title: Compilation unit is not on the build path of a Java project
body: <p>I joined an SVN server and checked out the source files a project from that server. In the Eclipse editors, when I hit <code>Ctrl + Space</code>, I got the following error message:</p>
  
  <p><img src="https://i.stack.imgur.com/aFU9R.png" alt="This compilation unit is not on the build path of a Java project"></p>
  
  <blockquote>
    <p>This compilation unit is not on the build path of a Java project</p>
  </blockquote>
  
  <p>What should I do to solve this problem?</p>