Using an Anonymous Inner Class for Event Handling

Lines 34–46 are one statement that declares the event listener’s class, creates an object of that class and registers it as the listener for imagesJComboBox’s ItemEvents. This event-listener object is an instance of an anonymous inner class—an inner class that’s declared without a name and typically appears inside a method declaration. As with other inner classes, an anonymous inner class can access its top-level class’s members. However, an anonymous inner class has limited access to the local variables of the method in which it’s declared. Since an anonymous inner class has no name, one object of the class must be created at the point where the class is declared (starting at line 35).

Lines 34–46 are a call to imagesJComboBox’s addItemListener method. The argument to this method must be an object that is an ItemListener (i.e., any object of a class that implements ItemListener). Lines 35–45 are a class-instance creation expression that declares an anonymous inner class and creates one object of that class. A reference to that object is then passed as the argument to addItemListener. The syntax ItemListener() after new begins the declaration of an anonymous inner class that implements interface ItemListener. This is similar to beginning a class declaration with

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The opening left brace at 36 and the closing right brace at line 45 delimit the body of the anonymous inner class. Lines 38–44 declare the ItemListener’s itemStateChanged method. When the user makes a selection from imagesJComboBox, this method sets label’s Icon. The Icon is selected from array icons by determining the index of the selected item in the JComboBox with method getSelectedIndex in line 43. For each item selected from a JComboBox, another item is first deselected—so two ItemEvents occur when an item is selected. We wish to display only the icon for the item the user just selected. For this reason, line 41 determines whether ItemEvent method getStateChange returns ItemEvent.SELECTED. If so, lines 42–43 set label’s icon.

The syntax shown in lines 35–45 for creating an event handler with an anonymous inner class is similar to the code that would be generated by a Java IDE. Typically, an IDE enables you to design a GUI visually, then it generates code that implements the GUI. You simply insert statements in the event-handling methods that declare how to handle each event.