Java OOPs Interview Questions 2026: 32 Q&A With Code

Last Updated: June 2026 | Level: Freshers to Mid-Level | Read Time: ~16 min

Object-oriented programming is the backbone of every Java technical round. Candidates report that OOP questions appear in almost every service-company interview (TCS, Infosys, Wipro, Accenture) and form the warm-up in product-company loops before the DSA grind begins. This guide collects 32 of the most repeated Java OOP questions, each with a full answer, runnable code, and the follow-up the interviewer usually asks next. Public preparation resources and the official Oracle Java documentation back the language behaviour shown here, confirm any version-specific detail on the official docs because behaviour can shift between Java releases.

Pair this with our Java Interview Questions 2026 master set and the Java Collections Interview Questions 2026 deep dive for full coverage.

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Table of Contents

1. Fundamentals (Q1 to Q10)
2. Inheritance and Polymorphism (Q11 to Q20)
3. Abstraction and Interfaces (Q21 to Q27)
4. Advanced and Tricky (Q28 to Q32)
5. Predict the Output
6. Quick Revision Table
7. Frequently Asked Questions

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Fundamentals

Q1. What are the four pillars of OOP in Java? Easy

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Q2. What is a class and what is an object? Easy

class Car {            // blueprint
    String model;
    void drive() { System.out.println(model + " driving"); }
}

Car c = new Car();     // object on the heap
c.model = "Nexon";
c.drive();             // Nexon driving

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Q3. What is encapsulation and how do you achieve it in Java? Easy

class Account {
    private double balance;                 // hidden state

    public double getBalance() { return balance; }

    public void deposit(double amt) {
        if (amt > 0) balance += amt;        // validated write
    }
}

The interviewer's follow-up is usually "why not make the field public?" Answer: a public field cannot enforce invariants such as a non-negative balance, and you cannot change the internal representation later without breaking callers.

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Q4. What is a constructor? Can it be private? Easy

class Logger {
    private static final Logger INSTANCE = new Logger();
    private Logger() {}                       // private constructor
    public static Logger get() { return INSTANCE; }
}

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Q5. What is the difference between a default constructor and a parameterised constructor? Easy

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Q6. What is the this keyword used for? Easy

class Point {
    int x, y;
    Point(int x, int y) { this.x = x; this.y = y; }  // field vs parameter
    Point(int x) { this(x, 0); }                      // constructor chaining
}

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Q7. What is the difference between instance variables and static variables? Medium

class Counter {
    static int total = 0;   // shared
    int id;                 // per object
    Counter() { id = ++total; }
}

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Q8. What is constructor overloading? Easy

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Q9. Can a constructor be inherited? Medium

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Q10. What is the order of execution: static block, instance block, constructor? Medium

class Demo {
    static { System.out.println("static block"); }
    { System.out.println("instance block"); }
    Demo() { System.out.println("constructor"); }
}
// new Demo(); twice prints:
// static block      (once)
// instance block / constructor   (each time)

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Inheritance and Polymorphism

Q11. What is inheritance and what types does Java support? Easy

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Q12. Why does Java not support multiple inheritance of classes? Medium

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Q13. What is the difference between method overloading and method overriding? Medium

class A { void show() { System.out.println("A"); } }
class B extends A {
    @Override void show() { System.out.println("B"); }   // overriding
    void show(int n) { System.out.println("B " + n); }    // overloading
}
A ref = new B();
ref.show();        // B  (runtime dispatch)

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Q14. What is dynamic method dispatch? Medium

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Q15. What is the super keyword used for? Easy

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Q16. Can you override a static method? Hard

class P { static void m() { System.out.println("P"); } }
class C extends P { static void m() { System.out.println("C"); } }
P ref = new C();
ref.m();          // P  (static = no runtime dispatch)

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Q17. Can you override a private method? Medium

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Q18. What happens if you override a method and change only the return type? Hard

class Shape {}
class Circle extends Shape {}
class A { Shape make() { return new Shape(); } }
class B extends A { @Override Circle make() { return new Circle(); } } // covariant, OK

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Q19. What is the difference between IS-A and HAS-A relationships? Medium

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Q20. What is upcasting and downcasting? Medium

Animal a = new Dog();          // upcast, implicit
if (a instanceof Dog d) {       // pattern match, Java 16+
    d.bark();                   // safe downcast
}

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Abstraction and Interfaces

Q21. What is abstraction and how is it achieved in Java? Easy

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Q22. What is the difference between an abstract class and an interface? Medium

