Scala / Java语法，返回接口实现

Question

What does this line of code mean in the code at the bottom?

return Promise.<SimpleResult>pure  //why is the dot

I am new to Scala and Java 6, what is the related topic/terminology?

If the framework doesn't find an action method for a request, the onHandlerNotFound operation will be called:

import play.*;
import play.mvc.*;
import play.mvc.Http.*;
import play.libs.F.*;

import static play.mvc.Results.*;

public class Global extends GlobalSettings {

    public Promise<SimpleResult> onHandlerNotFound(RequestHeader request) {
        return Promise.<SimpleResult>pure(notFound(
            views.html.notFoundPage.render(request.uri())
        ));
    }

}

Update :

play.libs.F.Java file

public static class Promise<A> {

    public static <A> Promise<A> pure(final A a) {
        return FPromiseHelper.pure(a);
    }
}

What is <A> and Promise<A> ?

Answer 1

Promise.pure() is a generic method, parameterized with some type T.

Promise.<SimpleResult>pure() calls this method with SimpleResult as the generic type. Most of the time, doing it is optional because the compiler infers the generic type from the arguments of the method or from the type of the variable the result is assigned to, like in

List<String> s = Collections.emptyList();

which is a shortcut for

List<String> s = Collections.<String>emptyList();

or

Set<String> Collections.singleton("hello");

which is a shortcut for

Set<String> Collections.<String>singleton("hello");

Answer 2

Sometimes type inference system can't fully serve its purpose and fails to automatically deduce the return type of the call of the generic method. To specify such a type, this construct exists - you help to infer the correct type of the expression.

Look at JLS for more. ( Formal definition )