The development of context-aware systems requires dynamic adaptation that challenges state-of-the-art programming language support. Context-oriented programming (COP) provides dedicated abstractions for first-class representation of context-dependent behavior. So far, COP has been implemented for dynamically-typed languages such as Lisp, Smalltalk, Python, Ruby, and JavaScript relying on reflection mechanisms, and for the statically-typed programming language Java based on libraries and pre-processors. ContextJ is our compiler-based COP implementation for Java that properly integrates COP's layer concept into the Java type system. In this paper, we introduce ContextJ's language constructs, semantics, and implementation. We present a case-study of a ContextJ-based desktop application.