title: cannot resolve symbol when read source code with android studio
body: <p>I try to read source code with android studio, and there are many <b>cannot resolve symbol</b> errors in it. Those errors include unsolved class(for example <code>android.util.MathUtils</code>, <code>android.annotation.NonNull</code> and <code>com.google.android.collect.Lists</code>) and variables(even I could see some of variable is defined in source). </p>
  
  <p>To fix this issue, i import necessary dependencies in <i>build.gradle</i>, but it's useless.</p>
  
  <p>what should i do to fix this issue.</p>