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

Given: 1. public class Person { 2. private String name; 3. public Person(String name) { this.name = name; } 4. public boolean equals(Person p) { 5. return p.name.equals(this.name); 6. } 7. } Which statement is true?

  1. The equals method does NOT properly override the Object.equals method.

  2. Compilation fails because the private attribute p.name cannot be accessed in line 5.

  3. To work correctly with hash-based data structures, this class must also implement the hashCode method.

  4. When adding Person objects to a java.util.Set collection, the equals method in line 4 will prevent duplicates.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The equals method does NOT properly override Object.equals because it takes a Person parameter instead of Object. A proper override must have the exact signature 'boolean equals(Object obj)'. This is an overload, not an override. Option B is incorrect because private fields of the same object type can be accessed. Option C is true but not the primary issue.

Multiple choice technology

Given: 1. import java.util.*; 2. public class Old { 3. public static Object get0(List list) { 4. return list.get(0); 5. } 6. } Which three will compile successfully? (Choose three.)

  1. Object o = Old.get0(new LinkedList());

  2. Object o = Old.get0(new LinkedList<?>());

  3. String s = Old.get0(new LinkedList<String>());

  4. Object o = Old.get0(new LinkedList<Object>());

  5. String s = (String)Old.get0(new LinkedList<String>());
Reveal answer Fill a bubble to check yourself
A,D,E Correct answer
Explanation

Option A compiles because LinkedList is raw and get0() returns Object. Option D compiles as LinkedList matches Object return. Option E compiles with the explicit cast to String. Option B fails because LinkedList> is wildcard-typed and get0() expects raw List. Option C fails because String != Object return type without casting.

Multiple choice technology

Given 10. class Foo { 11. static void alpha() { /* more code here / } 12. void beta() { / more code here */ } 13. } Which two statements are true? (Choose two.)

  1. Foo.beta() is a valid invocation of beta().

  2. Foo.alpha() is a valid invocation of alpha().

  3. Method beta() can directly call method alpha().

  4. Method alpha() can directly call method beta().
Reveal answer Fill a bubble to check yourself
B,C Correct answer
Explanation

Static methods like alpha() are called on the class name (Foo.alpha()), not instances. Instance methods like beta() can call static methods directly, but static methods cannot call instance methods without an object reference.

Multiple choice technology

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

Point is a static nested class inside Line. To instantiate it outside Line, use the fully qualified name Line.Point and call new Line.Point(). Static nested classes don't require an instance of the outer class.

Multiple choice technology

Given: 10. package com.sun.scjp; 11. public class Geodetics { 12. public static final double DIAMETER = 12756.32; // kilometers 13. } Which two correctly access the DIAMETER member of the Geodetics class? (Choose two.)

  1. import com.sun.scjp.Geodetics;

  2. import static com.sun.scjp.Geodetics;

  3. import static com.sun.scjp.Geodetics.*;

  4. package com.sun.scjp;
Reveal answer Fill a bubble to check yourself
A,C Correct answer
Explanation

DIAMETER is a public static final field. Option A imports the class to access it. Option C uses static import (import static) to import all static members, making DIAMETER accessible directly. Option B is invalid syntax for static import. Option D declares a package, not an import.

Multiple choice technology

Given: 10. public class Bar { 11. static void foo( int... x ) { 12. // insert code here 13. } 14. } Which two code fragments, inserted independently at line 12, will allow the class to compile? (Choose two.)

  1. foreach( x ) System.out.println(z);

  2. for( int z : x ) System.out.println(z);

  3. while( x.hasNext() ) System.out.println( x.next() );

  4. for( int i=0; i< x.length; i++ ) System.out.println(x[i]);
Reveal answer Fill a bubble to check yourself
B,D Correct answer
Explanation

The varargs parameter int... x is treated as an array. Option B uses enhanced for-loop to iterate over x. Option D uses traditional for-loop with x.length and array indexing. Option A is invalid syntax. Option C is wrong because arrays don't have hasNext()/next() methods.

Multiple choice technology

Given: 1. public class Plant { 2. private String name; 3. public Plant(String name) { this.name = name; } 4. public String getName() { return name; } 5. } 1. public class Tree extends Plant { 2. public void growFruit() { } 3. public void dropLeaves() { } 4. } Which statement is true?

  1. The code will compile without changes.

  2. The code will compile if public Tree() { Plant(); } is added to the Tree class.

  3. The code will compile if public Plant() { Tree(); } is added to the Plant class.

  4. The code will compile if public Plant() { this("fern"); } is added to the Plant class

  5. The code will compile if public Plant() { Plant("fern"); } is added to the Plant class.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Plant has only a parameterized constructor. Tree's no-arg constructor implicitly calls super(), which fails because Plant has no no-arg constructor. Adding public Plant() { this("fern"); } provides the missing no-arg constructor.

Multiple choice technology

Given: 1. public class TestSeven extends Thread { 2. private static int x; 3. public synchronized void doThings() { 4. int current = x; 5. current++; 6. x = current; 7. } 8. public void run() { 9. doThings(); 10. } 11.} Which statement is true?

  1. Compilation fails.

  2. An exception is thrown at runtime

  3. Synchronizing the run() method would make the class thread-safe.

  4. The data in variable "x" are protected from concurrent access problems.

  5. Declaring the doThings() method as static would make the class thread-safe.

  6. Wrapping the statements within doThings() in a synchronized(new Object()) { } block would make the classthread-safe.
Reveal answer Fill a bubble to check yourself
F Correct answer
Multiple choice technology

Given: 10. interface Foo {} 11. class Alpha implements Foo {} 12. class Beta extends Alpha {} 13. class Delta extends Beta { 14. public static void main( String[] args ) { 15. Beta x = new Beta(); 16. // insert code here 17. } 18. } Which code, inserted at line 16, will cause a java.lang.ClassCastException?

