title: debug jdk source can't watch variable what it is
body: <p>I'm debugging the JDK source like:</p>
  
  <pre><code> public static int codePointAt(CharSequence seq, int index) {
          char c1 = seq.charAt(index++);
          if (isHighSurrogate(c1)) {
              if (index &lt; seq.length()) {
                  char c2 = seq.charAt(index);
                  if (isLowSurrogate(c2)) {
                      return toCodePoint(c1, c2);
                  }
              }
          }
          return c1;
      }
  </code></pre>
  
  <p>and I want to see <code>c1</code> variable before I step into <code>if (isHighSurrogate(c1))</code>.
  However, when I debug watch <code>c1</code> variable
  it display :</p>
  
  <p><img src="https://i.stack.imgur.com/vft2x.png" alt="enter image description here"></p>
  
  <p>I really have tried added rt.jar source, and it really can step into breakpoint of JDK source, like:
  <img src="https://i.stack.imgur.com/moD7P.png" alt="enter image description here"></p>
  
  <p>but why <code>c1</code> variable can't display?</p>