Why can't I define these two constructors in Java for a single class?

Question

I am defining a constructor for a class, and I have these two definitions:

MyClass(Set<ClassA> setOfA) { ... }

MyClass(Set<ClassB> setOfB) { ... }

I get the following error:

MyClass(java.util.Set<ClassA>) is already defined in MyClass
    MyClass(Set<ClassB> setOfB)

If I specifically made one of them a HashSet instead of a Set, the code compiles. Why?

Answer 1

If you have

MyClass(Set<A> setOfA) { ... }

MyClass(Set<B> setOfB) { ... }

Type erasure turns them into:

MyClass(Set setOfA) { ... }

MyClass(Set setOfB) { ... }

So now they're the same, and the compiler is confused.

However, if one of them were a HashSet , you end up with this:

MyClass(Set setOfA) { ... }

MyClass(HashSet setOfB) { ... }

And now they're sufficiently different for the compiler to determine which to bind at compile time.

Answer 2

Because of type erasure , the Java compiler will (according to Java docs):

• Replace all type parameters in generic types with their bounds or Object if the type parameters are unbounded...
• Insert type casts if necessary to preserve type safety.
• Generate bridge methods to preserve polymorphism in extended generic types.

In other words, the Java compiler will convert the code to :

MyClass(Set setOfA) { ... }

MyClass(Set setOfB) { ... }

so both will have the same parameters. That's the cause of the error.

Answer 3

The problem here is what is called "type erasure". Boiled down, this means that generic types don't actually exist once the code is compiled, so your two constructor signatures MyClass(Set<A>) and MyClass(Set<B>) both look like MyClass(Set ).

The relevant description from Oracle can be found here: http://docs.oracle.com/javase/tutorial/java/generics/erasure.html