add calculateArea() method

src/main/java/com/ap/BasicCalculator.java

@@ -15,4 +15,9 @@ public class BasicCalculator implements Calculator {
     public int product(int a, int b) {
         return a * b;
     }
+
+    @Override
+    public double calculateArea(double a, double b) {
+        return a * b;
+    }
 }

src/main/java/com/ap/Calculator.java

@@ -0,0 +1,8 @@
+package com.ap;
+
+public interface Calculator {
+    int add(int a, int b);
+    int subtract(int a, int b);
+    int product(int a, int b);
+    double calculateArea(double a, double b);
+}