title: Why the append method for StringBuilder class work in one package but not the another package in the same project?
body: <p>I have created two packages in the same project, and I tried to use StringBuilder in both packages.</p>
  
  <p>However, when I try to use append method. It only works in the first package. </p>
  
  <p>In the second package, it always gives me the error "The method append(String) is undefined for the type StringBuilder".</p>
  
  <p>Why it would be like this? </p>
  
  <pre><code>'String s1 = "iPhone?";
  String s2 = "what?";
  StringBuilder bobTheBuilder = new StringBuilder();
  bobTheBuilder.append(s1);'
  </code></pre>
  
  <p>The code is exactly the same in two files, except that the 'append' method does not work in the second package.</p>