Hello there:

• if you need to do some random test with diferents ranges, make a diferent test methos for each of them.
• better use RepeatedTest than use a loop for repetitions
• don´t use spanish coments in the public solution
• sample test are empty
• perhaps better than use the same StringBuilder and clean it in each iteration is to use it only when you need, like new StringBuilder(number).reverse().toString()
• you don´t need a state in the solution, use static methods better than a object
• better and more clear is to use Random class and nextInt() to get the random values than Math.random()

Java translation Revision please:)