EitherTest.java

package sample.rest.demo.either;


import org.junit.Test;
import sun.reflect.generics.reflectiveObjects.NotImplementedException;

import java.util.ArrayList;
import java.util.List;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertTrue;

interface Either<L,R> {
    default boolean isLeft() {
        return false;
    }


    default boolean isRight() {
        return false;
    }

    default R get() {
        throw new NotImplementedException();
    }

    default L getLeft() {
        throw new NotImplementedException();
    }

    static <L,R> Either<L, R> left(L content){
        return new Left<>(content);
    }

    static <L,R> Either<L, R> right(R content){
        return new Right<>(content);
    }
}


class Left<L, R> implements Either<L, R> {

    private final L content;

    @Override
    public boolean isLeft() {
        return true;
    }

    Left(L content) {
        this.content = content;
    }

    @Override
    public L getLeft() {
        return content;
    }
}

class Right<L,R> implements Either<L, R> {

    private final R content;

    @Override
    public boolean isLeft() {
        return false;
    }

    @Override
    public boolean isRight() {
        return true;
    }

    Right(R content){

        this.content = content;
    }

    @Override
    public R get() {
        return content;
    }
}


public class EitherTest {


    @Test
    public void shouldHaveOnlyOneValeInEither(){

        double number = Math.random() * 10.0;

        int value = Double.valueOf(number).intValue();

        Either<List<Exception>, String> content = random(value);


        assertTrue(content.isLeft());
        assertEquals(1, content.getLeft().size());
        assertEquals(IllegalArgumentException.class, content.getLeft().get(0).getClass());



    }


    public Either<List<Exception>, String> random(int value){


        if (value % 2 == 0) {
            List<Exception> errors = new ArrayList<>();
            errors.add(new IllegalArgumentException());
            return Either.left(errors);
        }

        return Either.right(String.valueOf(value));

    }


}