from two classes. C. A class that inherits from one class, and implements an interface. D. All of the above are possible.

B. A class that inherits from two classes.

An interface may contain: A. private static data and public abstract methods. B. only public abstract methods. C. public static final data and public abstract methods. D. private static data and public final methods.

C. public static final data and public abstract methods.

Consider classes A, B and C, where A is an abstract superclass, B is a concrete class that inherits from A and C is a concrete class that inherits from B. Class A declares abstract method originalMethod, implemented in class B. Which of the following statements is true of class C? A. Method originalMethod cannot be overridden in class C—once it has been implemented in concrete class B, it is implicitly final. B. Method originalMethod must be overridden in class C, or a syntax error will occur. C. If method originalMethod is not overridden in class C but is called by an object of class C, an error occurs. D. None of the above.

D. None of the above.

Which of the following does not complete the sentence correctly?An interface . A. forces classes that implement it to declare all the interface methods. B. can be used in place of an abstract class when there is no default implementation to inherit. C. is declared in a file by itself and is saved in a file with the D. can be instantiated.

D. can be instantiated.

Declaring a method final means: A. it will prepare the object for garbage collection. B. it cannot be accessed from outside its class. C. it cannot be overloaded. D. it cannot be overridden.

D. it cannot be overridden.

When a superclass variable refers to a subclass object and a method is called on that object, the proper implementation is determined at execution time. What is the process of determining the correct method to call? A. execution time binding. B. execution binding. C. just-in-time binding. D. late binding

D. late binding.

11.1 True or false? A subclass is a subset of a superclass.

False. A subclass is an extension of a superclass and normally contains more details information than its superclass.

11.8 True or false? You can override a static method defined in a superclass.

False. You can only override accessible instance methods.

super

Keyword refers to the superclass and can be used to invoke the superclass's methods and constructors. This is used in two way: @To call a superclass constructor @To call a superclass method

Abstract method

Method with no body

Overriding a method

Overriding a method is recreating a method from a superclass in the subclass with the same signature and return type to specialize it for the subclass. @Must be prefaced with @Override key word. @Must have the same signature @Must have the same return type @Must be accessible by the subclass @Cannot be a static method

Describe the Comparable interface?

The Comparable interface contains a single method called compareTo, which should return an integer that is less than zero, equal to zero, or greater than zero if the executing object is less than, equal to, or greater than the object which it is being compared, respectively.

dynamic binding

The binding of a method invocation to its definition at run-time. Also called late binding.

11.9 How do you explicitly invoke a superclass's constructor from a subclass?

Use super() or super(args). This statement must be the first in the constructor in the subclass.

11.10 How do you invoke an overridden superclass method from a subclass?

Use super.method() or super.method(args).

How is overriding related to polymorphism?

When a child class overrides the definition of a parent's method, two versions of that method exist. If a polymorphic reference is used to invoke the method, the version of the method that is invoked is determined by the type of the object being referred to, not by the type of the reference variable.

constructor chaining

When constructing an object of a subclass, the subclass constructor first invokes its superclass constructor before performing its own tasks. If the superclass is a subclass, the superclass constructor invokes it's parent-class constructor before performing its own tasks; this continues until the ultimate parent class.

All of the following methods are implicitly final except: A. a method in an abstract class. B. a private method. C. a method declared in a final class. D. static method.

a method in an abstract class.

A(n)_________class cannot be instantiated. A. final. B. concrete. C. abstract. D. polymorphic.

abstract

area

amount of 2D space occupied by the shape; computed differently by different shapes

A ____ to a method can be polymorphic which gives the method flexible control of its arguments.

parameter

The binding of a method invocation to its definition is performed at ____ for a ____ reference.

run-time, polymorphic

Polymorphism

The ability for an object to take on many forms, EX: when a parent class reference is used to refer to a child class object.

polymorphism

The ability to define an operation that has more than one meaning by having the operation dynamically bound to methods of various objects.

Multiple Inheritance

The ability to derive a subclass from multiple classes. Java does not allow this, however does allow a superclass and an interface.

public class XYZ extends XYZparent

extends is what tells the compiler this class XYZ is extending the superclass XYZparent

Which keyword is used to specify that a class will define the methods of an interface? A. uses. B. implements. C. defines. D. extends.

