Default Listener Methods

From APIDesign

Recently I tried to apply the DefaultMethods API Design warning to work done by my colleagues and they (obviously) objected. That's OK, that happens all the time, but the worst thing this time was: they were right!

Do you remember the classical JavaBean pattern called listener and its associated adapter? Imagine for example MouseListener and MouseAdapter. The pair of classes was created to save users from always implementing all the listener methods and rather use the adapter. Turns out DefaultMethods allow us to merge these two types into one:

public interface MouseListener extends EventListener {
    public default void mouseClicked(MouseEvent e) {}
    public default void mousePressed(MouseEvent e) {}
    public default void mouseReleased(MouseEvent e) {}
    public default void mouseEntered(MouseEvent e) {}
    public default void mouseExited(MouseEvent e) {}
}

Such listener type is both a listener as well as an adaptor and there is nothing wrong with that!

Why?

Why does the philippic against using DefaultMethods in an API doesn't apply? The code that runs after one of the listener methods isn't (unless an unexpected exception happens or unless it never returns or runs too long - e.g. usual warnings when calling foreign code apply here) influenced by the code in the method - e.g. it shouldn't matter what the method does and thus adding new method with default empty implementation seems to be OK.

There is a necessary condition for that: such methods have to return void. If the methods returned anything observable, then the exception wouldn't apply. If the default body would like look like this:

/** @return true to "consume" the event */
public default boolean mouseExited(MouseEvent e) {
    return false;
}

then all the above arguments against these methods would apply again, as it would be impossible to distinguish between methods having default implementation or methods overridden with intention to keep the default implementation.

Up Side Down

Thanks Dušane for pointing the listener case out! As mentioned in TheAPIBook, slight change in the initial situation may turn the final advice up side down. Thus I gladly (have to) admit: using DefaultMethods in listener JavaBean pattern is perfectly fine.