Why is one class valid while the other is not?

Question

As you can see, having a non void return type is important.

class TestValid {

public String f(List<String> list) {
    return null;
}

public Integer f(List<Integer> list) {
    return null;
}

public void test() {
    f(Arrays.asList("asdf"));
    f(Arrays.asList(123));
}

}

class TestInvalid {

public void f(List<String> list) {
    System.out.println("strings");
}

public void f(List<Integer> list) {
    System.out.println("numbers");
}

}

Answer 1

After the type erasure part of compilation, List<String> and List<Integer> are both really type List; in TestInvalid you are making two methods with the same runtime signature.

Answer 2

In the second case, the methods f cannot be distinguished at runtime due to type-erasure.

http://java.sun.com/docs/books/tutorial/java/generics/erasure.html

Thus they both have precisely the same signature.

Answer 3

TestValid isn't valid to start with:

TestValid.java:9: name clash: f(List<Integer>) and f(List<String>)
have the same erasure
public Integer f(List<Integer> list) {
               ^

So the return type is a red herring. If return types were taken into account in determining signatures for overloading, then it would be valid, but currently it's as invalid as this:

// Equally invalid
public Integer f() {}
public String f() {}

If you genuinely managed to get TestValid to compile, I'd be interested to know which compiler you're using.

Answer 4

For TestValid class: the functions appear to be overloaded. You wont get a compile time error if the caller passes a parameter List object which is type defined.eg: new ArrayList() or new ArrayList(). as the signature (return type and input parameters) after type erasure is not same. However you will get compile time error if the you pass in new ArrayList().

The second definition violates the basic principle of overloading a function where two functions with same signature (return type and input parameters) after type erasure.

Answer 5

TestValid have been debated at another question