title: Java null pointer exception, when using JButton
body: <pre><code>code 1:
  String s = Character.toString(button[0]);
  bt[0].setText(s);//bt[]: JButton array;, button[] = char array
  code 2:
  String s = Character.toString(button[0]);
  bt[0] = new JButton(s);
  </code></pre>
  
  <p>Suppose, I already initialize bt's by new JButton[num];
  code 1 leads to null pointer exception, but code 2 run well.
  I think bt's already has a pointer value of null. then, i can modify them without restriction. but it isn't.
  I think code 2 initialize 2 times. but, it hasn't to be.</p>