Aspect	Abstract class	Interface
Instantiation	Cannot be instantiated	Cannot be instantiated
Methods	Abstract plus concrete	Abstract, default, static, private (Java 9+)
Fields	Any, including mutable state	public static final constants only
Inheritance	A class extends one	A class implements many
Constructor	Has constructors	No constructors
Use when	Related classes share state and code	Unrelated classes share a capability

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Q23. Can an interface have a constructor? Medium

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Q24. What are default methods and why were they added? Medium

interface Greeter {
    String name();
    default void hello() { System.out.println("Hi " + name()); }
}

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Q25. What happens with the diamond problem for default methods? Hard

interface A { default void run() { System.out.println("A"); } }
interface B { default void run() { System.out.println("B"); } }
class C implements A, B {
    @Override public void run() { A.super.run(); }   // explicit resolution
}

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Q26. Can an abstract class have a constructor and a main method? Medium

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Q27. What is a functional interface? Medium

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Advanced and Tricky

Q28. What is the difference between association, aggregation, and composition? Hard

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Q29. What is the Liskov Substitution Principle? Hard

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Q30. Why are equals() and hashCode() always overridden together? Hard

record User(int id, String name) {}   // equals + hashCode auto-generated

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Q31. What is the difference between shallow copy and deep copy? Hard

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Q32. What is polymorphism's benefit in real design? Medium

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Predict the Output

Snippet 1

class A {
    A() { System.out.println("A ctor"); print(); }
    void print() { System.out.println("A print"); }
}
class B extends A {
    int x = 10;
    @Override void print() { System.out.println("B print x=" + x); }
}
new B();

Output:

A ctor
B print x=0

Why: The superclass constructor runs before B's field initialiser. The overridden print() is dispatched to B, but x is still 0 because its initialiser has not executed yet. This partial-construction gotcha is a favourite advanced question.

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Snippet 2

class Parent { static String who() { return "Parent"; } }
class Child extends Parent { static String who() { return "Child"; } }
Parent p = new Child();
System.out.println(p.who());

Output:

Parent

Why: Static methods are not polymorphic. p has static type Parent, so Parent.who() is bound at compile time regardless of the runtime object.

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Snippet 3

class Box {
    int val = 5;
    Box add() { val++; return this; }
}
Box b = new Box();
System.out.println(b.add().add().add().val);

Output:

8

Why: Each add() returns this, enabling method chaining. val increments three times from 5 to 8. This pattern is the basis of fluent builders.

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Quick Revision Table

Concept	One-line takeaway
Overloading vs overriding	Compile-time vs runtime resolution
Static method override	Not possible, only hidden
Multiple inheritance	Classes no, interfaces yes
Abstract class vs interface	Shared state and code vs pure capability
equals + hashCode	Always override together
Upcast vs downcast	Implicit safe vs explicit risky
Composition vs inheritance	Prefer HAS-A for flexibility

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Frequently Asked Questions

Which OOP topic is asked most in Java interviews in 2026?

Candidates consistently report runtime versus compile-time polymorphism, the difference between abstract class and interface after Java 8 default methods, and why Java has no multiple inheritance of classes. Master those three and you cover most fresher OOP rounds.

Do I need to memorise the four pillars of OOP for Java rounds?

You need to explain each pillar with a Java example, not just name them. Interviewers usually follow up with code, for example asking you to show encapsulation with a private field plus a getter, or polymorphism with an overridden method.

Is method overloading runtime or compile-time in Java?

Overloading is resolved at compile time based on the static type of the arguments. Overriding is resolved at runtime based on the actual object. Mixing these up is the single most common OOP mistake candidates report.

How deep should my answer on inheritance go for a fresher round?

Cover the types Java supports, why multiple inheritance of classes is banned, and the IS-A versus HAS-A trade-off. If you can also state that composition is usually preferred over deep inheritance, you signal design maturity that most freshers miss.

Are records replacing the need to understand equals and hashCode?

Records generate equals, hashCode, and toString for you, but interviewers still ask you to explain the contract. Knowing why equal objects must share a hash code is what they are testing, the record is just a convenience.

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Related Articles

• Java Interview Questions 2026, the master 50-question set
• Java Collections Interview Questions 2026, List, Set and Map deep dive
• Java Multithreading Interview Questions 2026, concurrency rounds explained
• TCS Placement Papers 2026, Java OOP features heavily in TCS NQT

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Confirm any version-specific Java behaviour on the official Oracle documentation before your interview. This guide reflects candidate-reported question patterns and public preparation resources as of June 2026.