what does first <T> in "<T> Future<T> submit(Callable<T> task)" stands for?

Question

I'm new to Java and is trying to learn the concept of return type. I was reading Java API for the interface ExecutorService:

I know for submit(Callable<T> task), it returns a Future object of T type, however, there is another <T> in front of Future<T>, what does it represent?

Answer 1

The leading <T> is defining T as a type parameter (aka generics), in simple words, the function uses a generic type parameter called T where T is a type (at the time of writing this (Java 1.8), only reference types are supported)

<T> void fn(T t); // T is a type parameter

For example, Integer could be concrete type

<Integer>fn(1);

The compiler can often infer the type as follows

fn(1)

In your example, submitting a Callable<Integer> returns a Future<Integer>

Future<Integer> future = executor.submit(callableInteger);
Integer futureResult = future.get(); //type-saftey powered by generics 

You can check this out for a detailed explanation.

Answer 2

It's declaring a type variable that has scope only for the submit() method.

An Executor is not generic, but some of its methods use generic types, which are declared between the method modifiers and the return type.