string-calculator-with-different-input-types

String Calculator With Different Input Types

Write a calculator that calculates a sum of integers passed in a form of string. It should return a sum represented as a string.

Example input: 
"1, 2, 3"
Example output: 
"6"

Also, there are two different invalid input types:

1. StringCalculatorFunctional.sum("") -> "Calculation failed"
2. StringCalculatorFunctional.sum("bad input") -> "Calculation failed"

If a string contains integers, but also an element which isn't an integer, sum should be calculated from remaining numbers.

E.g.:
Input: "1, 2, bad input, 3"
Output: "6"

Hints:

1. Develop a helper method with the following signature:

   private static Optional<Integer> parseIntHelper(String number);
   

2. Convert split string into a stream
3. Use map-reduce paradigm:
   • http://www.techspot.co.in/2011/07/mapreduce-for-dummies.html
4. APIs for use:
   • https://docs.oracle.com/javase/8/docs/api/java/util/Arrays.html
   • https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html
   • https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html