JAVA CLASSES
How many times is sVATRate created when new objects are created if it is static?
Only one of sVATRate created ever, and each object references this state variable rather than having a new sVATRate created for each object
All value types
- byte - short - int - long - float - double - boolean - char
Method-local Inner Class
- can be instantiated only within the method where the inner class is defined.
this
-allows an object to refer to itself -the currently executing object
Public visibility
-anyone can edit variables anywhere -public methods provide services to clients
Class
-blueprint of an object -model or pattern from which objects are created
Static method
-can be called through class name -instantiation of an object is not needed to call method -cannot refer to instance variables since it was not called through an object, which holds those variables
Private visibility
-can only be accessed from inside the class -support other methods in the class (encapsulation)
nested classes
-class inside a class -nested class can use enclosed class's private variables but not vice versa
Interfaces
-class with abstract methods and constants (no code) -formal contract as to what a class must implement -semicolon after each method header, interface used in class line (public interface test) -class must implement interface in class line -ex. comparable (compareTo)
Objects
-contains states and behaviors -state = variables -behaviors = methods
Exceptions
-describes and unusual or erroneous situation -exceptions are thrown by a program, and may be caught by another part -stack-trace used to find errors -try{ statements} catch (name of exception){ statements}
message dialog
-displays output string
Instance data
-each instance of the object has its own version of the variable -variables declared at the beginning of the class -class declares type of data, but does not reserve memory -have default values
reference assignment
-for object references, assignment copies the memory location (copies pointer) -will delete any object not being pointed to (trash collector) -if you change one, both are changes -for primitive data types, a copy of the value is made
Objects as parameters
-for primitive data types, copy of actual parameter is stored into the formal parameter -when objected is passed to a method, actual and formal parameters become aliases (copy of pointer)
Static Nested Class
-is a nested class which is a static member of the outer class. -can be accessed without instantiating the outer class, using other static members. -does not have access to the instance variables and methods of the outer class.
Constructor
-method that has the same name as the class -no return type Public name (parameters)
Two expectations for constructors
-name must be same as its class name -must have no explicit return type
input dialog
-presents a prompt and single input text field
confirm dialog
-presents user w/ simple yes-or-no question
Overloading
-processing of using same method for multiple methods -parameters (signatures) are different
Void
-return type of a method -doesn't need to return anything
Aliases
-two or more references that refer to the same object
null
-variable does not currently point to anything -an object declared at class level (instance variable) is automatically initialized to null
Static variable
-variable shared among all instances of a class -objects share same variable -changed for one=changed for all
What is necessary in a class?
-variables and methods -constructor -getters and setters -toString
Inner Class
-write a class within a class. -can be private and once you declare an inner class private, cannot be accessed from an object outside the class.
exp of abstraction
-you understand its external behavior -you dont understand its inner details and you dont need to
constructor
A class contains constructors that are invoked to create objects from the class blueprint. Constructor declarations look like method declarations—except that they use the name of the class and have no return type. For example, Bicycle has one constructor: public Bicycle(int startCadence, int startSpeed, int startGear) { gear = startGear; cadence = startCadence; speed = startSpeed; }
instance method
A non-static method that has access to this and the instance variables.
You cannot begin variable names with what?
A number . Ex) 5data
Math.min()
Returns the smallest parameter Ex int m = Math.min(10, 20); // 10
Getter
Returns value
Math.ceil()
Rounds to nearest integer up when using double
Math.max()
Rounds to the nearest integer down when using dobule
Protected
Same as default, with the addition that subclasses can access protected methods and variables of the superclass (learn more about subclasses and superclass later)
"he"
String s1 = "The Beatles"; String s2 = s1.substring(1, 3)
11
String s1 = "The Beatles"; int len = s1.length()
Create a string declaration assigning the string "Hello" to the variable value.
String value = "Hello";
Examples of immutable types in java
String, Integer, Float, Double, Byte, Character, Long
API
Application programming interface, a collection of packages that comes with most IDE's
Attributes
Characteristics in an object. For example, a car can be red or blue
T/F arrays cannot contain primitive types
F
T/F default constructor is generated by the compiler even when another constructor defined in the class
F
T/F if a class is immutable, it needs setters
F
T/F static members can be associated with any specific instance of the class
F
T/F private variables can be seen in methods from other classes
F
T/F static variables can only be used in static methods
F
T/F boxing is the automatic conversion of a private variable to a wrapper class
F, it is the automatic conversion of a primitive type to the corresponding wrapper class
Method
Synonymous for behavior Coding def: a collection of statements that are grouped together to perform an operation
This class contains methods and objects that perform system level tasks.
System
Scanner objects work with...
System.in
Write the Java code to print the line "Hello World!" out to the system.
System.out.println("Hello World!");
T/F ArrayLists can contain other ArrayLists
T
T/F arrays can contain ArrayLists
T
T/F arrays can contain other arrays
T
T/F encapsulation restricts access to data within a class
T
T/F if there is no constructor defined, the compiler generates a default constructor
T
T/F a constructor can assign a value to a constant (variable defined final) with all variables
F, only for class variables. not variables defined within the constructor or within already created objects
T/F it is okay to have two constructors with the same name but different parameters
T
T/F the default equals method checks memory location
T
T/F you can make a class immutable by setting the variables to final
T
T/F Encapsulation controls access to variables (ie private vs public) using getters and setters
T
T/F Every class is a subclass of another class
T
T/F a constructor will control how the instance variables are initialized
T
The default constructor sets all of the class' boolean fields to?
False
Class
Templates for a set of objects, classes represent various concepts.
What does a driver program do?
Tests the capabilities of other classes.
What does a class's interface provide?
The information needed to use a class without revealing anything about its implementation
What are members of a class?
The instance variables and methods
How would you encapsulate data?
Through getter and setter methods
identical
Two values that are the same. In the case of objects, two variables that refer to the same object, also called aliases
How many classes are created when this program is compiled and what are the names of them?
Two, Vehicle.class and VehicleDemo.class
How many parameters does isFact have?
