Interface

The interface keyword is used to declare an interface.

The implements keyword specifies a list of interfaces, that class implements.

The default keyword allows to specify method with default implementation. Methods without implementation are called abstract. A non-abstract class must implement all abstract methods.

By default methods of interface are public. Since Java 9 interface can have private non-abstract method.

An interface can extend multiple interfaces. And an interface can not extend a class.

Empty interface is used as label or marker. For example, the Cloneable interface notes that calling the clone() method is valid. Otherwise, a CloneNotSupportedException will be thrown.

Like abstract classes, interfaces can be nested and have an anonymous implementation.

public interface MyInterface extends CharSequence, Cloneable {
    void method1();
    void method2();
    
    private void method3(){
        
    }
    
    default void method4(){
        
    }
}
public class Demo implements MyInterface{
    @Override
    public void method1() { }

    @Override
    public void method2() { }

    @Override
    public int length() {
        return 0;
    }

    @Override
    public char charAt(int index) {
        return 0;
    }

    @Override
    public CharSequence subSequence(int start, int end) {
        return null;
    }
    // ...
}
// anonymous implementation
CharSequence charSequence = new CharSequence() { 
        @Override
        public int length() {
            return 0;
        }

        @Override
        public char charAt(int index) {
            return 0;
        }

        @Override
        public CharSequence subSequence(int start, int end) {
            return null;
        }
    };