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Java and event handling

How events are passed to user interface components, how to create event handlers, and more

By Todd Sundsted, JavaWorld.com, 08/01/96

Page 2 of 3

Consider the following description of an imaginary user's interaction with the applet in Figure 2. The user clicks on the button labeled "One." The Java language run-time system gathers information about the event (the number of clicks, the location of the click, the time the click occurred, and the component that received the click) and packages that information in an instance of the Event class. The Java run-time system then begins at the component that was clicked (in this case, the Button labeled "One") and, via a call to the component's handleEvent() method, offers the component a chance to react to the event. If the component does not handle the event or handles the event incompletely (indicated by a return value of false), the Java run-time system offers the Event instance to the next higher component in the tree -- in this case an instance of the Panel class. The Java run-time system continues in this manner until the event is handled or the run-time system runs out of components to try. Figure 4 illustrates the path of this event as the applet attempts to handle it.

Figure 4: The path of an event

Each component making up the applet in Figure 2 adds a line to the TextArea object that indicates it received an event. It then allows the event to propagate to the next component in the tree. Listing 1 contains the code for a typical handleEvent() method. The complete source code for this applet is available here.

public boolean handleEvent(Event evt)
{
   if (evt.id == Event.ACTION_EVENT)
   {
      ta.appendText("Panel " + str + " saw action...\n");
   }
   else if (evt.id == Event.MOUSE_DOWN)
   {
      ta.appendText("Panel " + str + " saw mouse down...\n");
   }

   return super.handleEvent(evt);
}

Listing 1: A typical handleEvent() method

Event helper methods

The handleEvent() method is one place a programmer can put application code for handling events. Occasionally, however, a component will only be interested in events of a certain type (for example, mouse events). In these cases, the programmer can place the code in a helper method, rather than placing it in the handleEvent() method.

Here is a list of the helper methods available to programmers. There are no helper methods for certain types of events.

action(Event evt, Object what)
gotFocus(Event evt, Object what)
lostFocus(Event evt, Object what)
mouseEnter(Event evt, int x, int y)
mouseExit(Event evt, int x, int y)
mouseMove(Event evt, int x, int y)
mouseUp(Event evt, int x, int y)
mouseDown(Event evt, int x, int y)
mouseDrag(Event evt, int x, int y)
keyDown(Event evt, int key)
keyUp(Event evt, int key)

false to indicate that the helper method did not handle the event.

The implementation of the handleEvent() method provided by class Component invokes each helper method. For this reason, it is important that the redefined implementations of the handleEvent() method in derived classes always end with the statement

return super.handleEvent(e);

The code in Listing 2 illustrates this rule.

public boolean handleEvent(Event e)
{
   if (e.target instanceof MyButton)
   {
      // do something...
      return true;
   }

   return super.handleEvent(e);
}

Listing 2: Rule for ending statement in handleEvent() method
Failure to follow this simple rule will prevent the proper invocation of helper methods.