Two, a and b
Which lines represent the main program?
Line 6 and onwards
What does the dot operator do?
Links the name of an object with the name of a member.
What are static variable allowed to use?
Local variables and static variables...can't use anything that is instance-specific
What method is the starting point for any program?
Main method
Differences between a typical functional programming language and a typical imperative programming language
Nature of functions, type inference, classes and objects vs no units, specifying what to do vs how and what to do
How does Java have mutable state?
You can change the values of state after you've initialised it
Q: How would you describe the difference between this() and super()?
You can use this() for invoking the constructor of the class while super() helps to call the super class constructor.
Q: What difference you see between superclass and subclass?
You can use this() for invoking the constructor of the class while super() helps to call the super class constructor.
code reuse
You can write a method once, and use it multiple times, without having to rewrite the code each time
Encapsulation
a mechanism of wrapping the data (variables) and code acting on the data (methods) together as a single unit
accessor
a method that lets clients examine object state exp: distance, distance,FromOrigin
a module is a +++++ not a ++++++
a module is a partial program, not a complete program
java.util
a package of classes
a variable holds the object, or a reference to an object?
a reference to an object
module
a resable piece of software stored as a class
a void method does not need to contain?
a return statement
object
a runtime entity that contains data and responds to messages
class
a software package or template that describes the characteristics of similar objects
local variable
a variable defined inside a method, or block
field
a variable inside an object that is part of its state -each object has its own copy of each field
reference
a variable that ism declared to have a class type is a ____ to an object
reference variable
a variable which has a primitive type contains a value of that type
constructor syntax
access specifier(PUBLIC) -> name of class -> list of parameters including data type { constructor body }
method syntax
access specifier(public or private) -> return type of variable -> method name -> list of incoming variables including their type { method body }
instance field syntax
access specifier(public or private) -> type of variable -> name of variable
accessors
accesses an object and returns information about that object without altering the object
3 types of methods
accessor, mutator, helper
methods
actions of the object (bank account = withdrawals, deposits)
point method: translate(dx,dy)
adjusts the point's x and y by the givenamounts
helper method
aids the program in solving problems... usually done behind the scenes
server class
all the methods/data that is stored
private
can only be used inside that class
identity
distinguish from other objects in memory, as handled by the JVM
declaring a variable (does/does not) create an object
does not
immutable means
does not change
instance method(or object method)
exits inside each object of a class and gives behavior to each object
A constant made up of variables, operators, and method invocation
expression
What syntax is used to get one class to inherit data and behavior from another class?
extends <a class>
For loop syntax
for (int i=0; i<8; i++){ // or can decrement counter } int j=0; for(; j<8; j++) { // can initialise variable before for }
Looping over array with for loop
for (int i=0; i<arr.length; i++){ } for (String element : arr){ // for each element in array of type String[] }
Creating Format Strings
format(), that returns a String object rather than a PrintStream object - to create a formatted string that you can reuse, as opposed to a one-time print statement.
example of? class MountainBike extends Bicycle{ //new fields and methods defining the subclass MountainBike }
inheritance
constructors
initialization of an object (bank account = 2233, $600)
static variable, class variable
contains a value that is shared by all instances of the class
What syntax do you use to skip the current iteration in a loop?
continue;
When to use lambda expressions
Use it if you are encapsulating a single unit of behavior that you want to pass to other code, or if you need a simple instance of a functional interface and none of the preceding criteria apply
cos()
cosine
When to use local class
Use it if you need to create more than one instance of a class, access its constructor, or introduce a new, named type
When to use anonymous class
Use it if you need to declare fields or additional methods.
To return a string with an object, you would...
create toString method
What does this code do? String [] names = new String[20];
creates an array of 20 strings
Instantiating an object
creating object of a class, use the keyword new
constructor
initialize the instance variables of a newly instantiated object, only activated by keyword "new," all must have the same name as the class
Constants must be _____ by the program before being used.
initialized
What does the second line of code do? PairOfDice dice; //declares a variable of type PairOfDice dice = new PairOfDice();
initializes the object's instance variables and returns a reference to the object
constructor
initializes the state of new objects
Getter syntax
public string getVariable() The first letter of the variable is capitalized
Setter syntax
public void setVariable(String something)
How does the keyword super work with constructors?
super(<parameters>) at the beginning of the corresponding constructor in the subclass
How does the keyword super work with methods other than constructors?
super.<method name>(<parameters>)
default constructor
takes no arguments and provides reasonable initial values for an object
Math.pow()
takes two parameters and returns the first parameter raised to the power of the second parameter ex double p = Math.pow(2, 3); // 8.0
import java.util.Scanner;
tell java that the Scanner class will be used
access specifier
tells which other methods can call this method (public, private, static)
garbage collection
the JVM process of keeping track of which objects need to be stored and which can be deleted
outer class
the class that holds the inner class
methods
the code to manipulate the object data
encapsulation
the combining of data and behavior into a single software package
object reference
the identifier of the object
After executed, what does "dice" refer to? PairOfDice dice; //declares a variable of type PairOfDice dice = new PairOfDice();
the newly created object
'this' keyword refers to what?
the object being created
this
the object is an implicit parameter for the method and is referred with this keyword
implicit parameter
the object on which an instance method is called
In a JUnit test, what does the code below expect? assertFalse(myMethod(5, 15));
the output of the method to be false and triggers an error otherwise
state
the particular value of a instance variable at any moment which changes in response to messages sent to the object
turn(degrees)
the pen adds the indicated degrees to its current direction. positive degrees counterclockwise, negative degrees clockwise
home()
the pen jumps to the center of the graphics window without drawing and point north
down()
the pen lowers itself to the drawing surface
move(distance)
the pen moves the specified distance in the current direction. Can be integer or floating point. expressed in width x height
up()
the pen raises itself from the drawing surface
instantiation
the process of creating a new object
What does the blank in public ______ setName(String name){} represent
the return type -could be void or a type(double,string,int,point,etc)
what must be the name of the constructor?
the same name of the class in which it is defined
String
the second version finds the first occurrence of a
you can define constants with?
the static modifier and the final modifier
interface
the way for the client to interact with the server
two
there are ___ versions of indexOf
public methods
these are accessible to all client programs
private methods and variables
these can only be accessed by methods of that class
default constructors
these constructors have empty parameter lists
String Length
length() method : returns the number of characters contained in the string object.
toLowerCase
method returns a new String containing the original characters, but with all letters in lower case
visibility, return type, name, parameter
method structure
a reference to the current object
this
What modifiers can be used on a constructor definition?
three access modifiers: public, private, and protected
used to throw an exception from within a method. When a throw statement is encountered and executed, execution of the current method is stopped and returned to the caller
throw keyword
dereference
to access data or methods of an object with the dot notation, such as s.length
Strings
treated as objects.
