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: 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

Option B causes ClassCastException because x is a Beta object, and you cannot cast a Beta to Delta (Delta is a subclass of Beta, not the other way around). At runtime, when the JVM tries to cast the Beta object to Delta, it fails because the actual object is not a Delta. Options A, C, and D are valid casts that don't throw exceptions - Beta to Alpha is upcasting, Beta to Foo is valid (Beta implements Foo through Alpha), and the double cast in D is just a redundant upcast then downcast back to Beta.

Multiple choice technology programming languages

Given: 20. public class CreditCard { 21. 22. private String cardlD; 23. private Integer limit; 24. public String ownerName; 25. 26. public void setCardlnformation(String cardlD, 27. String ownerName, 28. Integer limit) { 29. this.cardlD = cardlD; 30. this.ownerName = ownerName; 31. this.limit = limit; 32. } 33. }

  1. The class is fully encapsulated.

  2. The code demonstrates polymorphism.

  3. The ownerName variable breaks encapsulation

  4. The cardlD and limit variables break polymorphism.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Encapsulation requires hiding variables using the private modifier and exposing them via public methods. Declaring ownerName as public exposes it directly, breaking encapsulation.

Multiple choice technology programming languages

Given: 1. package sun.scjp; 2. public enum Color { RED, GREEN, BLUE } 1. package sun.beta; 2. // insert code here 3. public class Beta { 4. Color g = GREEN; 5. public static void main( String[] argv) 6. { System.out.println( GREEN); } 7. } The class Beta and the enum Color are in different packages. Which two code fragments, inserted individually at line 2 of the Beta declaration, will allow this code to compile? (Choose two.)

  1. import sun.scjp.Color.*;

  2. import static sun.scjp.Color.*;

  3. import sun.scjp.Color; import static sun.scjp.Color.*;

  4. import sun.scjp.; import static sun.scjp.Color.;

  5. import sun.scjp.Color; import static sun.scjp.Color.GREEN;
Reveal answer Fill a bubble to check yourself
C,E Correct answer
Explanation

Line 4 declares a Color variable, requiring the Color type to be imported. Line 6 uses GREEN directly, requiring a static import. Option C imports both the type and all static members. Option E imports the type and just the GREEN constant. Option B only static imports, missing the type import needed for line 4 where Color is used as a type.

Multiple choice technology programming languages

Given: 10. public class Foo implements java.io.Serializable { 11. private int x; 12. public int getX() { return x; } 12.publicFoo(int x){this.x=x; } 13. private void writeObject( ObjectOutputStream s) 14. throws IOException { 15. // insert code here 16. } 17. } Which code fragment, inserted at line 15, will allow Foo objects to be correctly serialized and deserialized?

  1. s.writeInt(x);

  2. s.serialize(x);

  3. s.writeObject(x);

  4. s.defaultWriteObject();
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

To solve this question, the user needs to understand the concept of serialization in Java. Serialization is the process of converting a Java object into a stream of bytes so that it can be stored in a file or transmitted over a network. Deserialization is the reverse process of converting a stream of bytes back into a Java object.

The code given above defines a class Foo that implements the Serializable interface. This means that instances of Foo can be serialized and deserialized. The writeObject() method is a callback method that is called during serialization and allows the developer to customize the serialization process.

Option A: s.writeInt(x); This option will write the integer value of x to the ObjectOutputStream. However, this is not enough to fully serialize the Foo object, as there are other instance variables besides x that also need to be serialized.

Option B: s.serialize(x); This option is not valid, as there is no such method as serialize() in the ObjectOutputStream class.

Option C: s.writeObject(x); This option will write the object x to the ObjectOutputStream. However, this will not fully serialize the Foo object, as x is just one instance variable and there are others that also need to be serialized.

Option D: s.defaultWriteObject(); This option is the correct answer. The defaultWriteObject() method serializes all non-transient, non-static instance variables in the object. This includes the instance variable x, as well as any other instance variables that Foo may have. By calling this method, we ensure that the entire Foo object is serialized and can be correctly deserialized later.

Therefore, the answer is: D

Multiple choice technology programming languages

Which two are true? (Choose two.)

  1. An encapsulated, public class promotes re-use.

  2. Classes that share the same interface are always tightly

  3. An encapsulated class allows subclasses to overload methods, but

  4. An encapsulated class allows a programmer to change an
Reveal answer Fill a bubble to check yourself
A,D Correct answer
Explanation

Encapsulation promotes reusability by hiding implementation details and exposing only necessary interfaces. A well-encapsulated class allows programmers to modify internal implementation without breaking dependent code. Option B is incorrect because shared interfaces actually promote loose coupling, not tight coupling. Option C is incorrect because encapsulation doesn't restrict method overloading in subclasses.

