It becomes hard when the app uses reflection. When you want to remove some method foo, you not only have to make sure that it isn't referenced by some code, but

It becomes hard when the app uses reflection. When you want to remove some method foo, you not only have to make sure that it isn't referenced by some code, but also make sure that no code is passing the string "foo" to reflection API. This may be hard for many apps and is undecidable in general. Actually, even in plain, reflection-less java it may be quite complicated because subclass methods can be invoked by superclass-typed variables. For every method invocation you need to find all possible subclasses whose objects can be assigned to variable on which the method is invoked. You need to know which subclasses are ever passed as superclass/interface arguments to methods. You need to know which subclasses are assigned to superclass/interface member/static variables. You need to know which subclasses are stored in superclass collections. And so on.