Question

Here’s a question, this first code listing compiles just fine (JDK 1.6 | JDK 1.7):

ArrayList<String> a = new ArrayList<String>();
String[] s = a.toArray(new String[0]);

However, if I declare the List reference as a raw type:

ArrayList a = new ArrayList();
String[] s = a.toArray(new String[0]);

I get a compiler error saying the String[] is required but Object[] was found.

This means my compiler is interpreting the generic method as returning Object[] despite of receiving a String[] as its argument.

I doubled-checked the toArray(myArray) method signature:

<T> T[] toArray(T[] a);

Therefore it is a parameterized method whose type parameter <T> has no relation whatsoever with that of the List (i.e. <E>).

I have no idea how using a raw type here affects the evaluation of parameterized methods using independent type parameters.

• Does anyone has any idea why this code does not compile?
• Does anybody knows any reference where this behavior is documented?

Answer 1

It’s not exactly what you’d expect, but if you refer to a generic class in raw form, you lose the ability to use generics in any way for instance members. It’s not restricted to generic methods either, check out this:

 public class MyContainer<T> {

     public List<String> strings() {
         return Arrays.asList("a", "b");
     }
 }

 MyContainer container = new MyContainer<Integer>();
 List<String> strings = container.strings(); //gives unchecked warning!

This is the relevant part of the JLS (4.8):

The type of a constructor (§8.8), instance method (§8.4, §9.4), or non-static field (§8.3) M of a raw type C that is not inherited from its superclasses or superinterfaces is the raw type that corresponds to the erasure of its type in the generic declaration corresponding to C.