B. implements.

If the superclass contains only abstract method declarations, the superclass is used for: A. implementation inheritance. B. interface inheritance. C. Both. D. Neither.

B. interface inheritance.

It is a UML convention to denote the name of an abstract class in: A. bold. B. italics. C. a diamond. D. there is no convention of the UML to denote abstract classes—they are listed just as any other class.

B. italics

Constants declared in an interface are implicitly A. private. B. static. C. abstract. D. All of the above.

B. static.

polymorphic reference

A reference variable that can refer to different types of objects at different points in time.

11.5 How does a subclass invoke its superclass's constructor?

A subclass can explicitly invoke a suplerclass's constructor using the super keyword.

Which of the following statements about interfaces is false? A. An interface describes a set of methods that can be called on an object, providing a default implementation for the methods. B. An interface describes a set of methods that can be called on an object, not providing concrete implementation for the methods. C. Interfaces are useful when attempting to assign common functionality to possibly unrelated classes. D. Once a class implements an interface, all objects of that class have an is-a relationship with the interface type

A. An interface describes a set of methods that can be called on an object, providing a default implementation for the methods.

Which of the following statements about abstract superclasses is true? A. abstract superclasses may contain data. B. abstract superclasses may not contain implementations of methods. C. abstract superclasses must declare all methods as abstract. D. abstract superclasses must declare all data members not given values as abstract.

A. abstract superclasses may contain data.

Public Access Modifier

Accessible Everywhere

Inheritance

Allows one to define a general class (ie superclass) and later extend it to more specialized classes (ie subclasses) that inherits the properties and methods from the general class. @subclasses generally have more methods than superclass @Private data fields in the superclass are not accessible by a subclass, only by getters and setters @A subclass and superclass must have the same properties in an "is-a" @Not all "is-a" relationships are appropriate for inheritance. EX 1: A square is a rectangle, but should not extend from the rectangle class, rather should extend from geometricObject EX 2: Both people and trees have height and weight, but their classes should not be extensions. @Java only allows SINGLE INHERITANCE from a single superclass, however can be achieved using interfaces (abstract class)

Assigning a subclass reference to a superclass variable is safe: A. because the subclass object has an object of its superclass. B. because the subclass object is an object of its superclass. C. only when the superclass is abstract. D. only when the superclass is concrete.

B. because the subclass object is an object of its superclass.

For which of the following would polymorphism not provide a clean solution? A. A billing program where there is a variety of client types that are billed with different fee structures. B. A maintenance log program where data for a variety of types of machines is collected and maintenance schedules are produced for each machine based on the data collected. C. A program to compute a 5% savings account interest for a variety of clients. D. An IRS program that maintains information on a variety of taxpayers and determines who to audit based on criteria for classes of taxpayers.

C. A program to compute a 5% savings account interest for a variety of clients

A class that implements an interface but does not declare all of the interface's methods must be declared: A. public. B. interface. C. abstract. D. final.

C. abstract.

Classes and methods are declared final for all but the following reasons: A. final methods allow inlining the code. B. final methods and classes prevent further inheritance. C. final methods are static. D. final methods can improve performance.

C. final methods are static.

The UML distinguishes an interface from other classes by placing the word "interface" in above the interface name. A. italics. B. carets. C. guillemets. D. bold

C. guillemets.

Which of the following could be used to declare abstract method method1 in abstract class Class1 (method1 returns an int and takes no arguments)? A. public int method1(); B. public int abstract method1(); C. public abstract int method1(); D. public int nonfinal method1();

C. public abstract int method1();

How do class hierarchies and interface hierarchies intersect?

Class hierarchies and interface hierarchies do not intersect. A class can be used to derive a new class, and an interface can be used to derive a new interface, but these two types of hierarchies do not overlap.

super()

Constructors of the superclass are not inherited, they can only be invoked from the constructors of the subclasses by using super(). @ super() or super(args) must be the FIRST statement of the subclasses constructor.

