Why Does Java Restrict Method Overloading with Override-Equivalent Methods?

Method Overloading: Java's Restriction on Override-Equivalent Methods

Java prohibits the presence of two methods within a class with identical erasure despite different parameter types. This constraint stems from the language's effort to maintain compatibility with legacy code utilizing raw types.

The compilation error arises when two methods with varying parameter types, such as add(Set) and add(Set), attempt to coexist within the same class. Java considers these methods to have the same "erasure," which refers to the method signature after generic type parameters are eliminated during compilation.

This restriction exists to avoid conflicts in situations where a class inherits from a superclass using raw types. In such cases, the compiler must treat raw types as override-equivalent to generified types to ensure proper method overriding.

For example, consider a legacy class CollectionConverter with the method toList(Collection c) using raw types. If you extend this class and add a new method toList(Collection c), both methods would be override-equivalent and satisfy the override requirement.

However, if you later mistakenly add a third method toList(Collection c) to the subclass, the compiler faces ambiguity as it cannot determine which method to override. To resolve this ambiguity, Java enforces a rule against multiple override-equivalent methods.

It is important to note that this restriction is not a limitation of erasure but a design choice made to support compatibility with existing code. With generics added to method identifiers, ensuring uniqueness would have been possible at compile-time without the need for this rule. Nevertheless, Java maintains this restriction for the sake of compatibility.

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