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OOPs in Java — Questions and Answers

Java interviews return to OOP more than to any other topic because the language enforces it: everything lives inside a class. The differentiators that decide fresher interviews are abstract class versus interface, overloading versus overriding, why Java has no multiple class inheritance, and why String is immutable — all questions with precise, memorable answers.

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OOPs in Java— Concepts, Formulas & Shortcuts

  • Java supports encapsulation, abstraction, inheritance and polymorphism; it does not support multiple inheritance of classes.
  • An abstract class can hold state and constructors; an interface (pre-Java 8) holds only abstract methods and constants.
  • Overloading = same name, different parameter list, resolved at compile time. Overriding = same signature in a subclass, resolved at run time.
  • A class can extend one class but implement many interfaces — this is how Java avoids the diamond problem.
  • String is immutable, which makes it safe to share, cacheable in the string pool, and safe as a HashMap key.
  • final on a class prevents inheritance, on a method prevents overriding, and on a variable prevents reassignment.

OOPs in Java Practice Questions with Answers

Attempt each question first, then open the explanation. All 8 questions below are free to read and require no signup.

  1. Q1.Which OOP feature does Java NOT support for classes?

    Easy
    • AEncapsulation
    • BMultiple inheritance
    • CPolymorphism
    • DAbstraction
    +Show Answer & Explanation

    Answer: B. Multiple inheritance

    Explanation: Java allows a class to extend only one class, avoiding the diamond problem. Multiple inheritance of type is available through interfaces.

    OOPs in Java question 1 of 8

  2. Q2.Method overloading in Java is resolved:

    Moderate
    • AAt run time
    • BAt compile time
    • CBy the JVM at class-load time
    • DBy the garbage collector
    +Show Answer & Explanation

    Answer: B. At compile time

    Explanation: Overloading is static (compile-time) polymorphism — the compiler picks the method from the argument types.

    OOPs in Java question 2 of 8

  3. Q3.Which keyword prevents a method from being overridden?

    Easy
    • Astatic
    • Bfinal
    • Cprivate
    • Dabstract
    +Show Answer & Explanation

    Answer: B. final

    Explanation: A final method cannot be overridden by a subclass. (private methods are simply not visible for overriding, but final is the explicit mechanism.)

    OOPs in Java question 3 of 8

  4. Q4.An interface in Java (before Java 8) can contain:

    Moderate
    • AOnly abstract methods and constants
    • BInstance variables and constructors
    • CStatic blocks
    • DPrivate methods
    +Show Answer & Explanation

    Answer: A. Only abstract methods and constants

    Explanation: Pre-Java 8, interfaces held only public abstract methods and public static final constants. Java 8 added default and static methods.

    OOPs in Java question 4 of 8

  5. Q5.Why is the String class immutable in Java?

    Moderate
    • ATo save compilation time
    • BFor security, string-pool caching and thread safety
    • CBecause it is final
    • DTo allow inheritance
    +Show Answer & Explanation

    Answer: B. For security, string-pool caching and thread safety

    Explanation: Immutability lets strings be safely shared across threads, cached in the string pool, and used reliably as HashMap keys and in security-sensitive parameters.

    OOPs in Java question 5 of 8

  6. Q6.What is the output of System.out.println(10 + 20 + "Java");

    Moderate
    • A1020Java
    • B30Java
    • CJava30
    • DCompilation error
    +Show Answer & Explanation

    Answer: B. 30Java

    Explanation: Evaluation is left to right: 10 + 20 = 30 as integers, then concatenation with the String gives "30Java".

    OOPs in Java question 6 of 8

  7. Q7.Which of these can be achieved through an abstract class but NOT an interface?

    Difficult
    • ADefining a contract
    • BHolding instance state and a constructor
    • CMultiple implementation
    • DDeclaring constants
    +Show Answer & Explanation

    Answer: B. Holding instance state and a constructor

    Explanation: Only an abstract class can hold instance fields and constructors; interfaces cannot carry per-object state.

    OOPs in Java question 7 of 8

  8. Q8.Run-time polymorphism in Java is achieved by:

    Easy
    • AMethod overloading
    • BMethod overriding
    • CConstructor overloading
    • DUsing static methods
    +Show Answer & Explanation

    Answer: B. Method overriding

    Explanation: Overriding lets the JVM dispatch to the actual object's implementation at run time — dynamic method dispatch.

    OOPs in Java question 8 of 8

OOPs in Java — Frequently Asked Questions

What is the difference between an abstract class and an interface?+

An abstract class can have constructors, instance fields and concrete methods, and a class can extend only one. An interface defines a contract, supports multiple implementation, and since Java 8 can carry default and static methods but still no instance state.

Why does Java not support multiple inheritance of classes?+

To avoid the diamond problem, where a class inherits conflicting implementations of the same method from two parents. Interfaces give the benefit of multiple types without ambiguous state.

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