Method syntax
visibility modifier-return type-name-(parameters) -unless void return type, return must occur
method declared ____ does not return a value
void
Formal parameter
what is needed for method to proceed
Actual parameter
what is put in it
.equals()
what method do we want to use when comparing strings?
client class
what the user sees
return type
what type is returned, void means nothing
null pointer exception
when a program attempts to run a method with a null object
the reference changes
when you reassign a string variable, what happens
instance fields
where an object stores its data
Math class
which provides predefined methods for mathematical operations
While loop syntax
while (boolean_expression){ }
? operator
works as an if-then (someCondition ? valueTrue: valueFalse)
points store
x,y daya
Are strings objects?
yes
Can a constructor have a list of formal parameters?
yes
correct? if (std == null)
yes
new
you can instantiate an object by using the keyword
a class combines
• Data — identifiers that hold values. Data can be any type (int, float, String, etc.) • Methods — code that manipulates the data
nextBoolean()
returns the next input as a boolean
nextByte()
returns the next input as a byte
nextDouble()
returns the next input as a double
nextFloat()
returns the next input as a float
nextLong()
returns the next input as a long
nextShort
returns the next input as a short
next()
returns the next token in the input line as a String
when asking the user for something
scanner
how to initialize a scanner
scanner input = new Scanner();
the java class library is a?
set of packages
toString()
A method that returns the value of the Integer as a String.
doubleValue()
A method that returns the value of the Integer as a double.
intValue()
A method that returns the value of the Integer as an int.
initalizing a string input
String name = input.nextLine();
super
We can invoke a constructor of the superclass using the "________" keyword.
Anonymous Inner Class
- An inner class declared without a class name - declare and instantiate them at the same time. -used whenever you need to override the method of a class or an interface.
String class
- immutable, so that once it is created a String object cannot be changed. - to make a lot of modifications to Strings of characters then you should use String Buffer & String Builder Classes.
Testing for equality
-== operator compares object references, are they pointing to the same thing, returns true if aliases -.equals() method does the same thing (unless redefines), also will check is string has same characters
Data scope
-Data declared in the method can only be used in that method, class level can be used in other methods in the class -Data declared within a method (local data) have no default value
Where does the turtle graphics pen originate at?
0,0 cartesian, down, pointing north
primitive type
1 box of data that contains a value
Which lines represent the class?
1 through 5
Interfaces are useful for:
1- Declaring methods that one or more classes are expected to implement. 2- Capturing similarities between unrelated classes without forcing a class relationship. 3- Determining an object's programming interface without revealing the actual body of the class.
Q: Explain the difference between function overloading and overriding?
1- Overloading talks about the relationship between methods of the same class whereas the overriding focuses on the relationship between a superclass method and subclass method. 2- Overloading doesn't impact the inheritance from the superclass whereas the overriding impedes inheritance from the superclass. 3- In overloading, you have different methods sharing the same name whereas in overriding, the methods in subclasses replace the superclass versions. 4- Overloading requires the methods to be implemented with distinct signatures whereas overriding limits to use the same markup.
What are the two rules to remember when using static variables
1. Class methods can reference only the static variables and never the instance variables 2. Instance methods can reference static and instance variables
What are the two benefits to using an interface name?
1. Methods that use interface types are more general, in that they work with any classes that implement the interface 2. It's easier to maintain a program that uses interface types
Why do we declare abstract methods?
1. The abstract class which they are declared implements an interface, but the implementation of these methods is deferred to concrete classes 2. Even if there is no interface, these methods must be implemented in all subclasses, so declaring them abstract enforces this
naming conventions (2)
1. class names start with an uppercase letter 2. capitalize internal words
shadowing
2 variables with same name in same scope
paralell arrays
2+ arrays with related data at same indexes
What are the four numbers outputted? public class QuizClass { public static int num1; public int num2; ... public static void main(String [] args) { QuizClass a = new QuizClass(); QuizClass b = new QuizClass(); a.num1 = 1; a.num2 = 2; b.num1 = 3; b.num2 = 4; System.out.println(a.num1 + " " + a.num2 + " " + b.num1 + " " + b.num2); } }
3, 2, 3, 4 since num1 is static and will be the same for all instances
How many objects of type A will be created? pubic class A{ public A friend; public A(){ friend = null; } public A (A other){ friend = newA(other.friend); } public void setFriend(A newFriend){ friend = newFriend; } A firstVar = new A(); firstVar.setFriend(new A()); A secondVar = new A(firstVar); }
4
Between which lines could you insert a method?
4 and 5
Given the piece of code shown below, in which line(s) is (are) an object(s) created? 1 class Vehicle { 2 int passengers; 3 int fuelcap; 4 int mpg; 5 } 6 class VehicleDemo { 7 public static void main(String args[]) { 8 Vehicle minivan = new Vehicle(); 9 Vehicle truck = new Vehicle(); 10 int range;
8 and 9
Nested classes are divided into two types
:Non-static nested classes − These are the non-static members of a class. :Static nested classes − These are the static members of a class.
constructor name
= to the class name
Why should a programmer use the method equals instead of the operator == to compare two objects for equality?
== is too restrictive. In cases where two distinct objects
What are the two ways to compare objects for equality?
== or the instance method equals
client
A class that uses objects defined in another class.
