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Literate Train

The name for this project was automatically generated by Github. Follow me: @elvisoliveira

How to execute?

mvn spring-boot:run

How to test the code?

mvn test

How to test the app?

newman run ./postman_collection.json --environment ./postman_environment.json --iteration-data ./postman_data.json



How to run the UI?

There is a simple UI running on the root (/) controller. Since the default port is 1337, if you are running the app locally you can have access to it on

When the app starts a keypair is generated and delivered the file postman_data.json, you may use the info on it.

The Challange: Single Digit

A single digit is defined following this rules: Given an integer, we need to find the single digit of it.

  • If x has only one digit, then its single digit is x.
  • Otherwise, the single digit of x equals the single digit of the sum of the digits of x.

For instance, the single digit of 9875 will be calculated as:

Parameter Calc Return
9875 9 + 8 + 7 + 5 29
29 2 + 9 11
11 1 + 1 2
2 2 2
2 2 2

Given two numbers n and k, P must be defined as an string concatenation of n * k.


  • n = 9875
  • k = 4
  • p = 9875 9875 9875 9875
singleDigit = singleDigit(9875987598759875)
// 5 + 7 + 8 + 9 + 5 + 7 + 8 + 9 + 5 + 7 + 8 + 9 + 5 + 7 + 8 + 9 = 116
singleDigit = singleDigit(116)
// 1 + 1 + 6 = 8
singleDigit = singleDigit(8)

The singleDigit function must have the following parameters:

  1. n: A string representing an integer 1 <= n <= 10ˆ10000002
  2. k: An integer representing the number of concatenation times 1 <= k <= 10ˆ53
  3. The single digit function shall return an integer


  • A CRUD must be created for users
  • A user has a name, email, address and a list of single digit results already calculated
  • Each result list object must contain which parameters were inputted and the result of it


  • A memory cache must be created to store the last 10 calculations performed by the single digit function, this cache is user independent, that is, if a calculation has already been performed and it is on the cache, it must not be performed again. Using caching frameworks is not allowed


  • User name and email information must be encrypted with an asymmetric key RSA of size 2048. Each user may have a separated encryption key. The information will be encrypted with the public key and the client will decrypt using their private key


  • Endpoints must be made available for the users CRUD
  • An endpoint must be available for single digit calculation, this can be optionally associated with a user
  • An endpoint that retrieves all calculations for a given user must be created
  • An endpoint to send the user public key that will be used for encryption must be created. This API should receive as string that will contain the key

Building the app

  • The application must contain an in-memory database
  • Maven should be used to build, run tests and start the application
  • Unit tests must be created
  • Postman integrated tests must be created and it's collection must be at the root of the repository
  • This collection should be called "postman_collection.json"
  • An Open API Swagger file should be created with API specification


A programming challange in Java SpringBoot. Restful service that implements a cache based user manager.







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