java – How can this code on stream compile and work?

Question:

How can this code be executed and compiled if, with action.perform, the perform method is not even defined. And plus a command like editor at the moment m.record(editor::save); , and the macro list itself is intended for objects of type action? How can this be?

public class Macro {
    List<Actiona> macro;

    Macro(){
        macro = new ArrayList<>();
    }

    public void record(Actiona action){
        macro.add(action);
    }

    public void run(){
        macro.forEach(Actiona::perform);
    }

    public static void main(String[] args) {
        MockEditor editor = new MockEditor();
        Macro m = new Macro();
        m.record(editor::save);
        m.record(editor::close);
        m.run();
    }
}

public class MockEditor implements Editor {

    @Override
    public void save() {
        System.out.println("Save");
    }

    @Override
    public void close() {
        System.out.println("close");
    }
}

public interface Actiona {
    public void perform();
}

public interface Editor {
    public void save();
    public void close();
}

Answer:

This magic is easy to understand if you go to an IDE (say IDEA) and ask to replace the lambda with an anonymous class:

public static void main(String[] args) {
    MockEditor editor = new MockEditor();
    Macro m = new Macro();

    m.record(new Actiona() { // вместо m.record(editor::save);
        @Override
        public void perform() {
            editor.save();
        }
    });

    m.record(new Actiona() { // вместо m.record(editor::close);
        @Override 
        public void perform() {
            editor.close();
        }
    });

    m.run(); // Вызываем метод с двумя объектами Actiona

And suddenly everything becomes simple and clear, we implement the perform method by calling the editor.save() and editor.close() methods internally, so the behavior is quite correct.

A very good skill, as soon as you get confused in lambda functions, replace them mentally (or using the IDE) with anonymous classes and life becomes easier.