Java generics type binding. Why isn't this compiling?

Question

Given this generic function:

<T> List<T> function() { return null; }

Why does this compile

List<String> l = function();

While this does not?

List<String> l = (List<String>) function();

Answer 1

Because when you do a cast like this:

(List<String>) function()

the compiler can't infer the type parameter for the function() call, and falls back on binding T to Object.

While in

List<String> l = function();

it can infer the correct type for T.

Note that you may cast if you circumvent the job of the type inference by explicitly providing the type:

import java.util.List;

class Test {
    public static <T> List<T> function() { return null; }

    public static void main(String[] args) {
        List<String> l = (List<String>) Test.<String>function();
        //                              ^^^^^^^^^^^^^
    }
}

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I don't know the exact type inferencing rules for generic parameters. They are however specified in JLS Section 15.12.2.7.