Added example and test for functional interface.

core-java/functional-programming/functional-interfaces/src/main/java/io/reflectoring/function/custom/ArithmeticOperation.java

@@ -0,0 +1,13 @@
+package io.reflectoring.function.custom;
+
+/** The arithmetic operation functional interface. */
+public interface ArithmeticOperation {
+  /**
+   * Operates on two integer inputs to calculate a result.
+   *
+   * @param a the first number
+   * @param b the second number
+   * @return the result
+   */
+  int operate(int a, int b);
+}

core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/custom/ArithmeticOperationTest.java

@@ -0,0 +1,29 @@
+package io.reflectoring.function.custom;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+class ArithmeticOperationTest {
+
+  @Test
+  void operate() {
+    // Define operations
+    ArithmeticOperation add = (a, b) -> a + b;
+    ArithmeticOperation subtract = (a, b) -> a - b;
+    ArithmeticOperation multiply = (a, b) -> a * b;
+    ArithmeticOperation divide = (a, b) -> a / b;
+
+    // Use the operations
+    int addition = add.operate(10, 5); // Returns 15
+    int subtraction = subtract.operate(10, 5); // Returns 5
+    int multiplication = multiply.operate(10, 5); // Returns 50
+    int division = divide.operate(10, 5); // Returns 2
+
+    // Verify results
+    assertEquals(15, addition, "Result of addition is not correct.");
+    assertEquals(5, subtraction, "Result of subtraction is not correct.");
+    assertEquals(50, multiplication, "Result of multiplication is not correct.");
+    assertEquals(2, division, "Result of division is not correct.");
+  }
+}