Which statement best describes the relationship between superclass and subclass types? A. A subclass reference cannot be assigned to a superclass variable and a superclass reference cannot be assigned to a subclass variable. B. A subclass reference can be assigned to a superclass variable and a superclass reference can be assigned to a subclass variable. C. A superclass reference can be assigned to a subclass variable, but a subclass reference cannot be assigned to a superclass variable. D. A subclass reference can be assigned to a superclass variable, but a superclass reference cannot be assigned to a subclass variable.

D. A subclass reference can be assigned to a superclass variable, but a superclass reference cannot be assigned to a subclass variable.

Consider the abstract superclass below: public abstract class Foo{ private int a; public int b; public Foo( int aVal, int bVal ) { a = aVal; b = bVal; } // end Foo constructor public abstract int calculate(); } // end class Foo Any concrete subclass that extends class Foo: A. Must implement a method called calculate. B. Will not be able to access the instance variable a. C. Neither (a) nor (b). D. Both (a) and (b)

D. Both (a) and (b)

11.6 True or false? When invoking a constructor from a subclass, its superclass's no-arg constructor is always invoked.

False. If a subclass's constructor explicitly invoke a superclass's constructor, the superclass's no-arg constructor is not invoked.

How does inheritance support polymorphism?

In Java, a reference variable declared using a parent class can be used to refer to an object of the child class. If both classes contain a method with the same signature, the parent reference can be polymorphic.

Why is inheritance useful?

It enables polymorphism and code sharing.

What is polymorphism?

Polymorphism is the ability of a reference variable to refer to objects of various types at different times. A method invoked through such a reference is bound to different method definitions at different times, depending on the type of the object referenced.

11.3 What is single inheritance? What is multiple inheritance? Does Java support multiple inheritance?

Single inheritance allows a subclass to extend only one superclass. Multiple inheritance allows a subclass to extend multiple classes. Java does not allow multiple inheritance

Interface

Special class with no data and only abstract methods

Access Modifier

Specifies accessibility or scope of a field, method, constructor or class

Why is the StaffMember class in the firm example declared as abstract?

The StaffMember class is abstract because it is not intended to be instantiated. It serves as a placeholder in the inheritance hierarchy to help organize and manage the object polymorphically.

superclass / parent class / base class

The class a subclass inherits from.

child class / extended class / derived class

The class that inherits accessible data fields and methods from a superclass and may add new specialized data fields and methods and/or override superclass ones.

11.2 What keyword do you use to define a subclass?

The extends keyword is used to define a subclass that extends a superclass.

Interface hierarchy

The hierarchy formed when interfaces are derived from other interfaces. Interface hierarchies are distinct from class hierarchies.

Why is the pay method declared in the StaffMember class, given that it is abstract and has no body at that level?

The pay method has no meaning at the StaffMember level, so it is declared as abstract. But by declaring it there, we guarantee that every object of its children will have a pay method. This allows us to create an array of StaffMember objects, which is actually filled with various types of staff members, and to pay each one. The details of being paid are determined by each class, as appropriate.

super.method(parameters);

This is used to reference a method in a superclass. Usually these are inherited so this is not always necessary unless....

11.4 What is the output of running the class C in (a)? What problem arises in compiling the program in (b)? (a) class A { public A() { System.out.println("A's no-arg constructor is invoked"); } } class B extends A { } public class C { public static void main(String[] args) { B b = new B(); } } (b) class A { public A(int x) { } } (c) class B extends A { public B() { } } public class C { public static void main(String[] args) { B b = new B(); } }

a. A's no-arg constructor is invoked b. The default constructor of B attempts to invoke the default of constructor of A, but class A's default constructor is not defined.

perimeter

distance around the outside of the shape; computed differently based on shape

If you want an is-a relationship or polymorphism, but you don't want to give a subclass access to the code, inheritance won't give you the ______ you need.

encapsulation

An ____ is a collection of abstract methods and therefore cannot be instantiated.

interface

An _____ name can be used to declare an objects reference variable.

interface

An _____ reference can refer to any object of any class that implements that interface.

interface

The relationship between a listener and the component it listens to is established using _____.

polymorphism

binding

the process of determining which method definition is used to fulfill a given method invocation.


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