Java's interfaces have some limitations, including:

• No code re-use.
• No run-time decoration or class annotation (to my knowledge) that allows you to say that a specific class implements an interface.
• No shared namespace between class names and interface names, so if you want to substitute a class of a different name where library code does not use an interface, you cannot.

The first is the most annoying, though if you're using Java without a code generator, you've asked for it. The third is the most serious.