Events In Java

The last two methods of HelloComponent2, mouseDragged() and mouseMoved(), let us get information from the mouse. Each time the user performs an action, such as pressing a key on the keyboard, moving the mouse, or perhaps banging his or her head against a touch screen, Java generates an event. An event represents an action that has occurred; it contains information about the action, such as its time and location. Most events are associated with a particular GUI component in an application. A keystroke, for instance, can correspond to a character being typed into a particular text entry field. Pressing a mouse button can activate a particular button on the screen. Even just moving the mouse within a certain area of the screen can trigger effects such as highlighting or changing the cursor’s shape. To work with these events, we’ve imported a new package, java.awt.event, which provides specific Event objects that we use to get information from the user. (Notice that importing java.awt.* doesn’t automatically import the event package. Packages don’t really contain other packages, even if the hierarchical naming scheme would imply that they do.) There are many different event classes, including MouseEvent, KeyEvent, and ActionEvent. For the most part, the meaning of these events is fairly intuitive. A MouseEvent occurs when the user does something with the mouse, a KeyEvent occurs when the user presses a key, and so on. ActionEvent is a little special; we’ll see it at work later in our third version of HelloJava. For now, we’ll focus on dealing with MouseEvents. GUI components in Java generate events for specific kinds of user actions. For example, if you click the mouse inside a component, the component generates a mouse event. Objects can ask to receive the events from one or more components by registering a listener with the event source. For example, to declare that a listener wants to receive a component’s mouse-motion events, you invoke that component’s addMouseMotionListener() method, specifying the listener object as an argument. That’s what our example is doing in its constructor. In this case, the component is calling its own addMouseMotionListener() method, with the argument this, meaning “I want to receive my own mouse-motion events.” That’s how we register to receive events. But how do we actually get them? That’s what the two mouse-related methods in our class are for. The mouseDragged() method is called automatically on a listener to receive the events generated when the user drags the mouse—that is, moves the mouse with any button pressed. The mouseMoved() method is called whenever the user moves the mouse over the area without pressing a button. In this case, we’ve placed these methods in our HelloComponent2 class and had it register itself as the listener. This is entirely appropriate for our new text-dragging component. More generally, good design usually dictates that event listeners be implemented as adapter classes that provide better separation of GUI and “business logic.” We’ll discuss that in detail later. Our mouseMoved() method is boring: it doesn’t do anything. We ignore simple mouse motions and reserve our attention for dragging. mouseDragged() has a bit more meat to it. This method is called repeatedly by the windowing system to give us updates on the position of the mouse. Here it is:

 public void mouseDragged( MouseEvent e ) {  
 messageX = e.getX();  
 messageY = e.getY();  
 repaint();  
 }  

The first argument to mouseDragged() is a MouseEvent object, e, that contains all the information we need to know about this event. We ask the MouseEvent to tell us the x and y coordinates of the mouse’s current position by calling its getX() and getY() methods. We save these in the messageX and messageY instance variables for use elsewhere. The beauty of the event model is that you have to handle only the kinds of events you want. If you don’t care about keyboard events, you just don’t register a listener for them; the user can type all she wants and you won’t be bothered. If there are no listeners for a particular kind of event, Java won’t even generate it. The result is that event handling is quite efficient. While we’re discussing events, we should mention another small addition we slipped into HelloJava2:

 frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );  

This line tells the frame to exit the application when its close button is pressed. It’s called the “default” close operation because this operation, like almost every other GUI interaction, is governed by events. We could register a window listener to get notification of when the user pushes the close button and take whatever action we like, but this convenience method handles the common cases. Finally, we’ve danced around a couple of questions here: how does the system know that our class contains the necessary mouseDragged() and mouseMoved() methods (where do these names come from)? And why do we have to supply a mouseMoved() method that doesn’t do anything? The answer to these questions has to do with interfaces. We’ll discuss interfaces after clearing up some unfinished business with repaint().