java either validation

package man;

import static com.xxx.fp.Either.left;
import static com.xxx.fp.Either.right;
import static java.lang.String.format;

import com.xxx.fp.Either;
import java.util.function.Function;

public class Main {

    final static Function<String, Either<String, String>> notEmpty =
            s -> s.isEmpty() ? left("must not be empty") : right(s);

    static class User {
        String name;
        Integer age;

        public User(Integer age, String name) {
            this.age = age;
            this.name = name;
        }

        public Integer getAge() {
            return age;
        }

        public String getName() {
            return name;
        }
    }

    static Function<Integer, Either<String, Integer>> greaterThan(final Integer x) {
        return n -> n > x ? right(n) : left(format("should be greater than %d", x));
    }

    static Function<Integer, Either<String, Integer>> lessThan(final Integer x) {
        return n -> n < x ? right(n) : left(format("should be less than %d", x));
    }

    static <T, K> Function<T, Either<String, T>> using(final Function<T, K> l, Function<K, Either<String, K>> k) {
        return t -> k.apply(l.apply(t)).map(ignore -> t);
    }

    public void main(final String... args) {

        final Either<String, User> safeUser =
                Either.<String, User>right(new User(44, "Peter"))
                        .bind(using(User::getAge, greaterThan(18)))
                        .bind(using(User::getAge, lessThan(18)))
                        .bind(using(User::getName, notEmpty));

    }
}