  1. Alpha a = x;

  2. Foo f = (Delta)x;

  3. Foo f = (Alpha)x;

  4. Beta b = (Beta)(Alpha)x;
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

x is a Beta object. Casting it to Delta (a subclass) fails at runtime with ClassCastException because the actual object is not a Delta. You cannot cast an object to a subclass type it doesn't actually instantiate.

Multiple choice technology

Given: 11. class A { 12. public void process() { System.out.print("A,"); } 13. class B extends A { 14. public void process() throws IOException { 15. super.process(); 16. System.out.print("B,"); 17. throw new IOException(); 18. } 19. public static void main(String[] args) { 20. try { new B().process(); } 21. catch (IOException e) { System.out.println("Exception"); }} What is the result?

  1. Exception

  2. A,B,Exception

  3. Compilation fails because of an error in line 20.

  4. Compilation fails because of an error in line 14.

  5. A NullPointerException is thrown at runtime
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

When overriding a method, you cannot add new checked exceptions to the throws clause. Class A's process() declares no checked exceptions, so B cannot add "throws IOException". This violates Java's override rules. Line 20's catch block is fine - unused catch blocks are legal.

Multiple choice technology

Given: 12. import java.io.*; 13. public class Forest implements Serializable { 14. private Tree tree = new Tree(); 15. public static void main(String [] args) { 16. Forest f = new Forest(); 17. try { 18. FileOutputStream fs = new FileOutputStream("Forest.ser"); 19. ObjectOutputStream os = new ObjectOutputStream(fs); 20. os.writeObject(f); os.close(); 21. } catch (Exception ex) { ex.printStackTrace(); } 22. } } 23. 24. class Tree { } What is the result?

  1. Compilation fails

  2. An exception is thrown at runtime.

  3. An instance of Forest is serialized.

  4. An instance of Forest and an instance of Tree are both serialized
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Forest implements Serializable, but its field 'tree' is of type Tree, which does NOT implement Serializable. During serialization, Java attempts to serialize all non-transient object fields. When it tries to serialize the Tree object, it throws a NotSerializableException at runtime because Tree is not serializable.

Multiple choice technology

Given: 1. package geometry; 2. public class Hypotenuse { 3. public InnerTriangle it = new InnerTriangle(); 4. class InnerTriangle { 5. public int base; 6. public int height; 7. } Which statement is true about the class of an object that can reference the variable base?

  1. It can be any class

  2. No class has access to base

  3. The class must belong to the geometry package

  4. The class must be a subclass of the class Hypotenuse
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

InnerTriangle is a package-private class (no access modifier before 'class'), meaning only classes within the geometry package can access it. Since base is a public field inside InnerTriangle, any class that can access InnerTriangle can also access base. Therefore, only classes in the geometry package can reference the variable base.

Multiple choice technology

Which two statements are true about has-a and is-a relationships? (Choose two.)

  1. Inheritance represents an is-a relationship

  2. Inheritance represents a has-a relationship

  3. Interfaces must be used when creating a has-a relationship

  4. Instance variables can be used when creating a has-a relationship.
Reveal answer Fill a bubble to check yourself
A,D Correct answer
Explanation

Inheritance in Java represents an 'is-a' relationship - a subclass 'is a' type of its superclass. Has-a relationships represent composition and are typically implemented using instance variables (e.g., a Car 'has an' Engine). Interfaces are not required for has-a relationships - you use composition directly with instance variables. Options A and D are both correct statements.

Multiple choice technology

Given: 1. interface A { public void aMethod(); } 2. interface B { public void bMethod(); } 3. interface C extends A,B { public void cMethod(); } 4. class D implements B { 5. public void bMethod(){} 6. } 7. class E extends D implements C { 8. public void aMethod(){} 9. public void bMethod(){} 10. public void cMethod(){} 11. } What is the result?

  1. Compilation fails because of an error in line 3

  2. Compilation fails because of an error in line 7

  3. Compilation fails because of an error in line 9

  4. If you define D e = new E(), then e.bMethod() invokes the version of bMethod() defined in Line 5.

  5. If you define D e = (D)(new E()), then e.bMethod() invokes the version of bMethod() defined in Line 5.

  6. If you define D e = (D)(new E()), then e.bMethod() invokes the version of bMethod() defined in Line 9.
Reveal answer Fill a bubble to check yourself
F Correct answer
Multiple choice technology

Given: 1. public class Base { 2. public static final String FOO = "foo"; 3. public static void main(String[] args) { 4. Base b = new Base(); 5. Sub s = new Sub(); 6. System.out.print(Base.FOO); 7. System.out.print(Sub.FOO); 8. System.out.print(b.FOO); 9. System.out.print(s.FOO); 10. System.out.print(((Base)s).FOO); 11. } } 12. class Sub extends Base {public static final String FOO="bar";} What is the result?

  1. foofoofoofoofoo

  2. foobarfoobarbar

  3. foobarfoofoofoo

  4. foobarfoobarfoo

  5. barbarbarbarbar

  6. foofoofoobarbar
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Static members are accessed based on the reference type, not the actual object type. Base.FOO prints 'foo' (accessing Base's static variable). Sub.FOO prints 'bar' (accessing Sub's static variable). b.FOO prints 'foo' because b is declared as Base type. s.FOO prints 'bar' because s is declared as Sub type. ((Base)s).FOO prints 'foo' because the cast changes the reference type to Base. The result is 'foobarfoobarfoo'. Static members are not polymorphic - they depend on compile-time type.