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

@@ -4,4 +4,5 @@ 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);
 }