There's another solution that also works. I found it on this forum:

1. Go to the Build Path settings in the project properties.
2. Remove the JRE System Library
3. Add it back; Select "Add Library" and select the JRE System Library. The default worked for me.

This works because you have multiple classes in different jar files. Removing and re-adding the JRE lib will make the right classes be first. If you want a fundamental solution make sure you exclude the jar files with the same classes.