Swing/MnemonicToolTip のバックアップ(No.1)

class MnemonicToolTip extends JToolTip {
  private final JLabel mnemonicLabel = new JLabel();
  public MnemonicToolTip() {
    setLayout(new BorderLayout());
    mnemonicLabel.setForeground(Color.GRAY);
    mnemonicLabel.setBorder(BorderFactory.createEmptyBorder(0,2,0,2));
    add(mnemonicLabel, BorderLayout.EAST);
  }
  @Override public Dimension getPreferredSize() {
    Dimension d = super.getPreferredSize();
    if(mnemonicLabel.isVisible()) {
      d.width += mnemonicLabel.getPreferredSize().width;
    }
    return d;
  }
  @Override public void setComponent(JComponent c) {
    if(c instanceof AbstractButton) {
      AbstractButton b = (AbstractButton)c;
      int mnemonic = b.getMnemonic();
      if(mnemonic>0) {
        mnemonicLabel.setVisible(true);
        mnemonicLabel.setText("Alt+"+KeyEvent.getKeyText(mnemonic));
      }else{
        mnemonicLabel.setVisible(false);
      }
    }
    super.setComponent(c);
  }
}