title: Eclipse Oxygen - The project was not built since its build path is incomplete
body: <p>I just installed Eclipse Oxygen and tried to open an existing project into the workbench but I get this error:</p>
  
  <blockquote>
    <p>The project was not built since its build path is incomplete. Cannot
    find the class file for java.lang.Object. Fix the build path then try
    building this project</p>
  </blockquote>
  
  <p>I tried right clicking on the project - went to Properties - Java Build Path - Libraries - Add Library - JRE System Library and selected Workbench default JRE (jre1.8.0_60).</p>
  
  <p>I then cleaned and rebuilt the project but it then causes more errors to come up like:</p>
  
  <pre><code>ActionBar cannot be resolved to a type
  Activity cannot be resolved to a variable
  AdapterView cannot be resolved to a type
  ArrayAdapter cannot be resolved to a type
  </code></pre>