Abstract class

An abstract class is a class marked with the abstract keyword that cannot be instantiated directly. It exists to be extended by subclasses. It can declare abstract methods (no body) that subclasses must implement, and also hold concrete methods and instance fields.

Example

public abstract class Animal {
    protected final String name;

    public Animal(String name) { this.name = name; }

    public abstract String sound();  // subclasses must implement

    public void introduce() {        // shared concrete behaviour
        System.out.println(name + " says " + sound());
    }
}

public class Dog extends Animal {
    public Dog(String name) { super(name); }
    @Override public String sound() { return "Woof"; }
}

Abstract class vs interface

Modern guidance

Prefer interfaces with default methods where possible — they're more flexible. Use abstract classes when you genuinely need to share state (instance fields) or have a common constructor across subclasses.