Libraries are commonly used to increase productivity. As most software systems, they evolve over time and changes are required. However, this process may involve breaking compatibility with previous versions, leading clients to fail. In this context, it is important that libraries creators and clients frequently assess API stability in order to better support their maintenance practices. In this paper, we introduce APIDIFF, a tool to identify API breaking and non-breaking changes between two versions of a Java library. The tool detects changes on three API elements: types, methods, and fields. We also report usage scenarios of APIDIFF with four real-world Java libraries.