BAEL-5959: Monad in Java (#13278)

* BAEL-5959: Monad in Java

* Fix test name

core-java-modules/core-java-8/src/main/java/com/baeldung/monad/MonadSamples.java

@@ -0,0 +1,113 @@
+package com.baeldung.monad;
+
+import java.util.Optional;
+import java.util.function.Function;
+
+class MonadBaseExample {
+
+    public double multiplyBy2(double n) {
+        return n * 2;
+    }
+
+    public double divideBy2(double n) {
+        return n / 2;
+    }
+
+    public double add3(double n) {
+        return n + 3;
+    }
+
+    public double subtract1(double n) {
+        return n - 1;
+    }
+
+}
+
+class MonadSample1 extends MonadBaseExample {
+
+    public double apply(double n) {
+        return subtract1(add3(divideBy2(multiplyBy2(multiplyBy2(2)))));
+    }
+
+    public static void main(String[] args) {
+        final MonadSample1 test = new MonadSample1();
+        System.out.println(test.apply(2));
+        //6.0
+    }
+
+}
+
+class MonadSample2 extends MonadBaseExample {
+    public double apply(double n) {
+        final double n1 = multiplyBy2(n);
+        final double n2 = multiplyBy2(n1);
+        final double n3 = divideBy2(n2);
+        final double n4 = add3(n3);
+        return subtract1(n4);
+    }
+
+    public static void main(String[] args) {
+        final MonadSample2 test = new MonadSample2();
+        System.out.println(test.apply(2));
+        //6.0
+    }
+
+}
+
+class MonadSample3 extends MonadBaseExample {
+
+    public double apply(double n) {
+        return Optional.of(n)
+                .flatMap(value -> Optional.of(multiplyBy2(value)))
+                .flatMap(value -> Optional.of(multiplyBy2(value)))
+                .flatMap(value -> Optional.of(divideBy2(value)))
+                .flatMap(value -> Optional.of(add3(value)))
+                .flatMap(value -> Optional.of(subtract1(value)))
+                .get();
+    }
+
+    public static void main(String[] args) {
+        final MonadSample3 test = new MonadSample3();
+        System.out.println(test.apply(2));
+        //6.0
+    }
+}
+
+ class MonadSample4 extends MonadBaseExample {
+     public boolean leftIdentity() {
+         Function<Integer, Optional<Integer>> mapping = value -> Optional.of(value + 1);
+         return Optional.of(3).flatMap(mapping).equals(mapping.apply(3));
+     }
+
+     public boolean rightIdentity() {
+         return Optional.of(3).flatMap(Optional::of).equals(Optional.of(3));
+     }
+
+     public boolean associativity() {
+         Function<Integer, Optional<Integer>> mapping = value -> Optional.of(value + 1);
+         Optional<Integer> leftSide = Optional.of(3).flatMap(mapping).flatMap(Optional::of);
+         Optional<Integer> rightSide = Optional.of(3).flatMap(v -> mapping.apply(v).flatMap(Optional::of));
+         return leftSide.equals(rightSide);
+     }
+
+     public static void main(String[] args) {
+         final MonadSample4 test = new MonadSample4();
+
+         System.out.println(test.leftIdentity()); //true
+         System.out.println(test.rightIdentity()); //true
+         System.out.println(test.associativity()); //true
+     }
+ }
+
+class MonadSample5 extends MonadBaseExample {
+    public boolean fail() {
+        Function<Integer, Optional<Integer>> mapping = value -> Optional.of(value == null ? -1 : value + 1);
+        return Optional.ofNullable((Integer) null).flatMap(mapping).equals(mapping.apply(null));
+    }
+
+    public static void main(String[] args) {
+        final MonadSample5 test = new MonadSample5();
+        System.out.println(test.fail());
+    }
+}
+

core-java-modules/core-java-8/src/test/java/com/baeldung/monad/MonadSampleUnitTest.java

@@ -0,0 +1,39 @@
+package com.baeldung.monad;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class MonadSampleUnitTest {
+
+    @Test
+    public void whenNotUsingMonad_shouldBeOk() {
+        MonadSample1 test = new MonadSample1();
+        Assert.assertEquals(6.0, test.apply(2), 0.000);
+    }
+
+    @Test
+    public void whenNotUsingMonadButUsingTempVars_shouldBeOk() {
+        MonadSample2 test = new MonadSample2();
+        Assert.assertEquals(6.0, test.apply(2), 0.000);
+    }
+
+    @Test
+    public void whenUsingMonad_shouldBeOk() {
+        MonadSample3 test = new MonadSample3();
+        Assert.assertEquals(6.0, test.apply(2), 0.000);
+    }
+
+    @Test
+    public void whenTestingMonadProperties_shouldBeOk() {
+        MonadSample4 test = new MonadSample4();
+        Assert.assertEquals(true, test.leftIdentity());
+        Assert.assertEquals(true, test.rightIdentity());
+        Assert.assertEquals(true, test.associativity());
+    }
+
+    @Test
+    public void whenBreakingMonadProperties_shouldBeFalse() {
+        MonadSample5 test = new MonadSample5();
+        Assert.assertEquals(false, test.fail());
+    }
+}