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What is the difference between abstract class and interface in Java?

JavaOOPAbstract ClassInterface

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Question

What is the difference between abstract class and interface in Java? Explain with examples.

Answer

Abstract Class:

- Can have both abstract and non-abstract methods

- Can have instance variables

- Can have constructors

- Supports single inheritance

- Can have access modifiers (public, protected, private)

- Can have static and final methods

Interface:

- Can only have abstract methods (before Java 8) or default/static methods (Java 8+)

- Can only have constants (public static final variables)

- Cannot have constructors

- Supports multiple inheritance

- All methods are public by default

- Can have static and default methods (Java 8+)

Example:

// Abstract Class
abstract class Animal {
    protected String name;
    
    public Animal(String name) {
        this.name = name;
    }
    
    abstract void makeSound();
    
    public void sleep() {
        System.out.println(name + " is sleeping");
    }
}

// Interface
interface Flyable {
    void fly();
    default void land() {
        System.out.println("Landing...");
    }
}

Explanation

**Key Differences:** 1. **Inheritance:** A class can extend only one abstract class but can implement multiple interfaces. 2. **Method Implementation:** Abstract classes can have both abstract and concrete methods, while interfaces (before Java 8) could only have abstract methods. 3. **Variables:** Abstract classes can have instance variables, while interfaces can only have constants. 4. **Access Modifiers:** Abstract class methods can have any access modifier, while interface methods are public by default. 5. **Constructor:** Abstract classes can have constructors, interfaces cannot. **When to Use:** - Use **abstract class** when you want to share code among closely related classes - Use **interface** when you want to define a contract that multiple unrelated classes can implement