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issue iluwatar#335 Monad pattern introduced
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‎monad/index.md‎

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---
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layout: pattern
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title: Monad
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folder: monad
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permalink: /patterns/monad/
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categories: Presentation Tier
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tags:
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- Java
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- Difficulty-Advanced
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---
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## Intent
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Monad pattern based on monad from linear algebra represents the way of chaining operations
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together step by step. Binding functions can be described as passing one's output to another's input
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basing on the 'same type' contract.
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![alt text](./etc/monad.png "Monad")
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## Applicability
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Use the Monad in any of the following situations
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* when you want to chain operations easily
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* when you want to apply each function regardless of the result of any of them
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## Credits
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* [Design Pattern Reloaded by Remi Forax](https://youtu.be/-k2X7guaArU)
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* [Brian Beckman: Don't fear the Monad](https://channel9.msdn.com/Shows/Going+Deep/Brian-Beckman-Dont-fear-the-Monads)

‎monad/pom.xml‎

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<?xml version="1.0"?>
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<!--
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The MIT License
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Copyright (c) 2014 Ilkka Seppälä
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Permission is hereby granted, free of charge, to any person obtaining a copy
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of this software and associated documentation files (the "Software"), to deal
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in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
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The above copyright notice and this permission notice shall be included in
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all copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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THE SOFTWARE.
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-->
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<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.iluwatar</groupId>
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<artifactId>java-design-patterns</artifactId>
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<version>1.11.0-SNAPSHOT</version>
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</parent>
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<artifactId>monad</artifactId>
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<dependencies>
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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</project>
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‎monad/src/main/java/com/iluwatar/monad/App.java‎

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package com.iluwatar.monad;
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import java.util.Objects;
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public class App {
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/**
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* Program entry point.
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*
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* @param args @param args command line args
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*/
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public static void main(String[] args) {
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User user = new User("user", 24, Sex.FEMALE, "foobar.com");
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System.out.println(Validator.of(user).validate(User::getName, Objects::nonNull, "name is null")
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.validate(User::getName, name -> !name.isEmpty(), "name is empty")
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.validate(User::getEmail, email -> !email.contains("@"), "email doesn't containt '@'")
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.validate(User::getAge, age -> age > 20 && age < 30, "age isn't between...").get().toString());
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}
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}

‎monad/src/main/java/com/iluwatar/monad/Sex.java‎

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package com.iluwatar.monad;
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public enum Sex {
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MALE, FEMALE
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}

‎monad/src/main/java/com/iluwatar/monad/User.java‎

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package com.iluwatar.monad;
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public class User {
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private String name;
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private int age;
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private Sex sex;
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private String email;
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/**
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*
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* @param name - name
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* @param age - age
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* @param sex - sex
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* @param email - email
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*/
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public User(String name, int age, Sex sex, String email) {
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this.name = name;
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this.age = age;
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this.sex = sex;
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this.email = email;
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}
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public String getName() {
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return name;
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}
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public int getAge() {
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return age;
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}
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public Sex getSex() {
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return sex;
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}
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public String getEmail() {
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return email;
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}
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}

‎monad/src/main/java/com/iluwatar/monad/Validator.java‎

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package com.iluwatar.monad;
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import java.util.ArrayList;
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import java.util.List;
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import java.util.Objects;
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import java.util.function.Function;
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import java.util.function.Predicate;
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/**
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* Class representing Monad design pattern. Monad is a way of chaining operations on the
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* given object together step by step. In Validator each step results in either success or
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* failure indicator, giving a way of receiving each of them easily and finally getting
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* validated object or list of exceptions.
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* @param <T> Placeholder for an object.
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*/
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public class Validator<T> {
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private final T t;
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private final List<Throwable> exceptions = new ArrayList<>();
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/**
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* @param t object to be validated
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*/
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private Validator(T t) {
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this.t = t;
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}
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/**
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* Creates validator against given object
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*
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* @param t object to be validated
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* @param <T> object's type
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* @return new instance of a validator
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*/
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public static <T> Validator<T> of(T t) {
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return new Validator<>(Objects.requireNonNull(t));
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}
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/**
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* @param validation one argument boolean-valued function that
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* represents one step of validation. Adds exception to main validation exception
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* list when single step validation ends with failure.
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* @param message error message when object is invalid
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* @return this
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*/
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public Validator<T> validate(Predicate<T> validation, String message) {
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if (!validation.test(t)) {
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exceptions.add(new IllegalStateException(message));
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}
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return this;
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}
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/**
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* Extension for the {@link Validator#validate(Function, Predicate, String)} method,
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* dedicated for objects, that need to be projected before requested validation.
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* @param projection function that gets an objects, and returns projection representing
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* element to be validated.
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* @param validation see {@link Validator#validate(Function, Predicate, String)}
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* @param message see {@link Validator#validate(Function, Predicate, String)}
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* @param <U> see {@link Validator#validate(Function, Predicate, String)}
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* @return this
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*/
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public <U> Validator<T> validate(Function<T, U> projection, Predicate<U> validation,
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String message) {
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return validate(projection.andThen(validation::test)::apply, message);
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}
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/**
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* To receive validated object.
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*
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* @return object that was validated
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* @throws IllegalStateException when any validation step results with failure
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*/
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public T get() throws IllegalStateException {
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if (exceptions.isEmpty()) {
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return t;
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}
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IllegalStateException e = new IllegalStateException();
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exceptions.forEach(e::addSuppressed);
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throw e;
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}
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}

‎monad/src/test/java/com/iluwatar/monad/AppTest.java‎

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package com.iluwatar.monad;
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import org.junit.Test;
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public class AppTest {
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@Test
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public void testMain() {
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String[] args = {};
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App.main(args);
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}
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}

‎monad/src/test/java/com/iluwatar/monad/MonadTest.java‎

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package com.iluwatar.monad;
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import junit.framework.Assert;
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import org.junit.Rule;
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import org.junit.Test;
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import org.junit.rules.ExpectedException;
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import java.util.Objects;
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public class MonadTest {
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@Rule
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public ExpectedException thrown = ExpectedException.none();
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@Test
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public void testForInvalidName() {
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thrown.expect(IllegalStateException.class);
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User tom = new User(null, 21, Sex.MALE, "tom@foo.bar");
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Validator.of(tom).validate(User::getName, Objects::nonNull, "name cannot be null").get();
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}
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@Test
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public void testForInvalidAge() {
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thrown.expect(IllegalStateException.class);
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User tom = new User("John", 17, Sex.MALE, "john@qwe.bar");
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Validator.of(tom).validate(User::getName, Objects::nonNull, "name cannot be null")
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.validate(User::getAge, age -> age > 21, "user is underaged")
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.get();
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}
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@Test
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public void testForValid() {
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User tom = new User("Sarah", 42, Sex.FEMALE, "sarah@det.org");
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User validated = Validator.of(tom).validate(User::getName, Objects::nonNull, "name cannot be null")
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.validate(User::getAge, age -> age > 21, "user is underaged")
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.validate(User::getSex, sex -> sex == Sex.FEMALE, "user is not female")
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.validate(User::getEmail, email -> email.contains("@"), "email does not contain @ sign")
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.get();
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Assert.assertSame(validated, tom);
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}
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}

‎pom.xml‎

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<module>event-driven-architecture</module>
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<module>feature-toggle</module>
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<module>value-object</module>
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<module>monad</module>
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</modules>
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<dependencyManagement>

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