jresult

Result.java

import java.util.function.Function;

public abstract class Result<T> {

    public static <U> Result<U> good(U v) {
        return new Good<U>(v);
    }

    public static <U> Result<U> bad(String message) {
        return new Bad<U>(message);
    }

    public abstract <U> Result<U> then(Function<T, Result<U>> f);
    public abstract <U> U test(Function<T, U> ifGood, Function<String, U> ifBad);

    static final class Good<T> extends Result<T> {
        public final T Value;

        public Good(T v) {
            Value = v;
        }

        public <U> Result<U> then(Function<T, Result<U>> f) {
            return f.apply(Value);
        }

        public <U> U test(Function<T, U> ifGood, Function<String, U> ifBad) {
            return ifGood.apply(Value);
        }

        public String toString() {
            return "(Good, " + Value.toString() + ")";
        }
    }

    static final class Bad<T> extends Result<T> {
        public final String Message;

        public Bad(String message) {
            Message = message;
        }

        public <U> Result<U> then(Function<T, Result<U>> f) {
            return new Bad<U>(Message);
        }

        public <U> U test(Function<T, U> ifGood, Function<String, U> ifBad) {
            return ifBad.apply(Message);
        }


        public String toString() {
            return "(Bad, " + Message + ")";
        }
    }

}

ResultTest.java

import org.junit.Test;
import static org.junit.Assert.*;

public class LibraryTest {
    @Test public void testGood() {
        Result<String> t = Result
            .good("Hello")
            .then(m -> Result.good(m + " There!"));

        Boolean result = t.test(v -> true, msg -> false);
        assertTrue(result);
    }

    @Test public void testBad() {
        Result<String> t = Result
            .bad("Hello")
            .then(m -> Result.good(m + " There!"));

        Boolean result = t.test(v -> true, msg -> false);
        assertFalse(result);
    }
}