Yes, this is another entry about closure :)

In the closure proposal, the section "Closure conversion" describes how to use a closure instead of an anonymous class.
The rules are :

• the implemented interface must have one method
• the closure signature must be a subtype of the method signature

Implementing an ActionListener

ActionListener corresponds to a component default action, it's a one method interface, the method actionPerformed takes an ActionEvent that contains among other things the component that received the event.

  public interface ActionListener {
    void actionPerformed(ActionEvent e);
  }

So the client code can be this one :

 JButton button=new JButton("Ok");
 button.addActionListener(() {
   button.setText("Ko");
 });

Oh no !!!
This code is too optimistic, it assumes that because the event is not used, the parameter can be omit. But there is no such rule in the proposal. The correct code is the following :

 JButton button=new JButton("Ok");
 button.addActionListener((ActionEvent notUsed) {
   button.setText("Ko");
 });

The moral

It could be a good idea to add a closure conversion rule that permits parameters omission. Perhaps this rule can be extended to allow the omission of the last parameters from the right.

What do you think about that ?