Java generics type erasure of method parameters

Question

I got this one from a google I/O puzzler talk given by Joshua Bloch. Here's the code

 public class Glommer<T> {
      String glom(Collection<?> obj){
         String result = "";
         for(Object o : obj){
              result += o;
         }
         return result;
      }

      int glom(List<Integer> ints){
           int result = 0;
           for(int i : ints){
                result += i;
           }
           return result;
       }

      public static void main(String args[]){
           List<String> strings = Arrays.asList("1", "2", "3");
           System.out.println(new Glommer().glom(strings));
      }

this main method throws an exception because new Glommer is a raw type and hence all the generics in Glommer is erased, so it ends up calling int glom(List<Integer> ints) rather than String glom(Collection<?> obj).

My question is, even if I called glom() as new Glommer<Integer>().glom(strings) shouldn't it call the int glom(List<Integer> ints) method since due to type erasure, this method is effectively int glom(List ints) and strings is of type List not Collection?

Answer 1

The called method is defined at compilation time, not at runtime.

If you add a parameter to your constructor call, the compiler will have enough information to know that it has to call the first method. Otherwise, it's just as if generics didn't exist. In both case, the called method will always stay the same at runtime.

EDIT Some people seem to doubt, so here's another example:

public class Test {

    private static void test(Object object) {
        System.out.println("Object method");
    }

    private static void test(Integer integer) {
        System.out.println("Integer method");
    }

    public static void main(String[] args) {
        Object object = Integer.valueOf(0);
        test(object);
    }

}

The result is:

Object method

You pass an Integer to your method, but all that the compiler knows at compile time is that it's an object. The jvm doesn't automagically change the method call even though the Object is actually an Integer.

Answer 2

You can read more about Raw Types to understand it fully

Basically, raw types are for using legacy code, almost anything in a raw class will become raw itself, in this case those 2 methods.

So when it is raw there is a method that gets a List and one for Collection so its called the List one, if its not raw, the methods are not raw also and it will call the Collection one because it has the extra information