Oops.md

Java OOPs Questions and Answers

1. What are objects and classes in Java?

Answer:

• Class: A blueprint or template for creating objects. It defines properties (fields) and behaviors (methods).
• Object: An instance of a class. It represents a real-world entity with a state (attributes) and behavior (methods).

class Car {
    String brand;
    int speed;

    void accelerate() {
        speed += 10;
    }
}

Car car = new Car(); // Object of class Car

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2. How does method overloading work in Java? Give an example.

Answer:

• Method overloading allows multiple methods with the same name but different parameter lists (type, number, or order).
• It provides compile-time polymorphism.

class Calculator {
    int add(int a, int b) {
        return a + b;
    }

    double add(double a, double b) {
        return a + b;
    }
}

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3. What is method overriding, and how is it different from overloading?

Answer:

• Method Overriding: Redefining a method in a subclass that exists in the parent class. It provides runtime polymorphism.
• Difference:
  • Overloading happens within the same class, overriding occurs across a superclass and subclass.
  • Overloading is resolved at compile time, overriding at runtime.

class Parent {
    void display() {
        System.out.println("Parent method");
    }
}

class Child extends Parent {
    @Override
    void display() {
        System.out.println("Child method");
    }
}

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4. Explain the difference between an abstract class and an interface.

Answer:

Feature	Abstract Class	Interface
Inheritance	Single inheritance	Multiple inheritance
Method Types	Can have abstract and concrete	Only abstract methods (until Java 7), default and static methods (Java 8+)
Fields	Can have instance variables	Only constants
Keywords	abstract	interface

abstract class Animal {
    abstract void makeSound();
}

interface Vehicle {
    void start();
}

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5. What is multiple inheritance, and how is it implemented in Java?

Answer:

• Multiple inheritance refers to a class inheriting from more than one class or interface.
• Java does not support multiple inheritance with classes to avoid the Diamond Problem.
• It is achieved using interfaces.

interface A {
    void methodA();
}

interface B {
    void methodB();
}

class C implements A, B {
    public void methodA() {
        System.out.println("Method A");
    }

    public void methodB() {
        System.out.println("Method B");
    }
}

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6. What is the role of constructors in Java? Can we have a private constructor?

Answer:

• Role: Constructors initialize an object when it is created.
• Yes, we can have a private constructor. It is used in Singleton design patterns.

class Singleton {
    private static Singleton instance;
    private Singleton() {}

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

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7. How does the super keyword work with inheritance?

Answer:

• super is used to access methods or constructors of the parent class.
• It is used to:
  • Call the superclass constructor.
  • Access superclass methods and fields.

class Parent {
    Parent(String name) {
        System.out.println("Parent: " + name);
    }
}

class Child extends Parent {
    Child(String name) {
        super(name);
    }
}

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8. What are static and dynamic binding in Java?

Answer:

• Static Binding: Method calls resolved at compile-time (e.g., private, final, or static methods).
• Dynamic Binding: Method calls resolved at runtime (e.g., overridden methods).

class Parent {
    void display() {
        System.out.println("Parent method");
    }
}

class Child extends Parent {
    @Override
    void display() {
        System.out.println("Child method");
    }
}

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9. What is an inner class? Differentiate between a static inner class and a non-static inner class.

Answer:

• Inner Class: A class defined within another class.
• Static Inner Class:
  • Can be accessed without creating an instance of the outer class.
  • Does not have access to instance members of the outer class.
• Non-Static Inner Class:
  • Requires an instance of the outer class.
  • Can access all members of the outer class.

class Outer {
    class NonStaticInner {}
    static class StaticInner {}
}

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10. How are objects compared in Java? What is the role of the compareTo() method?

Answer:

• Objects are compared using:
  • == for reference comparison.
  • .equals() for value comparison.
• compareTo():
  • Used for natural ordering (e.g., sorting).
  • Returns:
    • Negative: Current object < argument.
    • Zero: Current object == argument.
    • Positive: Current object > argument.

class Person implements Comparable<Person> {
    String name;

    public int compareTo(Person other) {
        return this.name.compareTo(other.name);
    }
}