CSIT 112 | Fundamentals of Java Programming II | Java 2 Midterm

Q3: Write a program to calculate ∑ 𝒊𝑵 𝟏 . public int sum(int N) { int result=0; for (int i=1;i<=N;i++) result += i return result; } Q4. Draw the output of the following program. for (int j=0; j<2; j++) { for (int k=0; k<10; k++) System.out.print("*"); System.out.println();

********** **********

What is the output of executing the following class? public class Test { public static void main(String[] args) { int x = 0; System.out.print(inc(x) + " " + x); } int inc(int x) { Return x+1; } }

1 0

Given the nested if-else structure below, answer questions below. f (a > 0) if (b < 0) x = x + 5; else if (a > 5) x = x + 4; else x = x + 3; else x = x + 2; If x is currently 0, a = 5 and b = 5, what will x become after the above statement is executed?

3

What is the output of the following program? String msg = " hello "; msg = 1 + 2 + msg + 3 + 4; System.out.println(msg);

3 hello 34

Q20. Assume a class Triangle has been defined. It has three instance data, Point p1, p2, p3. The class also has a constructor that receives the 3 Points and initializes p1, p2, p3, a toString method that outputs the 3 Points, and a computeSurfaceArea method that calculates the surface area of the Triangle (as an int value). We want to extend this class to represent a 3-dimensional Pyramid, which consists of 4 Points. Since the Pyramid will consist of, in essence, 4 triangles, computeSurfaceArea for the Pyramid will compute 4 times the surface area of the Triangle made up of 3 of those 4 Points. Write the Pyramid class to handle the difference(s) between a Pyramid and the previously defined Triangle. Assume the instance data of Triangle are protected and all methods are public. Also assume that the class Point has a toString method.

ANS: public class Pyramid extends Triangle { protected Point p4; public Pyramid(Point a1, Point a2, Point a3, Point a4) { super(a1, a2, a3); p4 = a4; } public String toString() { return super.toString() + p4.toString(); } public int computeSurfaceArea() { return 4 * super.computeSurfaceArea(); } }

CONSIDER PART B CODE What is output of the code if Inherit()is defined as below. Inherit() { Figure f = new Figure(); Rectangle r = new Rectangle(); Box b = new Box(); f.display(); f = r; f.display(); f = b; f.display(); } a. Figure Rectangle Box b. Rectangle Box c. Figure Figure Figure d. Syntax error. This code will not compile or execute e. None of these

ANS: A This is a perfect example of polymorphism. The first invocation of the display method has a Figure as its referent. In the second invocation, the reference has been changed to Rectangle. Then for the third invocation it has been changed to Box.

CONSIDER PART B CODE What is output of the code if Inherit()is defined as below. Inherit() { Figure f = (Figure) new Box(); f.display(); Rectangle r = (Rectangle) f; r.display(); } 9 a. Rectangle Rectangle b. Rectangle Figure c. Figure Rectangle d. Figure Figure e. None of these

ANS: A f is a polymorphic reference variable, as is r. Trace through the inheritance structure to see that both references' invocation of the display() method will, indeed, display Rectangle.

CONSIDER PART A CODE You want addEm to now add all three values, return the sum, and changeEm to change x and y, but leave z alone. Which should you do? a. Redefine addEm and changeEm without referencing super.addEm() or super.changeEm(). b. Redefine addEm to return the value of z+super.addEm(), but leave changeEm alone. c. Redefine changeEm to call super.changeEm() and then set z = x + y, but leave addEm alone. d. Redefine addEm to return the value of z+super.addEm() and redefine changeEm to call super.changeEm() and then set z = x + y. e. Redefine changeEm to call super.changeEm() without doing anything to z, and redefine addEm to return super.addEm().

