title: Setting JDK in Eclipse
body: <p>I have two JDKs, for Java 6 and 7.</p>
  
  <p>I want to build my project using both. Initially we only built against 1.6.
  I see in my project setting I can select 1.5, 1.6 1.7 as the compiler level.</p>
  
  <p>How are these options added to the IDE? I never installed Java 1.5.
  Suppose I wanted Java 1.4. How would I get this to appear in the list? I have added two different JREs in my IDE preferences, but these are not what appears in the dropdown.</p>