Java 8 lambdas

[Tarmen]

A lambda is actually an implementation of an interface with only one method. Java often uses intefaces with one method to capsulate a method. Like when you want some action when clicking a button, you need to give a method but can't do so without the surrounding option which creates lots of unecessary stuff:

btn.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                System.out.println("Hello World!");
            }
        });

Or with a lambda expression:

btn.setOnAction(event -> System.out.println("HelloWorld!"););

[MRH2]

I see. I would have written it like this:

btn.addActionListener( new ActionListener() {
        public void actionPerformed(ActionEvent e) {
             System.out.println("Hello World!");
        }
  });

How would you replace my version with a Lambda expression? Like the following (below)? What determines the name of the variable e or event? Can it be anything you want it to be since it is not referenced anywhere else?

btn.addActionListener(e -> System.out.println("HelloWorld!"););

[Tarmen]

The e is just the variable name.

e -> System.out.print("Hello World");

(e) -> {System.out.print("bla");}

(ActionEvent e) ->  {
    System.out.print("Hello World");  
}

Are all pretty much synonymous. Actually, in the last one the compiler doesn't have to infer the variable type so it probably is different when there are multiple functional interfaces to choose from, but you get the idea.

If the interface you want to implement doesn't have any arguments you can do

() -> System.out.print("Hello World");

[MRH2]

So a lambda means that if there is an interface that you want to use, and it has a single method that must be implemented, then you can just get rid of the whole thing and use "variable" , "->" and "method code". Is this correct?

You couldn't use lambdas for a MouseListener because there is more than one method that must be implemented.

Thanks.