title: replace a button with another under android 4.0
body: <p>I have a "<code>start</code>" button on my content view, i want to replace it with a "<code>stop</code>" button when clicking on it.</p>
  
  <p>i have already defined a "<code>start</code>" and a "<code>stop</code>" button in <code>xml</code> , what should i do next? thanks a lot!</p>