What is a constructor and what is it used for?
A constructor is a pieces of code within a class. It must have the same name as the class, and it's used for setting initial conditions on variables, or for start up conditions on a program.
Integer(String s)
A constructor that constructs an Integer object that represents the specified String value as an int.
Integer(int n)
A constructor that constructs an Integer object that represents the specified int value.
Package
A group of similar types of classes
Interfaces
A java interface specifies the set of methods available to clients of a class. An interface provides a way of requiring a class to implement a set of methods and a way of informing clients about services regardless of implementation detail
getter
A method that returns the value of an instance variable.
instance
A member of a class. Every object is an instance of some class.
compareTo()
A method that accepts an Integer argument and returns 0 if the Integer is equal to the Integer argument, a value less than 0 if the Integer is numerically less than the Integer argument or a value greater than 0 if the Integer is numerically greater than the Integer argument.
equals()
A method that accepts an Integer argument and returns true if the argument is an Integer object that contains the same int value as this object.
setter
A method that assigns a value to an instance variable.
What is a final method?
A method that cannot be overriden by a subclass
modifier method
A method that changes the state (instance variables) of an object.
Constructor for a class
A method that gets called when the object is constructed to initialise the state of the class for the new object in a convenient way
mutator
A method that modifies an object's state exp: setLocation, translate
constructor
A special method that initializes the instance variables of a newly-constructed object.
Integer.MAX_VALUE
A static field that represents the maximum possible value an int can contain.
Integer.MIN_VALUE
A static field that represents the minimum possible value an int can contain.
pure method
A static method that depends only on its parameters and no other data.
Q: Describe the difference between a Sub-Class and an Inner Class?
A subclass is a class which gets inherited from another class termed as super class. It can easily access all public/protected methods and fields of its superclass. Inner class is a class which gets cradled within another class. An Inner class can access all variables and methods provided by the outer class.
data encapsulation
A technique for bundling multiple named variables into a single object.
classes
A template for creating objects of a given type
What is an argument? For the code below, give an example. What does the program do? class Fact { boolean isFact(int a, int b) { if( (b % a) == 0) return true; else return false; } } class IsFact { public static void main(String args[]) { Fact x = new Fact(); if(x.isFact(2, 20)) System.out.println("Yes it is"); if(x.isFact(3, 20)) System.out.println("this won't be displayed"); } }
A value(s) passed to a method. 3, 20 Decides if the first number is a factor of the second
Default
A variable or method declared with no access control modifier is available to any other class in the same package.
Implementation of an abstract method
Abstract methods are a means of requiring certain behavior in all subclasses
Private
Accessible only within the declared class itself
Protected method
Accessible to a class's descendants, but not to any other classes in the hierarchy
Class method
Activated when a message is sent to the class rather than to an object
one
All Java programs consist of at least ____ class.
What is the role of the visibility modifier protected in a class hierarchy?
Allows subclasses in hierarchy to access instance and class variables or methods or superclasses without allowing other clients in the system to access these items
instance variable
An attribute of an object; a non-static variable defined at the class level.
What is an object?
An instance (copy) of a class
More on interface
An interface by definition has all public members without any implementation. While an abstract class may group different flavors of class members like private, protected, etc. but has at least one abstract method.
Q: What difference you see between an Abstract class and an Interface?
An interface by definition has all public members without any implementation. While an abstract class may group different flavors of class members like private, protected, etc. but has at least one abstract method.
Q: What is an interface and why is it used?
An interface is similar to a class which may contain method's signature only but not bodies, and it is a formal set of method and constant declarations that must be defined by the class that implements it.
What are the two ways that the + operator can be used?
As a concatenation operator and as an addition operator
As Java and most imperative languages don't have type inference, what do we need to do as a programmer?
Assign every value a type when declaring that value, and specify every function's return type and the types of each of a function's arguments
While naming variables...
Case sensitive, constant variables are all caps and underscores, $ may be used.
Instance Variable
Belongs to an object and is allocated storage when the object is created
any group of zero or more statements between braces
Block
What will this output? Person[] people = new Person[3]; people[0] = new Person("Bob"); people[1] = new Person("Alice"); people[2] = new Person("Jane"); // make a copy Person[] peopleCopy = new Person[3]; for (int i = 0; i < 3; i++){ peopleCopy[i] = people[i]; } // now I can change some names people[2].setName("Jane Doe"); people[0] = new Person("Michelle"); System.out.println(peopleCopy[0] + ":" + peopleCopy[1] + ":" + peopleCopy[2]);
Bob:Alice:Jane Doe
How can a class extend its inherited characteristics?
By adding instance variables and methods and by overriding inherited methods
Scope of a variable declared as a method parameter?
Called a parameter variable and like a local variable it is visible only within that method.
static variables, variables that do not change
Class variables
template
Classes are a _________ (or blueprint) used to create specific objects.
What is the difference between a class and an object?
Classes define what properties and procedures each object of the type should have, while each object is a specific implementation with particular values
Describe a hierarchy
Classes inherit the instance variables and methods of the classes above them
What are concrete classes?
Classes that extend abstract classes and are already instantiated
encapsulation
Combining an object's data and methods into a single unit called a class
Identifiers that can hold only a single value
Constants
We can use the this reference to ___.
Construct additional constructors efficiently by making calls to another constructor.
The String object is the only object that can be declared without using a _____.
Constructor
What happens at run time when a method throws an exception
Control is passed immediately to the calling method and from there goes on up the call chainto the main method, where the JVM halts execution with a trace of this call chain
How can youobtain a true copy of an object?
Create a new instance of that object's class and then cop-y the original object's instance variables to the new object's variables.
constructors
Creates an object of the class
What two items are used to cause your method to end?
Curly bracket or return
Hiding internal State and requiring all interaction to be performed through an object's methods
Data encapsulation
The __________ is a constructor with no parameters.
Default constructor
shadowing
Defining a local variable or parameter with the same name and type as an instance variable.
