Is this code sample really returning the right parameterized types?

Question

I've read the Beautiful code with Google Collections, Guava and static imports article about Java collections, and the following snippet got my attention:

Map<String, Map<Long, List<String>>> map = Maps.newHashMap();

The thing is, I don't understand how it's possible that the newHashMap method can return a Map<String,Map<Long, List<String>>>. How did they write this code? When did it became possible? I lived under the impression that you need to explicitly declare the generics parameters in the constructor call.

Answer 1

Look at the source. All this method does is provides a nice shortcut:

public static <K, V> HashMap<K, V> newHashMap() {
  return new HashMap<K, V>();
}

This will work for any arguments because HashMap itself is generic.

Answer 2

You can declare generic parameters on a method call, not just a whole class. If you look at the source for that method:

public static <K, V> HashMap<K, V> newHashMap() {
    return new HashMap<K, V>();
}

You'll see it declares two generic parameters, K and V and declares the result to be a Map. The compiler is smart enough to figure out K and V from the LHS expression. In your case, K is String and V is Map<Long, List<String>>