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 {
      String glom(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.