java 的事件分发以及listener的实现
java.awt.component中对事件进行了分析，然后再调用listener的各个方法

protected void processMouseEvent(MouseEvent e) {
MouseListener listener = mouseListener;
if (listener != null) { //查是否有已的listener
int id = e.getID();
switch(id) {
case MouseEvent.MOUSE_PRESSED:
listener.mousePressed(e);
break;
case MouseEvent.MOUSE_RELEASED:
listener.mouseReleased(e);
break;
case MouseEvent.MOUSE_CLICKED:
listener.mouseClicked(e);
break;
case MouseEvent.MOUSE_EXITED:
listener.mouseExited(e);
break;
case MouseEvent.MOUSE_ENTERED:
listener.mouseEntered(e);
break;
}
}
}