What does the dot operator `.` (before the generic parameter) mean?

Question

I saw this code today:

    ImmutableMap<Class<? extends ClientCommand>, CommandProcessorInterface> immutableMap =
            ImmutableMap.<Class<? extends  ClientCommand>, CommandProcessorInterface>of(...

What does this syntax mean?

ImmutableMap.<Class ..

I knew generics was right after the class name. No?

What is the difference between:

ImmutableMap<Class.. and ImmutableMap.<Class..

Answer 1

It means you're invoking a generic static method, called of in the ImmutableMap class.

It's pretty much the same as you're invoking a static method, nested in some class:

SomeClass.staticMethod();

For the cases when your method has a type-parameter defined, you can explicitly provide the generic type and this is done like this:

SomeClass.<Type>genericStaticMethod();

And to answer you final question:

What is the difference between ImmutableMap<Class...> and ImmutableMap.<Class... ?

The first is usually used when creating an instance of a generic class. It's used to define the generic-type on class level, while the second is used to invoke a generic static method that's nested in some class.

Answer 2

It concerns a static method ImmutableMap.of. This has its own generic type parameters.

class Test {

static <T> T f() { return null; }

void t() {
    String s = Test.f();                 // Inferred from LHS.
    String t = Test.<String>f();         // Not needed.
    int n = Test.<String>f().length();   // Needed.
}

In your case it seems not really needed, but there I am on thin ice, as the generic type inference became a bit stronger in Java 8.