Why Does Java Restrict Methods with Identical Generic Type Erasure?

Method Erasure Ambiguity: Why Java Restricts Similar Method Signatures

The Java compiler throws the error "Method add(Set) has the same erasure add(Set) as another method in type Test" when two methods with identical generic type erasure are declared in the same class:

class Test {
    void add(Set<Integer> ii) {}
    void add(Set<String> ss) {}
}

This restriction stems from a desire to avoid conflicts in legacy code that uses raw types. Before generics, methods received and returned raw types, such as:

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

When extending such a class after introducing generics, the language designers deemed raw types "override-equivalent" to generic types. This allowed for overriding methods without changing their signatures, ensuring backward compatibility.

However, if multiple methods with different generic signatures were allowed, ambiguity could arise in cases where raw types are still used. For instance, if a class adds a generic method to an existing raw method overriding a superclass method, the compiler would not be able to resolve which method to invoke.

To eliminate this potential confusion, Java prohibits classes from having multiple methods with the same erasure, preventing the introduction of such ambiguities.

While this rule may seem inconvenient in scenarios like passing different generic set types, it is a necessary safeguard for maintaining compatibility with legacy code. Without it, upgrading code to use generics could break existing code that relies on raw types.

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