I dont use eclipse, but this solves one of the most common problems.
You make a project, You make a class, and then you try running it, and it says something like:
Unbound classpath container: 'JRE System Library [JavaSE-20]' in project Build path Build Path Problem
This is how its fixed:
https://stackoverflow.com/questions/22921800/unbound-classpath-container-jre-system-library-java-se-6-1-6-0-65-b14-462
Project > Properties > Java Path Build
Java Build Path > Libraries > JavaSE-? > Edit…
Select Alternate JRE
Now you can run your programs
