Dialog Focus

When a JDialog (or JFrame for that matter) is made visible, focus is placed on the first focusable component by default. There may be times when you want to change this behaviour. The obvious solution would be to invoke the requestFocusInWindow() method on the component you wish to receive focus. The problem is that this doesn’t work all the time.

Lets take a simple panel with two components and you want focus on the second component. The code might look something like:

JTextField textField = new JTextField(10);
JButton button = new JButton( "Button" );
JPanel panel = new JPanel();
panel.add( textField );
panel.add( button );
button.requestFocusInWindow();

The problem is this code won’t work because a component can’t request focus unless it has been added to a “realized” dialog. A realized dialog basically means that the Swing JDialog has been added to a peer component that represents a dialog on the underlying OS. This occurs when you invoke the pack() or setVisible( true ) methods on the JDialog.

So the simple solution to this problem is to invoke the requestFocusInWindow() method after packing the dialog:

JDialog dialog = new JDialog();
dialog.add( panel );
dialog.pack();
button.requestFocusInWindow();
dialog.setVisible( true );

Optionally, the requestFocusInWindow() method could be placed after the setVisible(…) method. However, remember that when using a modal dialog this will not work since the statement won’t be executed until after the dialog is closed.

The above solution works in many cases, but does have a drawback. You may not always have easy access to the component you wish to gain focus. An example of this situation would be when you use one of the JOptionPane.showXXX(…) methods. These methods allow you to construct a functional modal dialog with custom buttons simply by creating a panel and by invoking the appropriate convenience showXXX method. A simple example of creating a dialog with a contained panel and “Yes”, “No”, “Cancel” buttons would be:

JTextField userid = new JTextField(10);
JTextField password = new JTextField(10);
JPanel panel = new JPanel(new GridLayout(2,2));
panel.add( new JLabel( "Userid:") );
panel.add( userid );
panel.add( new JLabel( "Password:") );
panel.add( password );
JOptionPane.showConfirmDialog(null, panel);

In this case, if you run the above code you will notice that focus is on the “Yes” button. I suspect in most cases you would want focus on the “Userid:” text field. However in this case we don’t have access to the dialog that is created so we can’t use the approach from above to request focus on the text field after the dialog has been packed.

One solution for this problem is to use an AncestorListener. According to the AncestorListener API an event is fired when the component becomes visible or invisible, either by the setVisible() method or by being added or removed from the component hierarchy. For our purposes this means that the event is fired when the dialog containing the component is made visible, not when the component is added to the panel.

To take advantage of the AncestorListener generated events I created the RequestFocusListener class. This class simply handles the ancestorAdded event and invokes the requestFocusInWindow() method on the component. To use the RequestFocusListener class in the above example the code would be changed to:

JTextField userid = new JTextField(10);
userid.addAncestorListener( new RequestFocusListener() );

It doesn’t matter what panel the text field is added to, or what dialog the panel is added to, the text field will recieve focus. Also, this approach will work if the panel is added to a visible dialog, since the ancestorAdded event will still be generated.

All the suggestion made here will apply to JFrames as well as dialogs. Hopefully they will help resolve any focus issues you have related to initial focus when a dialog or frame is displayed.

Get The Code

RequestFocusListener.java

Related Reading

Java Tutorials: How to Make Dialogs
Java API: AncestorListener