Why can't I explicitly pass the type argument to a generic Java method?

Question

When the java compiler cannot infer the parameter type by itself for a static method, you can always pass it using the full qualified method name: Class . < Type > method();

Object list = Collections.<String> emptyList();

You can, if you pass in the type as a method parameter.

static <T> List<T> createEmptyList( Class<T> type ) {
  return new ArrayList<T>();
}

@Test
public void createStringList() {
  List<String> stringList = createEmptyList( String.class );
}

Methods cannot be genericised in the same way that a type can, so the only option for a method with a dynamically-typed generic return type -- phew that's a mouthful :-) -- is to pass in the type as an argument.

For a truly excellent FAQ on Java generics, see Angelika Langer's generics FAQ.

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Follow-up:

It wouldn't make sense in this context to use the array argument as in Collection.toArray( T[] ). The only reason an array is used there is because the same (pre-allocated) array is used to contain the results (if the array is large enough to fit them all in). This saves on allocating a new array at run-time all the time.

However, for the purposes of education, if you did want to use the array typing, the syntax is very similar:

static <T> List<T> createEmptyList( T[] array ) {
  return new ArrayList<T>();
}

@Test
public void testThing() {
  List<Integer> integerList = createEmptyList( new Integer[ 1 ] );
}