To compare two files in Eclipse, first select them in the Project Explorer / Package Explorer / Navigator with control-click. Now right-click on one of the files, and the following context menu will appear. Select Compare With / Each Other.

i really like the AnyEdit Plugin (http://andrei.gmxhome.de/anyedit/). It provides some adittional "Compare With" Functions:

• Compare / Replace with Clipboard Editors, Navigator, Project Explorer If some text is selected in editor, then only this selection will be compared, otherwise entire editor

• Compare / Replace with Workspace File... Editors, Navigator, Project Explorer If some text is selected in editor, then only this selection will be compared, otherwise entire editor

• Compare / Replace with External File... Editors, Navigator, Project Explorer If some text is selected in editor, then only this selection will be compared, otherwise entire editor

• Compare / Replace with Opened Editor...

Works very good to me.

I'm using Eclipse Kepler, so depending on your version your mileage may vary.

To compare two methods in the same class, do the following:

1. Open the class.
2. Show the Outline view (Window->Show View->Outline)
3. Hold CTRL and click on the two methods you wish to compare.
4. Right click on one of the highlighted methods.
5. Click on "Compare With->Each Other By Member"

You'll get a diff window similar to if you were comparing two files.

Update

Since answering this question, I have found a similar, but more powerful way to do this. Follow the steps above, except use the "Package Explorer" view rather than the outline view. The advantage of this is that you can choose to compare two methods that are in the same class or compare two methods that are in different classes. It's a great way to prove that two methods or subclasses are identical and can be refactored into a common shared resource.