Method has the same erasure as another method in type

This rule is intended to avoid conflicts in legacy code that still uses raw types.

Here's an illustration of why this was not allowed, drawn from the JLS. Suppose, before generics were introduced to Java, I wrote some code like this:

class CollectionConverter {
  List toList(Collection c) {...}
}

You extend my class, like this:

class Overrider extends CollectionConverter{
  List toList(Collection c) {...}
}

After the introduction of generics, I decided to update my library.

class CollectionConverter {
  <T> List<T> toList(Collection<T> c) {...}
}

You aren't ready to make any updates, so you leave your Overrider class alone. In order to correctly override the toList() method, the language designers decided that a raw type was "override-equivalent" to any generified type. This means that although your method signature is no longer formally equal to my superclass' signature, your method still overrides.

Now, time passes and you decide you are ready to update your class. But you screw up a little, and instead of editing the existing, raw toList() method, you add a new method like this:

class Overrider extends CollectionConverter {
  @Override
  List toList(Collection c) {...}
  @Override
  <T> List<T> toList(Collection<T> c) {...}
}

Because of the override equivalence of raw types, both methods are in a valid form to override the toList(Collection<T>) method. But of course, the compiler needs to resolve a single method. To eliminate this ambiguity, classes are not allowed to have multiple methods that are override-equivalent—that is, multiple methods with the same parameter types after erasure.

The key is that this is a language rule designed to maintain compatibility with old code using raw types. It is not a limitation required by the erasure of type parameters; because method resolution occurs at compile-time, adding generic types to the method identifier would have been sufficient.

Java generics uses type erasure. The bit in the angle brackets (<Integer> and <String>) gets removed, so you'd end up with two methods that have an identical signature (the add(Set) you see in the error). That's not allowed because the runtime wouldn't know which to use for each case.

If Java ever gets reified generics, then you could do this, but that's probably unlikely now.

This is because Java Generics are implemented with Type Erasure.

Your methods would be translated, at compile time, to something like:

Method resolution occurs at compile time and doesn't consider type parameters. (see erickson's answer)

void add(Set ii);
void add(Set ss);

Both methods have the same signature without the type parameters, hence the error.