GUI Components: Part 1 - Java: How to Program Early Objects - page 508

after new begins the declaration of an anonymous inner class that implements interface
ItemListener . This is similar to beginning a class declaration with

public class MyHandler implements ItemListener

The opening left brace at line 36 and the closing right brace at line 46 delimit the
body of the anonymous inner class. Lines 38-45 declare the ItemListener 's itemState-
Changed 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 44. For each item
selected from a JComboBox , another item is first deselected—so two ItemEvent s occur
when an item is selected. We wish to display only the icon for the item the user just
selected. For this reason, line 42 determines whether ItemEvent method getStateChange
returns ItemEvent.SELECTED . If so, lines 43-44 set label 's icon.

Software Engineering Observation 12.4
Like any other class, when an anonymous inner class implements an interface, the class
must implement every abstract method in the interface.

The syntax shown in lines 35-46 for creating an event handler with an anonymous
inner class is similar to the code that would be generated by a Java integrated development
environment (IDE). Typically, an IDE enables you to design a GUI visually, then it gen-
erates code that implements the GUI. You simply insert statements in the event-handling
methods that declare how to handle each event.

Java SE 8: Implementing Anonymous Inner Classes with Lambdas
In Section 17.9, we show how to use Java SE 8 lambdas to create event handlers. As you'll
learn, the compiler translates a lambda into an object of an anonymous inner class.

12.12 JList
A list displays a series of items from which the user may select one or more items (see the output
of Fig. 12.24). Lists are created with class JList , which directly extends class JComponent .
Class JList —which like JComboBox is a generic class—supports single-selection lists (which
allow only one item to be selected at a time) and multiple-selection lists (which allow any
number of items to be selected). In this section, we discuss single-selection lists.
The application of Figs. 12.23-12.24 creates a JList containing 13 color names.
When a color name is clicked in the JList , a ListSelectionEvent occurs and the appli-
cation changes the background color of the application window to the selected color. Class
ListTest (Fig. 12.24) contains the main method that executes this application.

1 // Fig. 12.23: ListFrame.java

2 // JList that displays a list of colors.

3 import java.awt.FlowLayout;

4 import java.awt.Color;

5 import javax.swing.JFrame;

6 import javax.swing.JList;

Fig. 12.23 | JList that displays a list of colors. (Part 1 of 2.)