Android™ How to Program, Second Edition

Registering Events

Every JComponent has an instance variable called listenerList that refers to an object of class EventListenerList (package javax.swing.event). Each object of a JComponent subclass maintains references to its registered listeners in the listenerList. For simplicity, we’ve diagramed listenerList as an array below the JTextField object in Fig. I.4.

When line 43 of Fig. I.1

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textField1.addActionListener( handler );

executes, a new entry containing a reference to the TextFieldHandler object is placed in textField1’s listenerList. Although not shown in the diagram, this new entry also includes the listener’s type (in this case, ActionListener). Using this mechanism, each lightweight Swing GUI component ...