$ is a legal character in Java identifiers, however using it tends to cause nearby regions to get highlighted incorrectly.

On Github.com:

interface Test {
  void aaaaa();
  void bbbbb();
  void ccccc$ddddd();
  void eeeee();
  void $$$$$();
}

Image:

In the wild: https://github.com/quat1024/apathy/blob/b2114a8bad81e9f7662513e38892af43901e840a/src/main/java/agency/highlysuspect/apathy/mixin/MinecraftServerAccessor.java