Computer Knowledge

Object-Oriented Programming

2,686 Questions

Object-oriented programming questions test core computer science concepts like classes, inheritance, polymorphism, and encapsulation. The focus includes Java program structures, method overloading, and memory allocation for objects. This topic is essential for technical sections in various recruitment tests.

Java class definitionsMethod overriding rulesPolymorphism conceptsGeneric type parametersMemory allocation in objects

Object-Oriented Programming Questions

Multiple choice technology programming languages

Given: 1. class TestA { 2. public void start() { System.out.println(”TestA”); } 3. } 4. public class TestB extends TestA { 5. public void start() { System.out.println(”TestB”); } 6. public static void main(String[] args) { 7. ((TestA)new TestB()).start(); 8. } 9. }

  1. TestA

  2. TestB

  3. Compilation fails.

  4. An exception is thrown at runtime.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Despite casting new TestB() to TestA type, Java uses virtual method dispatch based on the actual object type. TestB overrides start(), so calling start() invokes TestB's implementation, printing 'TestB'.

Multiple choice technology programming languages

Click the Exhibit button. 1. public interface A { 2. public void doSomething(String thing); 3. } 1. public class AImpl implements A { 2. public void doSomething(String msg) { } 3. } 1. public class B { 2. public A doit() { 3. // more code here 4. } 5. 6. public String execute() { 7. // more code here 8. } 9. } 1. public class C extends B { 2. public AImpl doit() { 3. // more code here 4. } 5. 6. public Object execute() { 7. // more code here 8. } 9. } Which statement is true about the classes and interfaces in the exhibit?

  1. Compilation will succeed for all classes and interfaces.

  2. Compilation of class C will fail because of an error in line 2.

  3. Compilation of class C will fail because of an error in line 6.

  4. Compilation of class AImpl will fail because of an error in line 2.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Class C's doit() method is valid because AImpl is a subclass of A, which is a valid covariant return type (narrowing the return type). However, execute() fails because you cannot widen a return type from String to Object when overriding - overriding methods must return the same type or a subtype, not a supertype. Option C correctly identifies line 6 as the compilation error.

Multiple choice technology programming languages

Given: 10. class Line { 11. public class Point { public int x,y; } 12. public Point getPoint() { return new Point(); } 13. } 14. class Triangle { 15. public Triangle() { 16. // insert code here 17. } 18. } Which code, inserted at line 16, correctly retrieves a local instance of a Point object?

  1. Point p = Line.getPoint();

  2. Line.Point p = Line.getPoint();

  3. Point p = (new Line()).getPoint();

  4. Line.Point p = (new Line()).getPoint();
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

getPoint() is an instance method of class Line, so you need a Line instance to call it. Options A and B try to call it statically on the class itself, which is invalid. Option C creates the instance correctly but fails to specify the full type Line.Point for the inner class. Option D correctly creates a Line instance and properly declares the variable with the inner class type Line.Point.

Multiple choice technology programming languages
  1. Given: 10. class Line { 11. public static class Point { } 12. } 13. 14. class Triangle { 15. // insert code here 16. } Which code, inserted at line 15, creates an instance of the Point class defined in Line?

  1. Point p = new Point();

  2. Line.Point p = new Line.Point();

  3. The Point class cannot be instatiated at line 15.

  4. Line 1 = new Line() ; 1.Point p = new 1.Point();
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Point is a static nested class defined inside the Line class. To instantiate it from outside the Line class, you must refer to it using the outer class name as 'Line.Point p = new Line.Point();'.

Multiple choice technology programming languages

Given: 11. class ClassA {} 12. class ClassB extends ClassA {} 13. class ClassC extends ClassA {} and: 21. ClassA p0 = new ClassA(); 22. ClassB p1 = new ClassB(); 23. ClassC p2 = new ClassC(); 24. ClassA p3 = new ClassB(); 25. ClassA p4 = new ClassC(); Which three are valid? (Choose three.)

  1. p0 = p1;

  2. p1 =p2;

  3. p2 = p4;

  4. p2 = (ClassC)p1;

  5. p1 = (ClassB)p3;

  6. p2 = (ClassC)p4;
Reveal answer Fill a bubble to check yourself
A,E,F Correct answer
Explanation

In Java, a subclass reference can be implicitly assigned to a superclass variable (p0 = p1). Downcasting requires an explicit cast, which is valid if the actual object matches the cast type (p1 = (ClassB)p3 and p2 = (ClassC)p4).