Behavior
Dependent on the object type
Postconditions
Describe what will result if the preconditions are satisfied
Class
Describes what an object will be. It is a blueprint, description, definition of an object
What would be the return type of the square root function you used before?
Double
Name 4 advantages of using static methods
Easier to debug (only depends on static state) Self documenting Groups methods that are related in a class without requiring an object The compiler can produce more efficient code since no specific object is involved (i.e. don't have to define an object first to use these methods)
More on abstract
Every abstract class must provide an instance method that defines its default behavior. While in an Interface, only the declaration of constants and instance methods is permissible, you can't implement the default behavior, and all methods are abstract by default.
What are the differences between functions in ML and functional languages and functions (strictly speaking procedures) in imperative languages, including Java?
Functions in ML don't have side effects whereas functions in imperative languages can use/alter larger system state (are procedures), procedures in OOP languages can have no arguments and return nothing (void) whereas functions in ML must have arguments and return a result that is only dependent on the inputs
Describe the difference between functional and imperative languages in terms of the nature of functions
Functions in a functional language have at least one input argument and must return a value, which is the direct result of operations on the inputs, whereas in imperative languages, functions (procedures) can take no arguments, have side effects (alter state initialised outside the function) and return nothing
aliasing
Having two references for the same object. This causes unintended problems
Three dimensions in object oriented
Identity Attributes Behavior
hides
If a subclass defines a static method with the same signature as a static method in the superclass, then the method in the subclass _______ the one in the superclass.
null reference, null pointer
If an object is uninitialized
When can related classes be used interchangeably in a program?
If they have the same interface
Two examples of a control flow statement
If-Then, Switch
What are the four ways in which methods in a subclass can be related to methods in a superclass?
Implementation, extension, overriding, and finality
How does Java organize classes?
In a hierarchy
How can interfaces be organized?
In an inheritance hierarchy
What is the difference between functional and imperative programming languages in terms of flow control?
In functional flow control is mainly through function calls (incl. recursion) whereas in imperative its mainly through loops, conditionals and method calls.
What is the difference between functional and imperative programming languages in terms of what you specify to the compiler?
In functional languages (subset of declarative), you tell the compiler what you want achieved by giving an example (i.e. tell the compiler what to do but not how), whereas in imperative you specify exactly how you want the desired result to be achieved.
The creation of sub classes using the "extends" keyword
Inheritance
Q: What do you know about anonymous and inner classes?
Inner class- are the ones having their definition within other classes, including those defined in methods. These can assume any accessibility including private, public, etc. Anonymous class- It is a class which has its definition inside a method with no name. You can instantiate or declare it in the same place, and it doesn't support explicit constructors.
non static variables, variables that change
Instance Variables
Objects
Instance of a class
What are the two basic parts of a class?
Instance variables and methods
Q: Are there any performance implications of Interfaces over abstract classes, if yes then specify?
Interfaces require additional indirections to find methods in the implementing class. This behavior makes them slower than abstract classes. Another fact that a QA engineer should know that a class can only extend a single abstract class whereas it can implement multiple interfaces. Also with interfaces, one has to address all of its methods which lead to extra efforts while developing an automation framework.
What is a class?
It is a template that defines the form of an object. It defines the data (variables) and code that acts on the data.
Why can't you specify a return type for a constructor?
It is called using the new keyword, which must return a reference to the newly constructed object
If this is used in a constructor, _____.
It must be the first statement.
What does the real power of object-oriented programming come from?
Its capacity to reduce code and to distribute responsibilities for such things as error handling in a software system
What does each object have?
Its own set of instance variables
java virtual machine
JVM
If you don't specify a constructor for a class, what happens in Java?
Java fills in a default constructor for you which takes no arguments and does nothing, but allows you to make objects of that class type.
final
Keeps a variable constant, any attempt to change it will cause an error ex. class MyClass { public static final double PI = 3.14; public static void main(String[ ] args) { System.out.println(PI); } }
To make a class immutable, what must you do to state that isn't primitive (i.e. accessing that states copies the reference rather than the actual value) or immutable (i.e. that state can change)?
Make a deep clone when accessing that state (i.e. in get method, return new date () / create entirely new array with same values as old one), and make a deep clone when setting that state (i.e. in constructor, create a new date() object
How would you make a mutable class immutable?
Make all state private and possibly final (so no can access state and change it); make all methods final (for weak immutability, i.e. can't change behaviour of class A but can extend behaviour through another class B that extends A) or make class final (for strong immutability i.e. can't extend class A); if a field isn't primitive or immutable, make a deep clone on the way in and the way out; make sure no method tries to change any internal state
What is the effect of polymorphism?
Makes classes easier to use because programmers need to memorize fewer method names
The switch statement in Java is efficient for what?
Making selections.
What is an overloaded method?
Many methods of the same names but of different return types and parameter lists.
When a variable references an object, it contains the ____.
Memory address of the object's location.
What is another name for the code contained within a class?
Method
overloading arguments
Method ____________ is having two or more methods in the same class with the same name but different _______________.
accessor methods
Methods used to obtain information about an object
Polymorphism
Methods with similar function in different classes that are given the same name
How is type inference a difference between functional and imperative programming languages?
Most functional languages have type inference whereas most imperative languages don't (have to specify types when declaring values, return type and type of arguments in functions)
Special properties that constructors have include:
Must have the same name as the class, have no return type (not even void) and may not return any values, and are typically public.
Abstract classes
Must never be instantiated. Their sole purpose is to define features and behavior common to their subclasses
Do parameters within a method have to be of the same type?
No
Does the class in the code above have a method associated with it?
No
If a class is immutable, is it necessary to perform a deep copy in the copy constructor?
No
Should a class have many themes?
No
Is there any point in copying/cloning an immutable type?
No - value can't be changed anyways
Does this code print out the line? Date taxDay = new Date915, "April", 2016); Date alsoTaxDay = new Date(15, "April", 2016); if (taxDay.equals(alsoTaxDay)){ System.out.println("They are the same"); }
No, because the default equals method does not check the value, it checks if the two live at the same memory location
Correct? ArrayList<char> lalaString = new ArrayList<char>();
No, primitive types cannot be stored in array list. Should be character instead
Correct? public class Point { public int x; public int y; public Point(){ x = y = 0; } public static int getX(){ return x; } public static int getY(){ return y; }
No, static methods cannot access class members
How many parameters does your method have?
