Personally, I have no idea, but I found this article by spending a few moments googling. It looks promising.
Never ascribe to malice that which can be adequately explained by stupidity.
Arvind Ik Chari
Joined: Dec 02, 2012
I found a solution on stackoverflow-- for reference I am posting that solution below... It also mentions Fred's solution but also says that that solution is more like a crack/hack- and that the recommended way to solve this problem is this--
Go to the Build Path settings in the project properties.
Remove the JRE System Library
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.