title: Downgrade java version in Eclipse
body: <p>I should downgrade my Java version in Eclipse. Indeed, I have Java10 and I should use java7. I am trying to change the settings in "Window -> Preferences -> Java -> Compiler" or in the project properties -> Java Compiler, but Eclipse does not underline String.join for example (String.join is available from Java8)</p>
  
  <p>Here are some screenshots:</p>
  
  <p>Window -> Preferences -> Java -> Compiler
  <a href="https://i.stack.imgur.com/u13k3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/u13k3.png" alt="enter image description here"></a></p>
  
  <p>Project Properties -> Java Compiler</p>
  
  <p><a href="https://i.stack.imgur.com/U7e1U.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/U7e1U.png" alt="enter image description here"></a>
  Any idea how to solve this issue ?</p>