None
Comment on the following two functions: public int helloworld(int a, int b) {...} public float helloworld(int a, int b) {...}
Not valid overloading of functions as functions of the same name must have different arguments at least and different return type is optional
The default constructor sets all of the class' reference variables to?
Null
A class that is not explicitly a subclass is automatically a subclass of what class?
Object
What class is not a subclass of any other class?
Object
What does OOP stand for?
Object Oriented Programming.
OOP
Object-Oriented Programming
What is the general form of the dot operator?
Object.member
What is immutable state?
Once it is set, the state cannot be changed (i.e. is constant)
What is a protected variable?
One that is accessible throughout the defining class and any of its subclasses
instance methods
Operates on individual objects of a class
Why do you want to make a deep clone when setting non-primitive, mutable state in a constructor of an immutable class?
Otherwise if you pass in a reference to another object of the same type as that state into the constructor, the reference will be copied across for that new object, so if you change the value stored at the original reference, the object's state will be changed as well as it points to this same reference. (not immutable)
Why do you want to make a deep clone when getting non-primitive, mutable state from an object which is an instance of an immutable class?
Otherwise you'll get the reference to that state (and could store without using new so reference is copied across), and if you then decide to change what you got saved under a new identifier name, you'll change the state for the object as well (from the class, so class not immutable). So you want to get deep clone so any changes made don't happen to original class's state.
What is Java's closest thing to polymorphism (writing one function that can be applied to multiple types), other than Generics?
Overloading
What is this an example of: public myfun(int a, int b) {...} public myfun(float a, float b) {...}
Overloading functions
public class PairOfDice { public int die1; public int die2; public _______(int val1, int val2){ die1 = val1; die2 = val2; }
PairOfDice
"args" the main method declaration
Parameters
In which two ways can the term interface be used
Part of the software system that interacts with human users and i a list of a class's public methods
What are the names of the instance variables in this class?
Passengers, fuelcap, mpg
static methods
Performs an operation for the entire class, not its individual objects
class
Previously, we defined a class as a collection of related methods. Now you know that a class is also a template for a new type of object.
These variable actually contain the value that they have been assigned.
Primitive variables
Inheritance
Provides a mechanism for reusing code and can greatly reduce the effort required to implement a new class
If more than one class is in a source code file, only one of them may be ______?
Public
Boolean
Returns possible values, true or false
Objects are ______ by variables.
Referenced
override
Replacing a default implementation of a method, such as toString.
What does a constructor in a UML not contain?
Return type
State the rules for passing a parameter of one class to a method that expects a parameter of another class
Same as the rule for assignment, the class of the actual parameter must be the same as or less inclusive than the class of the formal parameter
Two things to note about constructors
Same name as the class No return type
Object declaration syntax
Scanner in = new Scanner(System.in())
Provide an example Scanner statement:
Scanner keyboard = new Scanner (System.in);
setColor(aColor)
Sets the pen's color to aColor
Setter
Sets value
What is a key point for a well designed class?
Should define one entity (concept)
The this reference is ____.
Simply a name that an object can use to refer to itself.
object
Something that has both state and behavior - an individual instance based on a template
Preconditions
Specify the correct use of a method
Q: What is an abstract class and why is it used?
The abstract class serves as a template, and you can't instantiate it. It may contain static data, and you've to extend it to make it functional. Also, you must note that any class which has an abstract method automatically turns itself abstract.
What are primitive types?
The built in types that form the building blocks for more complicated types
Public
The class is accessible by any other class.
How is the class variable different from the instance variable below? public class Bicycle { private int cadence; private int gear; private int speed; //add inst var for the obj ID private int ID: //add class var for # Bicycle obs instantiated private static int numberOfBicycles = 0; ... }
The class variable is not related to any individual object, but to the class as a whole. It is related to all Bicycle objects. the ID number instance variable is related to one specific individual object.
What are instance variables?
The data members
Reference Types
The identity of the object and the fact that there can be multiple references to the same object are issues that arise when comparing two objects for equality and when copying an object
Finality
The method in the superclass is complete and cannot be modified by the subclasses = final
setDirection(degrees)
The pen points in the indicated direction
information hiding
The practice of making instance variables private to limit dependencies between classes.
scope
The region in which that variable or method is visible and can be accessed.
Dependency
The sender object's class depends on the receiver object's class
Overriding
The subclass method does not invoke the superclass method, instead, the subclass method is intended as a complete replacement of the superclass method
The method of a class are what?
The subroutine.
What happens when an object is assigned to a variable?
The variable contains a reference to the object
What is a parameter?
The variable that receives the argument (numbers).
How do the contents of the variables of minivan compare to the contents of the variables of truck?
They are independent of each other
What is static used for?
To associate state with a class, or in the case of methods, restrict the methods to only use local or static variable
What is the modifier static used for?
To designate class variables and methods
The word static can be used to do what?
To get a subroutine to work on its own within your program.
equivalent
Two objects that are "equal" but not necessarily identical, as defined by the equals method.
What can an interface never contain?
Variables
Write a piece of code that creates an instance of Vehicle called sportscar
Vehicle sportscar = new Vehicle();
What's the name of the class in the code above that represents the main program?
VehicleDemo
Class syntax
Very beginning! public class (name class here) {
Scope of a variable declared inside a method?
Visible only within that method and called a local variable
Scope of a variable declared inside the class but not in a method?
Visible to all methods of the class and called an instance field
What is the return type of the method you just wrote?
Void
Write a method called range that computes and prints out the range of the vehicle within the method itself.
Void range() { System.out.println("Range is " + fuelcap * mpg); }
When is an instance method activated?
