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

Abstract class — can have implementation. Interface — only abstract methods (until Java 8).

Abstract class:

• Can have implemented methods.
• Can have instance variables.
• Can have constructors.
• Single inheritance only.
• Can have any access modifier.

1public abstract class Animal {
2    protected String name;
3
4    // Implemented method
5    public void eat() {
6        System.out.println("Eating...");
7    }
8
9    // Abstract method
10    public abstract void makeSound();
11}

Interface (Java 8+):

• Can have default methods.
• Can have static methods.
• Can have private methods (Java 9+).
• No instance variables (only constants).
• Multiple inheritance.
• All methods public by default.

1public interface Animal {
2    // Default method
3    default void eat() {
4        System.out.println("Eating...");
5    }
6
7    // Abstract method
8    void makeSound();
9}

Java 8+ interface changes:

• Default methods.
• Static methods.
• Private methods (Java 9).