Java Closures versus MouseListener

The Closures for Java proposal simplifies the code for many purposes where anonymous class instance creation expressions are currently used. When the anonymous class's supertype is an interface with a single abstract method, a closure can be used directly. But if the supertype is a class, like java.util.TimerTask, or has more than one method, like java.awt.event.MouseListener, then you can't use a closure directly. You can still use an anonymous inner class directly, as always, but there are ways of using closures that may be more convenient. For TimerTask, a client can be written this way

void printHelloAfterDelay(java.util.Timer timer, long delay) {
    timer.schedule(TimerTask.of({ => System.out.println("Hello"); }), delay);
}

if we add the following utility method to TimerTask:

public TimerTask of(final Runnable block) {
    class ClosureTimerTask extends TimerTask {
        public void run() { block.run(); }
    }
    return new ClosureTimerTask();
}

Similarly, Peter von der Ahé showed me how to use the builder pattern, along with closures, to simplify handling mouse events:

void addSomeActions(java.awt.Component foo) {
    foo.addMouseListener(new MouseListenerBuilder() 
        .setMouseClicked({ MouseEvent e => System.out.println("Mouse clicked"); })
        .setMouseReleased({ MouseEvent e => System.out.println("Mouse released"); })
        .setMouseEntered({ MouseEvent e => System.out.println("Mouse entered " + e.getComponent()); }));
}

The implementation of MouseListenerBuilder is left as an exercise to the reader.