focus : Java Glossary

focus

the most recently clicked window or component has the focus of the user’s attention. When a button in a frame has focus, the frame also has focus. Keystrokes are directed to the component with the focus.

Tips

Component.requestFocus won’t work until after the frame is completely painted. This means you must usually put such code inside a WindowListener.
Like all Swing and AWT (Advanced Windowing Toolkit) calls, requestFocus calls should run the EDT (Event Dispatch Thread) thread.
Component.transferFocus lets you hand focus off to some other component.
Component.hasFocus lets you determine if a given Component has focus.
KeyboardFocusManager.getFocusOwner lets you ask which Component currently has focus.
Call getFocusOwner on all windows in your application. (Frame.getFrames() followed by calling getOwnedWindows on each of them recursively should get you a full list).
Java version 1.4 now handles focus and traversal with in totally new way with FocusTraversalPolicy.
JSpinners will drive you little crazy in FocusTraversalPolicy.getComponentAfter, because you don’t get passed the address of the JSpinner, but rather an associated JFormattedTextField that helps implement it. I consider this a bug. You have to use Jspinner.getEditor.getComponent(0) in FocusTraversalPolicy.getComponentAfter method.
Prior to 1.4 there were over a hundred outstanding bugs in focus handling. You had to use isManagingFocus( false ).
I hope this is too obvious to mention, but when you have a modal dialog up, no other window can have focus.

Sample Code

Here is how to write a simple Custom FocusTraversalPolicy in

Enabing Tab and Shift-Tab

Focus Box

Swing paints a thin box on a button if it has focus. You can suppress that with:

theButton.setFocusPainted( false );

Learning More

Oracle’s Javadoc on Focus Spec : available:

on the web at Oracle.com
in the current JDK 1.7.0_04 or in the old JDK 1.6.0_32 on your local Windows J: drive.

Oracle’s Product Info on Focus Traversal

PDF presentation
Tutorial

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