Java Generics return type issue

Question

I have the following method:

public <T extends Result> T execute(Command<T> command)
{
     return new LoginResult();
}

Here, Result is an interface, and the class LoginResult does implement this interface. However, I'm getting the error:

Incompatible types, required: T, found: com.foo.LoginResult

Yet if I change the method signature to:

public Result execute(Command<T> command)

Then the same return line works fine, without any error.

What's the issue here? And how can I return LoginResult from this method?

Edit: The reason I want to use generics, is so I can do something like the following:

Command<LoginResult> login = new Command<>();
 LoginResult result = execute( login );

Answer 1

You can't do this because the compiler can't confirm that LoginResult is of type T, since it's inferred at the call site (i.e. the caller decides which type parameter will be used).