Eclipse error: The type java.lang.Object cannot be resolved. It is indirectly referenced from the required .class files

This error popped up the other day when I was copying projects from an old workspace into a new, clean Eclipse workspace. It turned out that the reason the base class java.lang.Object could not be found is because the new workspace did not have Eclipse variables set, and they were referenced by the project in the old workspace. So in the project properties Build Path JRE path. Once I created the Eclipse classpath variables and rebuilt the project, the errors went away.