When a message is sent to the object
Exceptions for error handling
When a method's preconditions are violated, throw exceptions to catch the errors, thereby halting program execution at the point of the errors
Static variables and methods
When information needs to be shared among all instances of class, that information can be represented in terms of static variables and it can be accessed by means of static methods
When should a method thrown an exceptions?
When its preconditions are violated
subclass
When overriding a method, the entire method signature must be the same. Except that the return type may be a ________________ of the original return type.
What is a side effect?
When state outside the function/procedure being called is changed
Q: What if you declare a class w/o any access modifiers, where can you use this class in your program?
When we don't specify any access modifier for a class, then Java assign package level access to it. It implies that you can access such a class from other classes and interfaces within the boundary of that package.
Reference variable
When you create an object using the constructor
What is overloading a function?
When you have functions with the same name but different arguments and maybe even a different return type (but NOT just a different return type)
Where does execution resume after a call to a method?
With the line of code following the call
Can a class implement more than one interface?
Yes
Correct? public class QuizClass { public int x; public static String text; public int square (int x) { return x * x; } public String getName() { return text; } }
Yes
Can the parameter-list for a constructor be empty?
Yes if the constructor has no parameters
Correct? public class Point{ public final int x; public final int y; public Point(int newx, int newy){ x = newx; y = newy; } }
Yes, a constructor can assign value to a final/constant
Is the return type of a method important?
Yes, boolean, int, and double can not mix
Correct? public boolean equals(Date otherDate){ if (day == otherDate.getDay() && month.equals(otherDate.getMonth()) && year == otherDate.getYear()){ return true; } else{ return false; } }
Yes, use .equals to compare Strings and use == to compare integers
The default constructor sets all of the class' numeric fields to?
Zero
Constructors
____________ are NOT inherited by a subclass. But they can be invoked.
reference type
a box that contains a pointer to an object
nested class
a class within another
What is PairOfDice()? PairOfDice dice; //declares a variable of type PairOfDice dice = new PairOfDice();
a constructor
abstraction
a distancing between ideas and details -we can use objects without knowing how they work
private fields
a field that cannot be accessed from outside the class
Packages
a group made up of similar types of classes, along with sub-packages
Extension
a. the subclass method does not exist in the superclass b. the subclass method invokes the same method in the superclass and also extends the superclass's behavior with its own operations
object
an entity that combines state and behavior
an object
an instance of the class
an object is
an instance of the class
Instance members must be associated with ___
an object
instance of a class
an object in the class
classes declared and instantiated at the same time
anonymous class
fields
attributes of an object (bank account = bank account #, amount of money)
Class variable
belongs to a class, its storage is allocated at program startup and is independent of the number of instances created
Aggregation
between a container class and the classes of the object - has-a relationship
What are primitive types in Java?
boolean - 1 bit byte - 8 bits as a signed integer (-128 to 127) short - 16 bits as a signed integer int - 32 bits as a signed integer long -64 bits as a signed integer float - 32 bits as as floating point number double - 64 bits as a floating point number
helper methods
break a complex task performed by a method into subtasks
What syntax do you use to jump out of a for loop or while loop?
break;
What are the signed primitives?
byte, short, int, long (i.e. all numbers apart from float and double so all integers basically)
Java compiles a program by turning it into
bytecode
mutator method
changes or alters the fields of an object (bank account = withdraw, deposit)
mutators
changes the state of an object by modifying at least one of its instance variable
'l'
char aLetter; String myName = "Charlie Brown"; aLetter = myName.charAt(4)
blueprint from which individual objects are created
class
static member variables and static member methods are sometimes called ___ variables and methods
class
class variables and methods belong to the ____ itself rather than to ____ of that class
class instances
Return value syntax
class MyClass { static int sum(int val1, int val2) { return val1 + val2; } public static void main(String[ ] args) { int x = sum(2, 5); System.out.println(x); } } //output 7 *Note: The return is saying to add val 1 and val 2 (parameters), as you can see in the next function starting with public, there is a variable with x and the sum has (2,5) those two will be added due to the method prior
Declaring a method
class Myclass{ static void sayHello(){ //sayHello is the method System.out.println("Hello World!") } public static void main (String [ ] args) { sayHello(); sayHello(); sayHello(); } } /* output: Hello World! Hello World! Hello World! */
Nested Classes Syntax
class Outer_Demo { class Nested_Demo { } }
class is visible to all classes everywhere
class declared with the modifier "public"
class only visible within its own package
class has no modifier (package-private)
name, instance variables, constructors, methods
class signature
a class can have multiple
constructors
methods
classes provide _____ that are used to extract data values from the class
How can you access a static variable?
classname.staticvariablename
public
client can see data
private
client cannot see data
Concatenating Strings
concatenating two strings , returns a new string that is string1 with string2 added to it at the end. - "My name is ".concat("Zara"); or - "Hello," + " world" + "!"
signature of a method
consists of the method's name and a list of the parameter types
Scanner scanInput; scanInput = new Scanner(System.in);
declare an instance of the scanner class and connect it to the keyboard
object, null
declaring a variable does not create an ____, reference values are initialized with the value ____
3 types of constructors
default, overloaded, copy (ALWAYS PUBLIC)
behavior
defined by methods of the objects class
Inner Classes (Non-static Nested Classes): 3 types
depending on how and where you define them. -Inner Class -Method-local Inner Class -Anonymous Inner Class
visibility modifiers
determine what the client can see: private or public
Driver program
drive other parts of programs
If an equals method has not been implemented, what will this code output? (assume class is called Car and constructor for make of the car has been implemented) Car firstCar = new Car("Dodge"); Car secondCar = new Car("Dodge"); System.out.println(firstCar.equals(secondCar));
false because the default equals method checks the memory location
order in code
fields constructors methods
Constants are declared using what keyword?
final
example of? static final double PI = 3.141592653589793;
final modifier
the value of this field cannot change
final modifier
accessor method
gets information to the user (bank account = get balance)
accessor methods are referred to as?
getter methods
encapsulation
hiding implementation detail for clients
whitespace character
how does scanner's next() method know where the String ends
point method:distance(p)
how far away the point is from point p
What statement must we declare in order to use the Scanner class?
