Top 15 Java & OOPs Interview Questions (With Answers & Examples) for Beginners

Whether you're preparing for your first developer role or brushing up for a technical round, these 15 frequently asked Java and OOPs questions will sharpen your basics and boost your confidence.

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1️⃣ What is a Class in Java?

A class is a blueprint for creating objects. It defines properties (variables) and behaviors (methods).

class Car {
    String brand;
    void drive() {
        System.out.println("Car is driving");
    }
}

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2️⃣ What is an Object in Java?

An object is an instance of a class. It holds data and can perform actions.

Car myCar = new Car(); // Object created

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3️⃣ What is Encapsulation?

Encapsulation means binding data with methods and restricting direct access to variables.

✅ Achieved using:

• private fields

• public getters/setters

class Person {
    private int age;
    public void setAge(int age) {
        if(age > 0) this.age = age;
    }
    public int getAge() { return age; }
}

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4️⃣ What is Abstraction?

Abstraction means hiding internal logic and exposing only what’s necessary.

✅ Achieved using:

• abstract classes

• interfaces

• Encapsulated logic with public methods

Example: You see getBalance() but not how it fetches from the DB.

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5️⃣ Difference between Encapsulation and Abstraction?

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6️⃣ How do Access Modifiers work in Java?

• private: Accessible only in the class

• default: Package-private

• protected: Accessible in package + subclass

• public: Accessible everywhere

Used to control visibility and enforce encapsulation.

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7️⃣ How is Encapsulation Implemented in Java?

Using:

• Private variables

• Public getters/setters

• Validation inside setters

This helps secure data and control access.

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8️⃣ How is Abstraction Implemented in Java?

Using:

• abstract classes (can have method bodies)

• interfaces (pure abstraction until Java 8+)

Example:

interface Animal {
    void makeSound(); // Abstract method
}

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9️⃣ What are Libraries in Java?

Java comes with rich built-in libraries like:

• java.util.* (collections)

• java.io.* (file handling)

• java.net.* (networking)

Also, many external libraries like Apache, Jackson, etc.

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🔟 Why Can Java Run Across Platforms?

Because of the Java Virtual Machine (JVM).

Java code → Compiled to bytecode → Run by JVM on any OS
"Write once, run anywhere" 🔁

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1️⃣1️⃣ Procedural vs OOPs – What’s Better?

✅ OOPs makes code scalable, modular, and easier to maintain.

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1️⃣2️⃣ One Key Reason for Java's Popularity?

Java's platform independence + automatic memory management (GC) made it ideal for:

• Web apps

• Enterprise software

• Android apps

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1️⃣3️⃣ How Does Java Handle Memory?

Java uses Garbage Collection (GC) to automatically free unused memory. No need to manually allocate or deallocate like in C/C++.

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1️⃣4️⃣ What is Polymorphism?

Polymorphism means many forms—same method name, different behavior.

• Compile-time: Method overloading

• Runtime: Method overriding

class Shape {
    void draw() { System.out.println("Drawing shape"); }
}
class Circle extends Shape {
    void draw() { System.out.println("Drawing circle"); }
}

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1️⃣5️⃣ What is Inheritance?

Inheritance allows one class to inherit fields/methods from another.

class Animal {
    void sound() { System.out.println("Some sound"); }
}
class Dog extends Animal {
    void sound() { System.out.println("Bark"); }
}

✅ Promotes code reuse and hierarchy.

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🚀 Wrap-Up & What’s Next?

These 15 questions are must-know for any Java/OOPs interview. Practice writing small programs and understand the concepts deeply—not just definitions.

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