title: "This compilation unit is not on the build path of a java project" -Eclipse
body: <p>I am new to the eclipse environment. I clone my project from the repository(Hg) by 
  New->Project->Mercurial->Clone existing project. Here my problem is "This compilation unit is not on the build path of a java project" message comes when I editing the coding. Auto completion not working. Kindly let me know how solve this problem... </p>
  
  <p>Note:
   I searched in google as well as in stackoverflow. But still I am not clear in that.  </p>