use stream instead of for-loop to sum array

src/main/java/com/ordestiny/tdd/kata/StringCalculator.java

@@ -1,6 +1,7 @@
 package com.ordestiny.tdd.kata;
 
 import java.text.DecimalFormat;
+import java.util.Arrays;
 import java.util.regex.Pattern;
 
 public class StringCalculator {
@@ -73,18 +74,15 @@ private double getSum(String numbers, String delimiter) {
 
  String[] arrayOfStringNumbers = numbers.split(delimiter);
 
- return sumStringDigits(arrayOfStringNumbers);
+ return sum(arrayOfStringNumbers);
  }
 
- private double sumStringDigits(String[] digitsArray) {
- double result = 0.0;
-
- for (String digit : digitsArray) {
- double num = Double.parseDouble(digit);
- result += num;
- }
-
- return result;
+ private double sum(String[] numbers) {
+ Double sum = Arrays.stream(numbers)
+ .map(number-> Double.valueOf(number))
+ .reduce(0.0, Double::sum);
+
+ return sum;
  }
 
  private String format(double value) {