Odd scenario where type inference doesn't work

Question

I've come across an odd scenario where type inference doesn't work and instead I'm greeted with a message:
incompatible types
found : java.lang.Object
required: java.util.Set
Here's the scenario (built with JDK6, javac):
public class GenericTest {
     public static void test() {
          Set testList = GenericTest.method1(new Class[] { ArrayList.class });
     public static <I> I method1(Class<List>[] params) {
          return null;
}Yet when I remove the type information from the method1 params, the class builds.
public class GenericTest {
     public static void test() {
          Set testList = GenericTest.method1(new Class[] { ArrayList.class });
     public static <I> I method1(Class[] params) {
          return null;
}Why would removing the type information from the parameter have any effect whatsoever on the return type?

Answer 1

ilmaestro wrote:
Though casting the parameter as you mentioned resolves the error (because it eliminates the generic), it still doesn't explain why changes to the parameter effect an error message having to do with the return type.It is due the call to the method with the "erased" type Class[] (not a Class<?>[], for example) and a return type resolution. Unchecked conversion (from the raw type to the parametrized type) is applied to the method argument and the return type of the method is determined as an erasure of the declared return type (JLS 3rd edition, [Method Result and Throws Types|http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.12.2.6], first bullet, second sub-bullet). Erasure of the return type is a java.lang.Object, which is incompatible with the java.util.Set.
In the second example, method parameter is already a raw type, and unchecked conversion is not required. The return type of the method is I and is correctly inferred to the java.util.Set.
If the method is called with a non-raw type, then return types are inferred correclty. For example, following code compiles fine.
public class GenericTest {
  public static void test() {
    Set testList = GenericTest.method1((Class<List>[]) (new Class[] { ArrayList.class }));
  public static <I> I method1(Class<List>[] params) {
    return null;
}This example is not "correct" due to the cast to Class<List>[], because such array can't contain Class<ArrayList> as an element (Class<ArrayList> is not a child of Class<List>). And this makes definition Class<List>[] almost useless, because such array should contain only List.class values.