ANS: B Since super.changeEm() will not affect z and we don't want changeEm()to affect z, there is no need to redefine changeEm()for BClass. Any reference to changeEm()will reference AClass's version which is the same one that BClass will use. But addEm needs redefining because the inherited method addEm calls AClass's addEm which only adds x and y together. Now, we want to add x, y and z together. We can either redefine addEm enntirely, or use super.addEm()and add z to the value returned. The latter approach is better since it reuses previously written code.

CONSIDER PART A CODE Which of the following would best redefine the toString method for BClass? a. public String toString(int z) { return " " + x + " " + y + " " + z; } b. ) public String toString() { 7 return super.toString(); } c. public String toString() { return super.toString() + " " + z; } d. public String toString() { return super.toString()+" " x+" "+y+" "+z; } e. public String toString() { return " " + x + " + y + " " + z; }

ANS: C The best way to redefine a method is to reuse the parent class's method and supplement it with the code needed to handle the changes in the subclass. Answer C does this. Answer E accomplishes the same task, but does not reuse any of the parent class's code. Answer B returns only x and y, not z, and answer D returns x, y, and z, while answer A is syntactically invalid because the toString method does not accept a parameter

CONSIDER PART B CODE What is output of the code if Inherit()is defined as below. Inherit() { Figure f = new Figure(); Rectangle r = new Rectangle(); Box b = new Box(); f.display(); f = r; f.display("one"); f = b; f.display("two"); } a. Figure Rectangle b. Figure Rectangle Figure Box c. Figure Figure Figure Figure d. Syntax error; this code will not compile e. None of these

ANS: D Examine the f variable, and its invocation of the display method. Does f's display method accept any parameters? No, so, this code has a syntax error and it will not even compile.

CONSIDER PART A CODE You want to extend AClass to BClass. BClass will have a third int instance data, z. Which of the following would best define BClass's constructor? a. public BClass(int a, int b, int c) { super(a, b, c); } b. public BClass(int a, int b, int c) { x = a; y = b; z = c; } c. public BClass(int a, int b, int c) { z = c; } d. public BClass(int a, int b, int c) { super(a, b); z = c; } e. public BClass(int a, int b, int c) 6 { super(); }

ANS: D Inheritance is useful because it allows you to reuse code. For this problem, you would want to reuse as much of AClass as possible. Answer D allows you to use AClass's constructor, which expects two int parameters. This sets x and y to be a and b respectively. But since AClass does not have an instance data z, you need to directly assign z to be c in the constructor. The answer in B will also work, but does not reuse any code and so is not as desirable as D. The answers in A and E are syntactically invalid since they do not pass the correct number of parameters to AClass's constructor. Finally, the answer in C only sets z correctly and leaves x and y uninitialized

Q14. What is super class and subclass?

Ans: A class from where a subclass inherits features is called superclass. It is also called base class or parent class. A class that inherits all the members (fields, method, and nested classes) from other class is called subclass. It is also called a derived class, child class, or extended class

Q10. How to achieve Data hiding programmatically?

Ans: By declaring data members (variables) as private, we can achieve or implement data hiding. If thevariables are declared as private in the class, nobody can access them from outside the class.The biggest advantage of data hiding is we can achieve security.

Q11. What are getter and setter methods in Java?

Ans: In Java, setter method is a method that is used for updating the values of a variable. This method is also known as mutator method. Getter method is a method that is used to retrieve the value of a variable or return the value of the private member variable. This method is also known as an accessor method

Q15. How is Inheritance implemented/achieved in Java?

Ans: Inheritance can be implemented or achieved by using two keywords: 1. extends: extends is a keyword that is used for developing the inheritance between two classes and two interfaces. 2. implements: implements keyword is used for developing the inheritance between a class and interface.