import java.util.Scanner;
. operator
indicates that the accessor is a method of the class to which the object variable belongs
JUnit testing should be applied to
individual methods
An object created from a class is called an ______ of that class
instance
PlayerData class creates an object. that object is an ____ of the PlayerData class. name and age are ____ _____ in the object.
instance instance variables
methods that the object contains
instance methods
variables that the object contains
instance variables
public ____ getArea() { return width * height; }
int
each point object contains two pieces of data
int named x and int named y
What are the parameters of the constructor? public class PairOfDice { public int die1; public int die2; public PairOfDice(int val1, int val2){ die1 = val1; die2 = val2; } public void roll(){ die1 = (int)(Math.random()*6) + 1; die2 = (int)(Math.random()*6) + 1; } }
int val1, int val2
a group of related methods with empty bodies, uses "implements" method
interface
! operator
inverts the value of a Boolean
calling a method
invoking the code to perform a service for an object
toUpperCase
is similar toLowerCase, but returns the original string with all letters in upper case
java import for a scanner
java.util.Scanner
does not
just declaring a variable _____ create an object
like a class variable, but for only one method
local variables
scanner initalization goes in the
main method
instance variable
maintains information in an object
@param
marks a parameter of a method
@throws
marks an exception thrown by the method
@return
marks the returned value of a method
accessors
messages that access the object's state, used to see if a mutator works correctly
mutators
messages that change an object's state
subroutine
method
instance variable declared private is not visible from...
methods within other classes
Write a pieces of code that invokes the range method you wrote on minivan.
minivan.range();
setter method is also referred to as?
mutator method
mutator
mutator method is used to set a value of a private field. It follows a naming scheme prefixing the word "set" to the start of the method name.
class inside of another class
nested class
Objects are created with what operator?
new
What keyword do we need to instantiate an object and what does it return?
new, returns a reference to the newly constructed object
Can a constructor be declared static?
no
can a constructor return void?
no
Correct? public void myMethod(){ int x; if (x>10) { System.out.println("larger than 10"); } }
no because x was never initialized
Does the code below create a new object? public Date (Date oldDate) { this(oldDate.getDay(), oldDate.getMonth(); oldDate.getYear(); } assume Date method is defined earlier in code
no, because it is a copy constructor that sets the values of the current object being created
Will this return a String of the date? Date taxDay = new Date(15, "April", 2016); System.out.println("Tay day is " + taxDay);
no, it will print the location in memory where the object taxDay is stored
Correct? public class QuizClass{ int num; String text; public static void printNice() { System.out.println(text + " " + num); } }
no, text and num are not intialized
files
nstances of Scanner can be connected to other "things", such as ____.
what is an actual value stored in the variable?
null
point is an
object
Static variables are instantiated ____ per ____ not per object
once, not
instance methods
operates on individual objects of a class
a namespace that organizes a set of related classes and interfaces
package
name space that organizes a set of related classes and interfaces
package
when a method takes in something
parameter
formal parameters
parameters that are listed in a method's definition
actual parameters
parameters that are values passed to a method when it is invoked (arguments)
parameter
passes information to a method
block
piece of code enclosed in a { } pair. When exited, the memory for a local variable is automatically dumped
The out object contains what two methods?
print and println
point method:display
print the coordinates
To make data not seen outside the class, you declare it as ___
private
static field syntax
private static String bankName = "TD Bank"
main method
public static void main (String[] args){}
a class is a
program entity that represents either: 1. a program/ module 2. a template for a new type of object
object oriented programming
programs that preform their behavior as interactions between objects
object variables
provides a memory location to a specific object instance, and does not store the objects themselves
change using 'this' public class Point{ public int x = 0; public int y = 0; //constructor public Point(int a, int b){ x = a; y = b; } }
public class Point{ public int x = 0; public int y = 0; //constructor public Point(int x, int y){ this.x = x; this.y = y; } }
What is the prototype/signature of the following function public int add(int x; int y){ return x+y; }
public int add(int x; int y)
constant syntax
public static final double OVERDRAFT_FEE = 5 (YOU REFER TO THIS AS BankAccount.OVERDRAFT_FEE)
String value = scanInput.next();
read a value from the keyboard
null
reference values are initialized with the value
this
refers to the implicit parameter inside your class
scope of variables
region of the program within which the variable can validly appear in lines of code
information hiding
restriction of access
a constructor has no __ type
return
What is the form of a piece of code to return a calculation from a method?
return value;
nextInt()
returns an int that contains the next integer typed on the keyboard
indexOf
returns an int that is the index of the first occurrence of a character
Math.abs()
returns the absolute value of its parameter
nextLine()
returns the input line as a String
default constructor
sets all instance fields to their default values (String = "null", integer = "0", double = "0.0", boolean = false)
overloaded constructor
sets the instance fields to the user input
point method: setLocation(x,y)
sets the point's x andy to the given values
public void setTitle(String newTitle){ title = newTitle; }
setter method
sin()
sine
static field
something that is kept the same with whoever uses the program... always declared as private (bank name = TD Bank)
constants
something that is unable to be changed, is indicated with all capitals in variable name
constructor
special method that creates an object and assigns initial values to the data
Constructors
special methods invoked when an object is created and are used to initialize them.
sqrt()
square root
two characteristics of objects
state and behavior
exp class: string
string class is a template for creating string obkects
immutable
strings are ___ objects which means their value cannot change
primitive type
strings are not a
literal representation
strings are unique since they have a
How can an exception be handled under program control?
try-catch statement
overloaded methods
two or more methods in the same class that have the same name but different parameters
declaration syntax for
type name;
The statement below is an example of int x = new Integer(5);
unboxing
make variables common to all objects
use the static modifier
local variables
used to provide temporary working storage for data in a method, instantiated in the method
accessor
used to return the value of a private field. It follows a naming scheme prefixing the word "get" to the start of the method name
chaining constructors
used when a class has several constructors
copy constructor
uses a prewritten constructor to initialize other objects
