Java Type Hierarchy in Eclipse shows "Source not found"

I am finding Java classes by using ctrl + shift + t, I click on the class I wish to see (example Scanner or Random in java.Utils) the type hierarchy for and it opens a tab that says, "Source not found." I read an answer that said my build path was not correct, but my programs uses these with no problem. Can I fix this so I can start to study type hierarchy or am I doing something wrong? Page after I click the class in menu