Combo boxes -- JComboBox

All of the components we have examined in this chapter have been derived from JTextComponent; however, there are other components in Swing which handle text.

These components are generally not parented by JTextComponent, they are usually composed of at least one JTextComponent child with the addition of some other functionality.

The combo box (also known as a pull-down list).

A combo box is the combination in a single component of

• a text field,
• a button, and
• a list

A JComboBox

The actions of the combo box are as follows:

• By default, the selected text is displayed in the text field, which acts as the current selection box.
• The button to the right of the text field controls the display of the list box, and is used to select a preconfigured list of items.
• the combo box has the addition of editing capability in the selection text field,
• the ability to display icons along with, or in place of, the text is also available.

When the use combo boxes? Use combo boxes to conserve GUI Space:

• Combo boxes are ideal for allowing the user to choose a single item from a long list of options, without consuming a lot of dialog space.
• If you are running out of room in a dialog, and you need a UI that allows for either selecting from a list of choices or typing in place, use a combo box.
• Consider deploying the scroll bar when the item list reaches ten items, or if you need to display a hierarchy.
• The Swing JComboBox will automatically add scroll bars after approximately seven items, but you can change this by setting the preferred size of the instance.

As simple example program, that produced the above combo box follows:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

class CombBoxExample
		extends 	JFrame
		implements  ItemListener
{
	private	JComboBox	combo;

	final   String[]    sList =
	                    {
	                        "Canada",
	                        "USA",
	                        "Australia",
	                        "Bolivia",
	                        "Denmark",
	                        "Japan"
	                    };
	public CombBoxExample()
	{
		setTitle( "ComboBox Application" );
		setSize( 300, 110 );
		setBackground( Color.gray );
		getContentPane().setLayout( new BorderLayout() );

		JPanel topPanel = new JPanel();
		topPanel.setLayout( null );
		getContentPane().add( topPanel, BorderLayout.CENTER );

        // Create a combobox
		combo = new JComboBox();
		combo.setBounds( 20, 35, 260, 20 );
		topPanel.add( combo );

		// Populate the combobox list
		for( int iCtr = 0; iCtr < sList.length; iCtr++ )
		    combo.addItem( sList[iCtr] );

        // Allow edits
        combo.setEditable( true );

        // Watch for changes
        combo.addItemListener( this );
	}

	public void itemStateChanged( ItemEvent event )
	{
		if( event.getSource() == combo
				&& event.getStateChange() == ItemEvent.SELECTED )
		{
			System.out.println( "Change:"
							+ combo.getSelectedItem() );
		}
	}

	public static void main( String args[] )
	{
		// Create an instance of the test application
		CombBoxExample mainFrame	= new CombBoxExample();
		mainFrame.setVisible( true );
	}
}