title: Android studio: Warning while I try to move Class to another package
body: <p>This is an error msg.</p>
  
  <pre><code>Method Object.toString(), 
  referenced in method SettingActivity.saveDataButtons(), 
  will not be accessible in module personal-health-assistant back up 29 oct  
  
  Method String.trim(), 
  referenced in method SettingActivity.setNullCurrentFocusedEditText(), 
  will not be accessible in module personal-health-assistant back up 29 oct  
  
  Method String.length(), 
  referenced in method SettingActivity.setNullCurrentFocusedEditText(), 
  will not be accessible in module personal-health-assistant back up 29 oct
  </code></pre>
  
  <p>I want to know </p>
  
  <ol>
  <li>How can this happen? Why are some methods cannot will not be accessible in module?</li>
  <li>Is it matter?</li>
  <li>How to solve this warning?</li>
  </ol>