Multiple choice technology programming languages

Which of the following is correct syntax for an Abstract class ?

  1. abstract double area() { }

  2. abstract double area()

  3. abstract double area();

  4. abstract double area(); { }
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

An abstract method in Java has no method body - just the signature followed by a semicolon. Option B shows the correct syntax for an abstract method declaration. Option A incorrectly includes a method body with curly braces, which would make it a concrete method. Option C uses semicolon incorrectly with braces.

Multiple choice technology programming languages

Which two are true? (Choose two.)

  1. A finalizer may NOT be invoked explicitly.

  2. The finalize method declared in class Object takes no action.

  3. super.finalize() is called implicitly by any overriding finalize method.

  4. The finalize method for a given object will be called no more than

  5. The order in which finalize will be called on two objects is based on
Reveal answer Fill a bubble to check yourself
B,D Correct answer
Explanation

B is true: Object.finalize() does nothing by default (empty implementation). D is true: The JVM guarantees finalize() is called at most once per object. A is false: finalize() can be called explicitly. C is false: super.finalize() must be called explicitly. E is false: finalization order is unpredictable.

Multiple choice technology programming languages

Which of the following statements are true?

  1. The return type for a method can be any Java type, including void

  2. An important principal of object oriented programming is implementation hiding

  3. When you perform mathematical calculations on the unlike data type, java will perform a implicit conversion to unify the types

  4. All the Above
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Option A is true because Java methods can return any type including primitives, objects, void, and custom types. Option B is true because encapsulation (implementation hiding) is a core OOP principle that hides internal details. Option C is true because Java performs implicit type widening/conversion when operands of different types are used in arithmetic operations. Since A, B, and C are all true, D 'All the Above' is correct.

Multiple choice technology programming languages

Through _________ , you can control what parts of a program can access the members of a class

  1. Encapsulation

  2. Class

  3. Inheritance

  4. Polymorphism
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Encapsulation is the object-oriented programming concept of wrapping data and code into a single unit and restricting access to class members using modifiers like private, protected, and public. Distractors like inheritance, class, or polymorphism do not directly restrict access to members.

Multiple choice technology programming languages

In order for a source code file, containing the public class Test, to successfully compile, which of the following must be true?

  1. It must have a package statement

  2. It must be named Test.java

  3. It must import java.lang

  4. It must declare a public class named Test
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Java requires that public class names match the source file name exactly. If a source file contains a public class named Test, the file must be named Test.java. Package statements and imports are optional - java.lang is imported automatically by default.

Multiple choice technology programming languages

Given: classA {} class B extends A {} class C extends A {} class D extends B {} Which three statements are true? (Choose three.)

  1. The type List is assignable to List<A>.

  2. The type List<Object> is assignable to List<?>.

  3. The type List<D> is assignable to List<? extends B>.

  4. The type List<? extends B> is assignable to List<? extends A>.

  5. The type List<Object> is assignable to any List reference.
Reveal answer Fill a bubble to check yourself
B,C,D Correct answer
Explanation

In Java generics, List can be assigned to List> because the unbounded wildcard accepts any type. List can be assigned to List extends B> because D extends B. List extends B> can be assigned to List extends A> because B extends A. Option A is false because generics are invariant - List is not assignable to List. Option E is false because List cannot be assigned to List due to invariance.

Multiple choice technology programming languages

Given: 10. class Nav{ 11. public enum Direction { NORTH, SOUTH, EAST, WEST } 12. } 13. public class Sprite{ 14. // insert code here 15. } Which code, inserted at line 14, allows the Sprite class to compile?

  1. Direction d = NORTH;

  2. Nav.Direction d = NORTH;

  3. Direction d = Direction.NORTH;

  4. Nav.Direction d = Nav.Direction.NORTH;
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The Direction enum is nested inside the Nav class, so it must be accessed as Nav.Direction. To use the enum constant NORTH, the full qualified name Nav.Direction.NORTH is required. Options A, B, and C miss either the outer class reference or the enum type.

Multiple choice technology programming languages

3) How can you call one constructor from another if a class has multiple constructors

  1. Cant do

  2. use this ()

  3. new keyword

  4. using objectname
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

To call one constructor from another in the same class, you use the this() keyword as the first statement. For example, this(5) calls a constructor taking an int parameter. Option A is incorrect because constructor chaining is supported. Option C would create a new object, not call another constructor.

Multiple choice technology web technology

5) Can you change the reference of the final object

  1. No the reference cannot be change

  2. Using object class

  3. Using interface

  4. Using thread
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A final object reference cannot be changed to point to a different object once initialized. This is what final means for reference types. However, the object's internal state CAN be modified if it's mutable. The question asks about changing the reference itself, not the object's contents.