Multiple choice technology programming languages

Given: 10. abstract class A { 11. abstract void al(); 12. void a2() { } 13. } 14. class B extends A { 15. void a1() { } 16. void a2() { } 17. } 18. class C extends B { void c1() { } } and: A x = new B(); C y = new C(); A z = new C(); Which four are valid examples of polymorphic method calls? (Choose four.)

  1. x.a2();

  2. z.a2();

  3. z.c1();

  4. z.a1();

  5. y.c1();

  6. x.a1();
Reveal answer Fill a bubble to check yourself
A,B,D,F Correct answer
Explanation

Polymorphic method calls work when the reference type has the method and the actual object's class overrides it. x.a2() and z.a2() are polymorphic - A declares a2(), B overrides it, and x/z reference B/C objects. x.a1() and z.a1() are polymorphic - A declares abstract a1(), B/C implement it. z.c1() and y.c1() are NOT polymorphic - reference type C has c1(), but reference z is type A (which doesn't have c1()), and y.c1() is just a normal call on C reference, not polymorphic.

Multiple choice technology programming languages

Click the Exhibit button. 1. public class Car { 2. private int wheelCount; 3. private String vin; 4. public Car(String vin) { 5. this.vin = vin; 6. this.wheelCount = 4; 7. } 8. public String drive() { 9. return “zoom-zoom”; 10. } 11. public String getInfo() { 12. return “VIN: “+ vin + “wheels: “+ wheelCount; 13. } 14. } And: 1. public class MeGo extends Car { 2. public MeGo(String vin) { 3. this.wheelCount = 3; 4. } 5. } What two must the programmer do to correct the compilation errors? (Choose two.)

  1. insert a call to this() in the Car constructor

  2. insert a call to this() in the MeGo constructor

  3. insert a call to super() in the MeGo constructor

  4. insert a call to super(vin) in the MeGo constructor

  5. change the wheelCount variable in Car to protected

  6. change line 3 in the MeGo class to super.wheelCount = 3;
Reveal answer Fill a bubble to check yourself
D,E Correct answer
Explanation

MeGo's constructor tries to access this.wheelCount, but wheelCount is private in Car. Even in a subclass, private members are not accessible - they must be protected or public. Option E correctly identifies this. Also, MeGo's constructor must call super(vin) to invoke Car's constructor since Car has no no-arg constructor - Java doesn't automatically insert super() when the parent lacks a no-arg constructor. Option D correctly identifies this.

Multiple choice technology programming languages

Given: 1. public class Blip { 2. protected int blipvert(int x) { return 0; } 3. } 4. class Vert extends Blip { 5. // insert code here 6. } Which five methods, inserted independently at line 5, will compile? (Choose five.)

  1. public int blipvert(int x) { return 0; }

  2. protected long blipvert(int x, int y) { return 0; }

  3. private int blipvert(long x) { return 0; }

  4. protected long blipvert(int x) { return 0; }

  5. protected int blipvert(long x) { return 0; }

  6. protected long blipvert(long x) { return 0; }
Reveal answer Fill a bubble to check yourself
A,B,C,E,F Correct answer
Explanation

When overriding: A is valid (can widen access from protected to public, same signature). B compiles (different method - overloaded with two parameters). C compiles (different method - overloaded with long parameter). D fails - same signature but changing return type from int to long is not allowed. E compiles (overloaded with long parameter). F compiles (overloaded with long parameter). So A, B, C, E, F all compile.

Multiple choice technology programming languages

Which four are true? (Choose four.)

  1. Has-a relationships should never be encapsulated.

  2. Has-a relationships should be implemented using inheritance.

  3. Has-a relationships can be implemented using instance variables.

  4. Is-a relationships can be implemented using the extends keyword.

  5. Is-a relationships can be implemented using the implements keyword.

  6. An array or a collection can be used to implement a one-to-many has-a relationship.
Reveal answer Fill a bubble to check yourself
C,D,E,F Correct answer
Explanation

Has-a relationships (composition/aggregation) represent 'has-a' connections between classes and should use instance variables to hold references - option C is correct. Arrays/collections implement one-to-many has-a relationships - option F is correct. Has-a should NOT use inheritance (that's for is-a) - options A and B are wrong. Is-a relationships represent inheritance and use 'extends' for classes - option D is correct, or 'implements' for interfaces - option E is correct. So C, D, E, F are true.

