Word Frequency

The brief

For the following task, please develop code that is both production quality code and fully unit-tested.

Write a solution that counts the number of occurrences of words in given Strings. The number of occurrences of each word should be stored in a HashMap. For example if the String the man in the moon is passed in then the HashMap will contain:

    “the”:	2  
    “man”:	1  
    “in”:	1     
    “moon”:	1

If subsequently the String the man on the moon is passed in then the HashMap will now contain:

    “the”:	4
    “man”:	2
    “in”:	1
    “on”:	1
    “moon”:	2

All input should be validated to prevent empty or null Strings being processed – appropriate exceptions should be thrown in these scenarios.

The solution should also be thread-safe so that occurrences are correctly updated when concurrently invoked.

The approach

You will be provided with a skeletal interface solution.