BAEL-5960 - Functors in Java (#13125)

core-java-modules/core-java-functional/src/main/java/com/baeldung/functional/functors/EnumFunctor.java

@@ -0,0 +1,23 @@
+package com.baeldung.functional.functors;
+
+public enum EnumFunctor {
+    PLUS {
+        public int apply(int a, int b) {
+            return a + b;
+        }
+    }, MINUS {
+        public int apply(int a, int b) {
+            return a - b;
+        }
+    }, MULTIPLY {
+        public int apply(int a, int b) {
+            return a * b;
+        }
+    }, DIVIDE {
+        public int apply(int a, int b) {
+            return a / b;
+        }
+    };
+
+    public abstract int apply(int a, int b);
+}

core-java-modules/core-java-functional/src/main/java/com/baeldung/functional/functors/Functor.java

@@ -0,0 +1,23 @@
+package com.baeldung.functional.functors;
+
+import java.util.function.Function;
+
+public class Functor<T> {
+    private final T value;
+
+    public Functor(T value) {
+        this.value = value;
+    }
+
+    public <R> Functor<R> map(Function<T, R> mapper) {
+        return new Functor<>(mapper.apply(value));
+    }
+
+    boolean eq(T other) {
+        return value.equals(other);
+    }
+
+    public T getValue() {
+        return value;
+    }
+}

core-java-modules/core-java-functional/src/test/java/com/baeldung/functional/functors/FunctorsUnitTest.java

@@ -0,0 +1,50 @@
+package com.baeldung.functional.functors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.function.Function;
+
+import org.junit.jupiter.api.Test;
+
+class FunctorsUnitTest {
+    @Test
+    public void whenProvideAValue_ShouldMapTheValue() {
+        Functor<Integer> functor = new Functor<>(5);
+        Function<Integer, Integer> addThree = (num) -> num + 3;
+        Functor<Integer> mappedFunctor = functor.map(addThree);
+        assertEquals(8, mappedFunctor.getValue());
+    }
+
+    @Test
+    public void whenApplyAnIdentityToAFunctor_thenResultIsEqualsToInitialValue() {
+        String value = "baeldung";
+        //Identity
+        Functor<String> identity = new Functor<>(value).map(Function.identity());
+        assertTrue(identity.eq(value));
+    }
+
+    @Test
+    public void whenApplyAFunctionToOtherFunction_thenResultIsEqualsBetweenBoth() {
+        int value = 100;
+        long expected = 100;
+        // Composition/Associativity
+        Function<Integer, String> f = Object::toString;
+        Function<String, Long> g = Long::valueOf;
+
+        Functor<Long> left = new Functor<>(value).map(f)
+            .map(g);
+        Functor<Long> right = new Functor<>(value).map(f.andThen(g));
+
+        assertTrue(left.eq(expected));
+        assertTrue(right.eq(expected));
+    }
+
+    @Test
+    public void whenApplyOperationsToEnumFunctors_thenGetTheProperResult() {
+        assertEquals(15, EnumFunctor.PLUS.apply(10, 5));
+        assertEquals(5, EnumFunctor.MINUS.apply(10, 5));
+        assertEquals(50, EnumFunctor.MULTIPLY.apply(10, 5));
+        assertEquals(2, EnumFunctor.DIVIDE.apply(10, 5));
+    }
+}