Multiple choice technology programming languages

Given: 11. public class Key { 12. private long id1; 13. private long 1d2; 14. 15. // class Key methods 16. } A programmer is developing a class Key, that will be used as a key in a standard java.util.HashMap. Which two methods should be overridden to assure that Key works correctly as a key? (Choose two.)

  1. public int hashCode()

  2. public boolean equals(Key k)

  3. public int compareTo(Object o)

  4. public boolean equals(Object o)

  5. public boolean compareTo(Key k)
Reveal answer Fill a bubble to check yourself
A,D Correct answer
Explanation

For a class to work correctly as a HashMap key, it must override hashCode() and equals(Object). HashMap uses hashCode() to bucket entries and equals() to compare keys. The equals() method signature must be 'public boolean equals(Object o)' - overriding Object.equals() requires the Object parameter type, not Key. Option B's 'equals(Key k)' would be overloading, not overriding. Options A and D are correct - hashCode() and equals(Object) must both be overridden consistently.

Multiple choice technology programming languages

True or false: It is preferable to write: List dogs = new ArrayList(); instead of List dogs = new ArrayList(); because raw types enable developers to avoid needless verbiage.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Using generic types (List) is strongly preferable over raw types (List). Raw types disable type checking, allowing runtime ClassCastException errors that generics would catch at compile-time. The 'verbiage' of generic types provides type safety - it's not needless. Joshua Bloch's Effective Java explicitly advises against raw types.

Multiple choice technology programming languages

Joshua Bloch says that the strangest thing about the Java platform is _____

  1. Poor Unicode support.

  2. java.lang.Cloneable does not contain clone() method.

  3. The byte type is signed.

  4. The creators of the Java programming language modeled the syntax after C and C++.

  5. java.io.InputStream is an abstract class and not an interface.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Joshua Bloch (author of Effective Java) has famously noted that the byte type being signed (range -128 to 127) is the strangest aspect of Java. This is because most hardware and languages use unsigned bytes (0-255), forcing awkward conversions when working with binary data. Other options are not quotes he's identified as 'strangest'.

Multiple choice technology programming languages

Given classes defined in two different files: 1. package util; 2. public class BitUtils { 3. private static void process(byte[] b) { } 4. } 1. package app; 2. public class SomeApp { 3. public static void main(String[] args) { 4. byte[] bytes = new byte[256]; 5. // insert code here 6. } 7. } What is required at line 5 in class SomeApp to use the process method of BitUtils?

  1. process(bytes);

  2. BitUtils.process(bytes);

  3. app.BitUtils.process(bytes);

  4. util.BitUtils.process(bytes);

  5. import util.BitUtils. *; process(bytes);

  6. SomeApp cannot use the process method in BitUtils.
Reveal answer Fill a bubble to check yourself
F Correct answer
Explanation

The process() method in BitUtils is declared as private static. Private methods are not accessible outside their own class, even with fully qualified names or imports. SomeApp cannot access this method regardless of how it's called.

Multiple choice technology programming languages

class Abc { int i=initialize(369); int j=initialize(); public int initialize() { return 318; } public void initialize(int i) { this.i=i; } public static void main(String a[]) { Abc abc=new Abc(); System.out.println("i="+abc.i); abc.initialize(369); System.out.println("after i="+abc.i); System.out.println("j="+abc.j); } }

  1. Error: Non static method intialize cannot accessed from static context main

  2. Error: Method initialize is already defined

  3. i=0 after i=369 j=318

  4. Error: incompatible types
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Line int i=initialize(369) tries to call initialize(int) which has void return type, but int i expects an int value. This is a type mismatch - void methods cannot return values to initialize variables.

Multiple choice technology programming languages

Given classes defined in two different files: 1. package util; 2. public class BitUtils { 3. public static void process(byte[]) { /* more code here */ } 4. } 1. package app; 2. public class SomeApp { 3. public static void main(String[] args) { 4. byte[] bytes = new byte[256]; 5. // insert code here 6. } 7. } What is required at line 5 in class SomeApp to use the process method of BitUtils?

  1. process(bytes);

  2. BitUtils.process(bytes);

  3. util.BitUtils.process(bytes);

  4. SomeApp cannot use methods in BitUtils.

  5. import util.BitUtils.*; process(bytes);
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Since SomeApp does not import the util package or the BitUtils class, it must use the fully qualified class name util.BitUtils.process(bytes); to call the static method.