Q6. Will the following code snippet compile successfully? If yes, what is the output of the following program? public class Myclass { 14 private int x = 10; static int m1() { int y = x; return y; } public static void main(String[] args) { m1(); } }

Ans: No, the above code will not be compiled because x is an instance variable and instance member cannot be accessed from static region

Q12. In the following code, radius is declared as private in the class Circle, and myCircle is an instance of class Circle. Does the code cause any error problems? If so, explain why? class Circle { private double radius = 1; /** Find area of the circle */ public double getArea() { 17 return radius * radius * Math.PI; } public static void main(String[] args) { Circle myCircle = new Circle(); System.out.println("Radius is " + myCircle.radius); System.out.println("Area of cirle: " +myCircle.getArea()); } }

Ans: No, the above code will not create any problem. The code will be compiled successfully. The output is: Radius is 1.0, Area of cirle: 3.141592653589793.

Q16. Are constructor and instance initialization block inherited to subclass?

Ans: No. A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructor and instance initialization block of the superclass are not members and thus not inherited by its subclass. However, they can be invoked from the subclass.

Q17. Find out the output of the following program. public class A { void m1() { System.out.println("m1 in class A"); } } public class B extends A { void m1() { super.m1(); System.out.println("m1 in class B"); } } public class C extends B { void m1() { System.out.println("m1 in class C"); 19 super.m1(); } } public class Test { public static void main(String[] args) { C c = new C(); c.m1(); } }

Ans: Output: m1 in class C m1 in class A m1 in class B

Q8. what is the output of the following code snippet? public class Myclass { Myclass() { System.out.println("constructor"); } 15 static void m1() { System.out.println("static method"); } void m2(){ System.out.println("instance method"); } static { System.out.println("static block"); } { System.out.println("instance block"); } public static void main(String[] args) { Myclass obj = new Myclass(); m1(); obj.m2(); } }

Ans: Output: static block instance block 16 constructor static method instance method.

Q18. What is Polymorphism in Java OOPs?

Ans: Polymorphism in java is one of the core concepts of object-oriented programming system. Polymorphism means "many forms" in Greek. That is one thing that can take many forms. Polymorphism is a concept by which we can perform a single task in different ways. That is, when a single entity (object) behaves differently in different cases, it is called polymorphism. In other words, if a single object shows multiple forms or multiple behaviors, it is called polymorphism.

Q7. How is static variable different from the instance variable?

Ans: The difference between static variable and instance variable is as follows: a) A static variable is also called class variable whereas, an instance variable is also called non-static variable. b) Class variable can be accessed inside a static block, instance block, static method, instance method, and method of the inner class whereas, instance variable can be accessed only inside the instance members, and method of the inner class. c) Class variable is always resolved during compile time whereas, instance variable is resolved during the runtime. 4) Static variable cannot be serialized in Java whereas, instance variable can be serialized

Q9. What is Encapsulation in Java? Why is it called Data hiding?

Ans: The process of binding data and corresponding methods (behavior) together into a single unit is called encapsulation in Java. In other words, encapsulation is a programming technique that binds the class members (variables and methods) together and prevents them from being accessed by other classes, thereby we can keep variables and methods safes from outside interference and misuse. If a field is declared private in the class then it cannot be accessed by anyone outside the class and hides the fields within the class. Therefore, Encapsulation is also called data hiding

Q13. What is Inheritance in Java?

Ans: The technique of creating a new class by using an existing class functionality is called inheritance in Java. In other words, inheritance is a process where a child class acquires all the properties and behaviors of the parent class.

Q5. Can we access static members if no instance of the class is constructed?

Ans: Yes, we can access the static members if no instance of class exists because they are not tied to a specific instance. They are shared across all instances of the class.

The instruction super(); does which of the following?

It calls the constructor as defined in the current class's parent class.

Assume list is an array of int values, temp is some previously initialized int value, and c is an int initialized to 0. What does the following code do? for (j=0; j < list.length; j++) if (list[j] < temp) c++;

It counts the number of elements in list that are less than temp

All classes in Java are directly or indirectly subclasses of the __________ class

Object

Q19. How to achieve/implement static polymorphism (compile time polymorphism) in Java? How to achieve/implement dynamic polymorphism in Java?

