Tested w/ SwingSet2.

With Swing components, whether or not an underline which means a mnemonic is dispayed is alternated by ALT key. In most components, the behavior is same as CapsLock key to alternate lowercase characters and uppercase characters. Once the key is pressed, the condition is not changed until the key is pressed again.

However, in ColorChooser,
  - Underlines for tab titles("Swatches", "HSB", "RGB") are never hidden. They are always displayed.
  - In RGB tab, underlines are displayed only while ALT key is pressed.
In FileChooser,
  - Underlines are displayed only while ALT key is pressed.

I guess all components should support the alternation in the same way.