JList 예제

Dynamically Updating a List

The previous example demonstrated the most basic list box example which included only static data and supported no selection management.

However, there is much more to the JList class. The more advanced list box application, DynListExample.java to demonstrate some of the functionality provided by this useful Swing component.

A list box will be able to dynamically updated by the user:

• Initially the list is empty.
• The User can add new text to the list by entering text in a text field and clicking the Add button.
• The user may want to delete items, we will create a Remove button and, to it, attach code to delete list items.
• we will also intercept list selections and transfer the selected item back into the text field.

 

DynListExample Program Output

The DynListExample.java program has the following main additions to the previous ListExample.java program:

• A string vector has replaced the static string array so that the code can dynamically insert and remove strings.
• The list may grow beyond the bounds of the application frame so the list instance has been added to a scrolling pane, and it will automatically attach scroll bars when required.
• A ListSelectionListener is implemented which is responsible for detecting user list selections. The list box attaches the listener code using the following line:

  listbox.addListSelectionListener( this );
  

• The valueChanged() method is called any time the user selects an item from the list. When this occurs, the code determines the current list selection and copies its text to the text field.
• The valueChanged() method, like other action handlers shown previously, uses the event.getSource() call to determine the component generating the event:

  if( event.getSource() == listbox
  && !event.getValueIsAdjusting() )
  

  However note that List selection events are generated in clusters:

  • one for each item that is being deselected,
  • one for each selected item, and one final event that indicates that the selection values are no longer changing.

  This final event is the one the program needs to detect, while all others can be disregarded. To accomplish this, the code uses the event's getValueIsAdjusting() method.

The full code listing for DynListExample.java is as follows:

// Imports
import java.util.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

class DynListExample
		extends 	JFrame
		implements	ActionListener,
					ListSelectionListener
 {
	// Instance attributes used in this example
	private	JPanel		topPanel;
	private	JList		listbox;
	private	Vector		listData;
	private	JButton		addButton;
	private	JButton		removeButton;
	private	JTextField	dataField;
	private	JScrollPane scrollPane;


	// Constructor of main frame
	public DynListExample()
	{
		// Set the frame characteristics
		setTitle( "Advanced List Box Application" );
		setSize( 300, 100 );
		setBackground( Color.gray );

		// Create a panel to hold all other components
		topPanel = new JPanel();
		topPanel.setLayout( new BorderLayout() );
		getContentPane().add( topPanel );

		// Create the data model for this example
		listData = new Vector();

		// Create a new listbox control
		listbox = new JList( listData );
		listbox.addListSelectionListener( this );

		// Add the listbox to a scrolling pane
		scrollPane = new JScrollPane();
		scrollPane.getViewport().add( listbox );
		topPanel.add( scrollPane, BorderLayout.CENTER );

		CreateDataEntryPanel();
	}


	public void CreateDataEntryPanel()
	{
		// Create a panel to hold all other components
		JPanel dataPanel = new JPanel();
		dataPanel.setLayout( new BorderLayout() );
		topPanel.add( dataPanel, BorderLayout.SOUTH );

		// Create some function buttons
		addButton = new JButton( "Add" );
		dataPanel.add( addButton, BorderLayout.WEST );
		addButton.addActionListener( this );

		removeButton = new JButton( "Delete" );
		dataPanel.add( removeButton, BorderLayout.EAST );
		removeButton.addActionListener( this );

		// Create a text field for data entry and display
		dataField = new JTextField();
		dataPanel.add( dataField, BorderLayout.CENTER );
	}

	// Handler for list selection changes
 	public void valueChanged( ListSelectionEvent event )
 	{
 		// See if this is a listbox selection and the
 		// event stream has settled
		if( event.getSource() == listbox
						&& !event.getValueIsAdjusting() )
		{
			// Get the current selection and place it in the
			// edit field
			String stringValue = (String)listbox.getSelectedValue();
			if( stringValue != null )
				dataField.setText( stringValue );
		}
 	}

	// Handler for button presses
	public void actionPerformed( ActionEvent event )
	{
		if( event.getSource() == addButton )
		{
			// Get the text field value
			String stringValue = dataField.getText();
			dataField.setText( "" );

			// Add this item to the list and refresh
			if( stringValue != null )
			{
				listData.addElement( stringValue );
				listbox.setListData( listData );
				scrollPane.revalidate();
				scrollPane.repaint();
			}
		}

		if( event.getSource() == removeButton )
		{
			// Get the current selection
			int selection = listbox.getSelectedIndex();
			if( selection >= 0 )
			{
				// Add this item to the list and refresh
				listData.removeElementAt( selection );
				listbox.setListData( listData );
				scrollPane.revalidate();
				scrollPane.repaint();

				// As a nice touch, select the next item
				if( selection >= listData.size() )
					selection = listData.size() - 1;
				listbox.setSelectedIndex( selection );
			}
		}
	}

	// Main entry point for this example
	public static void main( String args[] )
	{
		// Create an instance of the test application
		DynListExample mainFrame	= new DynListExample();
		mainFrame.setVisible( true );
	}
}