Java 8 Lambda Expressions - what about multiple methods in nested class

Question

I'm reading about the new features at: http://www.javaworld.com/article/2078836/java-se/love-and-hate-for-java-8.html

I saw the example below:

Using Anonymous Class:

button.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent ae) {
        System.out.println("Action Detected");
    }
});

With Lambda:

button.addActionListener(e -> {
    System.out.println("Action Detected");
});

What would someone do with a MouseListener if they wanted to implement multiple methods within the anonymous class, e.g.:

public void mousePressed(MouseEvent e) {
    saySomething("Mouse pressed; # of clicks: "
               + e.getClickCount(), e);
}

public void mouseReleased(MouseEvent e) {
    saySomething("Mouse released; # of clicks: "
               + e.getClickCount(), e);
}

... and so on?

Answer 1

From JLS 9.8

A functional interface is an interface that has just one abstract method, and thus represents a single function contract.

Lambdas require these functional interfaces so are restricted to their single method. Anonymous interfaces still need to be used for implementing multi-method interfaces.

addMouseListener(new MouseAdapter() {

    @Override
    public void mouseReleased(MouseEvent e) {
       ...
    }

    @Override
    public void mousePressed(MouseEvent e) {
      ...
    }
});

Answer 2

You can use multi-method interfaces with lambdas by using helper interfaces. This works with such listener interfaces where the implementations of unwanted methods are trivial (i.e. we can just do what MouseAdapter offers too):

// note the absence of mouseClicked…
interface ClickedListener extends MouseListener
{
    @Override
    public default void mouseEntered(MouseEvent e) {}

    @Override
    public default void mouseExited(MouseEvent e) {}

    @Override
    public default void mousePressed(MouseEvent e) {}

    @Override
    public default void mouseReleased(MouseEvent e) {}
}

You need to define such a helper interface only once.

Now you can add a listener for click-events on a Component c like this:

c.addMouseListener((ClickedListener)(e)->System.out.println("Clicked !"));