The project associated with the warning has a specific execution environment specified on the build path. An execution environment specifies what runtime is required to run the project. The current jre library picked for the project isn't strictly compatible with the specified runtime. I believe that CDC is a specification for connected devices that use a subset of the standard jre. Your workspace probably has an actual jre/jdk installed. Since the CDC would be a subset of the standard, its possible that you would use things that aren't compatible with the target runtime.