Declare Calculator as a static class

stringCalculator/src/main/java/fr/ippon/stringcalculator/Calculator.java

@@ -3,7 +3,10 @@ package fr.ippon.stringcalculator;
 import java.math.BigDecimal;
 import java.util.function.BinaryOperator;
 
-public class Calculator {
+public final class Calculator {
+
+  private Calculator() {
+  }
 
   public static String sum(String first, String second) {
     return operate(first, second, BigDecimal::add);
@@ -14,8 +17,8 @@ public class Calculator {
     return operate(first, second, BigDecimal::subtract);
   }
 
-  private static String operate(String first,
-      String second, BinaryOperator<BigDecimal> operation) {
+  private static String operate(String first, String second,
+      BinaryOperator<BigDecimal> operation) {
     notNull(first);
     notNull(second);