[undertow]

Code:

    Button b = new Button("Hello"); 

    b.addActionListener(new ActionListener()
    {
         public void actionPerformed(ActionEvent e)
         {
             //something in response to click
         }
    });

    Frame f = new Frame("Java Frame"); 
    f.add(b, BorderLayout.NORTH ); 
    f.pack(); 
    f.show();

I would also suggest looking into SUN's documentation on actionListener interface and the Action Object.