Java weird operator ()-> meaning and String operations [duplicate]

Question

Today I met part of weird code which I do not understand.

What can mean something like this ()-> for example

method( ()-> System.out.println("Hello") );

another question is, what is an output of String:

[AB][CD]*EF+(X/Y)

of course there were not any instance of A, B etc.But I there were answers like

ABCDEFX, ACEFXX, ACEFXA,

I do not remember them

Can anybody help me?

Answer 1

Suppose you have an interface that declares one method:

public static interface MyFunctionalInterface {
    void m1();
}

And you have a method that receives an object of that type as a parameter:

public void method(MyFunctionalInterface i) { ... }

You can implement that interface and use it immediately using anonymous inner classes like this:

method( new MyFunctionalInterface() {
    public void m1() {
        System.out.println("Hello");
    }
});

In Java 8 you can replace that with a lambda expression such as the one you showed:

method( () -> System.out.println("Hello"); );

The empty parameters represent the m1() method, with no parameters.

Suppose the functional interface you were using had a method with one parameter (if your method had the form method2(ActionListener s) { ... } for example), then you would use:

method2( e -> System.out.println("Hello"); );

which would be the same as doing this:

method2( new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        System.out.println("Hello");
    }
});

There are many tutorials about Lambda expressions in Java 8. This one is a good quick-start.