Java Generics bug on methods? [duplicate]

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.

Answer 2

When you don't use generics compiler treats it as a raw type and hence every generic type becomes Object and so you cannot pass String[] because it needs Object[]
So here is the deal - If you use

List l = new ArrayList<String>();

You are using raw type and all its instance members are replaced by its erasure counterparts. In particular each parameterized type appearing in an instance method declaration is replaced with its raw counterpart. See JLS 4.8 for details.