A vote counting and results generating system

In order to run tests please invoke

 mvn test

In order to start the application please invoke

 mvn spring-boot:run

Application will listen on port 8080

To register a void please use following REST-ful endpoint

 POST /vote

 {
    {"candidate" :"B", "voter":"B"}    
 }

To get the voting results please use following REST-ful endpoint

GET /results

 {
    {
      "A": 4,
      "B": 1,
      "C": 4,
      "D": 2
    }    
 }

I have made the following assumptions:

All voters are registered on the fly in a thread safe manner
There are 'hardcoded' 4 candidates A, B, C, D
AS there is only one pull no poll id is being passed in the register vote request
If registering 2 or 3 votes for the same candidate by the same voter corresponing 2 or 3 saparate requests must be made
In the code I used some synchronization to make code thread safe. If using database optimistic locking such synchronization wouldn't be needed

Provide feedback