Static polymorphism can be achieved by method overloading. Other examples of compile time polymorphism are constructor overloading and method hiding. 20 1 Dynamic or runtime polymorphism can be achieved by overriding an inherited method or implementing an interface.

The difference between a checked and an unchecked exception is

an unchecked exception requires no throws clause

To determine the type that a polymorphic variable refers to, the decision is made

by the Java run-time environment at run time

If a programmer writes a class that he wants to be extended by another programmer, then this programmer must

change private methods and instance data to be protected

Q1: Try, catch, and finally Please complete the following program to catch an "ArithmeticException"; Specifically, a. please catch and print the exception information "Exception caught:Division by zero" on user's terminal; b. Print the message "I am in final block" in the final block. class Division { public static void main(String[] args) { int a = 10, b = 5, c = 5, result; // apply a "try..catch..final" structure to following statements result = a / (b - c); System.out.println("result" + result); System.out.println("I am in final block"); } }

class Division { public static void main(String[] args) { int a = 10, b = 5, c = 5, result; try { result = a / (b - c); System.out.println("result" + result); } catch (ArithmeticException e) { System.out.println("Exception caught:Division by zero"); } finally { System.out.println("I am in final block"); } } }

Java does not support multiple inheritance but some of the abilities of multiple inheritance are available by

implementing interfaces

Q2. Implement the Speaker interface: public interface Speaker { public void speak(); public void announce(String str); } Then, create three classes that implement Speaker in various ways: o SpeakerOfTheHouse: speak method prints "I am Speaker of the House.", and the announce method takes the name of a bill in the form of a string and prints it in the sentence "The [bill] has passed!" o SportsAnnouncer: speak method prints "Goal!", and the announce method takes the name of a team in the form of a string and prints the sentence "The [team] have scored a goal!" o Actor: speak method prints "I've been nominated for three Academy Awards.", and the announce method takes the name of a movie in the form of a string and prints the sentence, "I'm currently staring in [movie]." Create a driver class, and in the main method, prompt the user to enter three Strings -- the name of a bill, the name of a sports team, and the name of a movie. Then, create an object from each of the classes described above and call the speak and announce methods of each object, using the strings provided by the user.

import java.util.Scanner; interface Speaker { public void speak(); public void announce(String str); } class SpeakerOfTheHouse implements Speaker { public void speak(){ System.out.println("I am Speaker of the House."); } public void announce(String str){ System.out.println("The "+str+" has passed!"); } } 12 class SportsAnnouncer implements Speaker { public void speak(){ System.out.println("Goal!"); } public void announce(String str){ System.out.println("The " + str + " have scored a goal!"); } } class Actor implements Speaker { public void speak(){ System.out.println("I've been nominated for 3 Academy Awards."); } public void announce(String str){ System.out.println("I'm currently starring in "+str+"."); } } class Driver { public static void main(String args[]){ SpeakerOfTheHouse henryClay = new SpeakerOfTheHouse(); SportsAnnouncer howardCosell = new SportsAnnouncer(); Actor meryllStreep = new Actor(); Scanner scanner = new Scanner(System.in); System.out.print("Enter the name of a bill:"); String bill = scanner.nextLine(); System.out.print("Enter the name of a sports team:"); String team = scanner.nextLine(); System.out.print("Enter the name of a movie:"); String movie = scanner.nextLine(); Speaker[] speakers = {henryClay, howardCosell, meryllStreep}; String[] phrases = {bill, team, movie}; 13 for(int i=0; i<3; i++){ speakers[i].speak(); speakers[i].announce(phrases[i]); } } }

A finally clause will execute

in any circumstance

Abstract methods are used when defining

interface classes

A variable declared to be of one class can later reference an extended class of that class. This variable is known as

polymorphic

An exception can produce a "call stack trace" which lists

the active methods in the opposite order that they were invoked