issue #335 Monad pattern introduced

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

@@ -0,0 +1,19 @@
+package com.iluwatar.monad;
+
+import java.util.Objects;
+
+public class App {
+
+  /**
+   * Program entry point.
+   *
+   * @param args @param args command line args
+   */
+  public static void main(String[] args) {
+    User user = new User("user", 24, Sex.FEMALE, "foobar.com");
+    System.out.println(Validator.of(user).validate(User::getName, Objects::nonNull, "name is null")
+        .validate(User::getName, name -> !name.isEmpty(), "name is empty")
+        .validate(User::getEmail, email -> !email.contains("@"), "email doesn't containt '@'")
+        .validate(User::getAge, age -> age > 20 && age < 30, "age isn't between...").get().toString());
+  }
+}

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

@@ -0,0 +1,5 @@
+package com.iluwatar.monad;
+
+public enum Sex {
+  MALE, FEMALE
+}

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

@@ -0,0 +1,39 @@
+package com.iluwatar.monad;
+
+public class User {
+
+  private String name;
+  private int age;
+  private Sex sex;
+  private String email;
+
+  /**
+   *
+   * @param name - name
+   * @param age - age
+   * @param sex - sex
+   * @param email - email
+   */
+  public User(String name, int age, Sex sex, String email) {
+    this.name = name;
+    this.age = age;
+    this.sex = sex;
+    this.email = email;
+  }
+
+  public String getName() {
+    return name;
+  }
+
+  public int getAge() {
+    return age;
+  }
+
+  public Sex getSex() {
+    return sex;
+  }
+
+  public String getEmail() {
+    return email;
+  }
+}

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

@@ -0,0 +1,81 @@
+package com.iluwatar.monad;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import java.util.function.Function;
+import java.util.function.Predicate;
+
+/**
+ * Class representing Monad design pattern. Monad is a way of chaining operations on the
+ * given object together step by step. In Validator each step results in either success or
+ * failure indicator, giving a way of receiving each of them easily and finally getting
+ * validated object or list of exceptions.
+ * @param <T> Placeholder for an object.
+ */
+public class Validator<T> {
+  private final T t;
+  private final List<Throwable> exceptions = new ArrayList<>();
+
+  /**
+   * @param t object to be validated
+   */
+  private Validator(T t) {
+    this.t = t;
+  }
+
+  /**
+   * Creates validator against given object
+   *
+   * @param t   object to be validated
+   * @param <T> object's type
+   * @return new instance of a validator
+   */
+  public static <T> Validator<T> of(T t) {
+    return new Validator<>(Objects.requireNonNull(t));
+  }
+
+  /**
+   * @param validation one argument boolean-valued function that
+   *                   represents one step of validation. Adds exception to main validation exception
+   *                   list when single step validation ends with failure.
+   * @param message    error message when object is invalid
+   * @return this
+   */
+  public Validator<T> validate(Predicate<T> validation, String message) {
+    if (!validation.test(t)) {
+      exceptions.add(new IllegalStateException(message));
+    }
+    return this;
+  }
+
+  /**
+   * Extension for the {@link Validator#validate(Function, Predicate, String)} method,
+   * dedicated for objects, that need to be projected before requested validation.
+   * @param projection function that gets an objects, and returns projection representing
+   *                   element to be validated.
+   * @param validation see {@link Validator#validate(Function, Predicate, String)}
+   * @param message    see {@link Validator#validate(Function, Predicate, String)}
+   * @param <U>        see {@link Validator#validate(Function, Predicate, String)}
+   * @return this
+   */
+  public <U> Validator<T> validate(Function<T, U> projection, Predicate<U> validation,
+                                   String message) {
+    return validate(projection.andThen(validation::test)::apply, message);
+  }
+
+  /**
+   * To receive validated object.
+   *
+   * @return object that was validated
+   * @throws IllegalStateException when any validation step results with failure
+   */
+  public T get() throws IllegalStateException {
+    if (exceptions.isEmpty()) {
+      return t;
+    }
+    IllegalStateException e = new IllegalStateException();
+    exceptions.forEach(e::addSuppressed);
+    throw e;
+  }
+}

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

@@ -0,0 +1,13 @@
+package com.iluwatar.monad;
+
+import org.junit.Test;
+
+public class AppTest {
+
+  @Test
+  public void testMain() {
+    String[] args = {};
+    App.main(args);
+  }
+
+}

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

@@ -0,0 +1,42 @@
+package com.iluwatar.monad;
+
+
+import junit.framework.Assert;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+
+import java.util.Objects;
+
+public class MonadTest {
+
+  @Rule
+  public ExpectedException thrown = ExpectedException.none();
+
+  @Test
+  public void testForInvalidName() {
+    thrown.expect(IllegalStateException.class);
+    User tom = new User(null, 21, Sex.MALE, "tom@foo.bar");
+    Validator.of(tom).validate(User::getName, Objects::nonNull, "name cannot be null").get();
+  }
+
+  @Test
+  public void testForInvalidAge() {
+    thrown.expect(IllegalStateException.class);
+    User tom = new User("John", 17, Sex.MALE, "john@qwe.bar");
+    Validator.of(tom).validate(User::getName, Objects::nonNull, "name cannot be null")
+        .validate(User::getAge, age -> age > 21, "user is underaged")
+        .get();
+  }
+
+  @Test
+  public void testForValid() {
+    User tom = new User("Sarah", 42, Sex.FEMALE, "sarah@det.org");
+    User validated = Validator.of(tom).validate(User::getName, Objects::nonNull, "name cannot be null")
+        .validate(User::getAge, age -> age > 21, "user is underaged")
+        .validate(User::getSex, sex -> sex == Sex.FEMALE, "user is not female")
+        .validate(User::getEmail, email -> email.contains("@"), "email does not contain @ sign")
+        .get();
+    Assert.assertSame(validated, tom);
+  }
+}