Is it permissible to include methods with implementation in the interface?

In Java, starting from version 8, it is allowed to include methods with implementations in interfaces — these are called default methods. They enable adding new methods to interfaces without breaking existing implementations. Such methods are declared with the default keyword and contain a body.

Example:

public interface MyInterface {
    void abstractMethod();

    default void defaultMethod() {
        System.out.println("Default implementation");
    }
}

This is useful for extending interfaces without the need to change all classes that implement them.