title: java string.trim method implement detail
body: <pre><code>public String trim() {
      int len = value.length;
      int st = 0;
      char[] val = value;    /* avoid getfield opcode */
  
      while ((st &lt; len) &amp;&amp; (val[st] &lt;= ' ')) {
          st++;
      }
      while ((st &lt; len) &amp;&amp; (val[len - 1] &lt;= ' ')) {
          len--;
      }
      return ((st &gt; 0) || (len &lt; value.length)) ? substring(st, len) : this;
  }
  </code></pre>
  
  <ol>
  <li>why <code>val[st] &lt;= ' '</code>,not <code>val[st] == ''</code>.</li>
  <li>what is <code>(st &gt; 0) || (len &lt; value.length)</code> meaning.</li>
  </ol>