fix: add MultiplyFunction (#5)

Adds MultiplyFunction implementing CalcFunction, following the
Add/Subtract pattern, wired into LoopTestApplication.

src/main/java/com/example/LoopTestApplication.java

@@ -2,6 +2,7 @@ package com.example;
 
 import com.example.util.AddFunction;
 import com.example.util.CalcFunction;
+import com.example.util.MultiplyFunction;
 import com.example.util.SubtractFunction;
 
 public class LoopTestApplication {
@@ -12,5 +13,7 @@ public class LoopTestApplication {
         System.out.println("ADDITION: " + add.calc(2, 5, 6));
         CalcFunction subtract = new SubtractFunction();
         System.out.println("SUBTRACTION: " + subtract.calc(2, 5, 6));
+        CalcFunction multiply = new MultiplyFunction();
+        System.out.println("MULTIPLICATION: " + multiply.calc(2, 5, 6));
     }
 }

src/main/java/com/example/util/MultiplyFunction.java

@@ -0,0 +1,15 @@
+package com.example.util;
+
+public class MultiplyFunction implements CalcFunction {
+
+    public double calc(double... input) {
+        if (input.length == 0) {
+            return 0;
+        }
+        double result = input[0];
+        for (int i = 1; i < input.length; i++) {
+            result *= input[i];
+        }
+        return result;
+    }
+}

src/test/java/com/example/util/MultiplyFunctionTest.java

@@ -0,0 +1,30 @@
+package com.example.util;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class MultiplyFunctionTest {
+
+    private final MultiplyFunction multiplyFunction = new MultiplyFunction();
+
+    @Test
+    void multipliesMultipleValues() {
+        assertEquals(60.0, multiplyFunction.calc(2.0, 5.0, 6.0));
+    }
+
+    @Test
+    void returnsSingleValue() {
+        assertEquals(5.0, multiplyFunction.calc(5.0));
+    }
+
+    @Test
+    void returnsZeroForNoInput() {
+        assertEquals(0.0, multiplyFunction.calc());
+    }
+
+    @Test
+    void multipliesNegativeValues() {
+        assertEquals(-2.0, multiplyFunction.calc(1.0, -2.0));
+    }
+}