Java 7 generics type inference fails

Question

I have the following method:

protected <T> Func1<ApiResponse<T>, Observable<T>> checkApiSuccess() {
    return new Func1<ApiResponse<T>, Observable<T>>() {
        @Override
        public Observable<T> call(ApiResponse<T> response) {
            if (response.isSuccess()) {
                return Observable.from(response.getResult());
            }

            return Observable.error(new ApiException(response.getError()));
        }
    };
}

Why does this work:

Func1<ApiResponse<LoginResult>, Observable<LoginResult>>
        checkApiSuccessFunc = checkApiSuccess();

return apiService
        .login(params)
        .flatMap(checkApiSuccessFunc);

but this doesn't:

return apiService
        .login(params)
        .flatMap(checkApiSuccess());

I get the compile-time error no suitable method found for flatMap(Func1<ApiResponse<Object>>, Observable<Object>). Somehow when using the method directly the type T can't be mapped to LoginResult anymore.

Answer 1

The type inference probably can't determine that the T should be a LoginResult. It may work with Java 8 where the type inference system has been improved.

You could try using a target type to help the compiler:

flatMap(this.<LoginResult> checkApiSuccess())

Answer 2

In

Func1<ApiResponse<LoginResult>, Observable<LoginResult>>
        checkApiSuccessFunc = checkApiSuccess();

return apiService
        .login(params)
        .flatMap(checkApiSuccessFunc);

The compiler infers from the declared type of checkApiSuccessFunc what the return type of checkAPISuccess() is, and from this infers everything inside it.

But in

return apiService
        .login(params)
        .flatMap(checkApiSuccess());

There is really not enough information to infer the type of checkApiSuccess(). flatMap expects the function to take an <? super T> parameter. So it expects something which is a superclass of whatever login emits, and since you have nothing that limits it, it infers Object. You haven't mentioned what the signature function that this return is called for is.