How does this code that fills an array with 10 distinct random numbers work?

See my comments in the code itself:

public static void main(String[] args){
    //make a new array of 10 integers
    int a[] = new int[10];

    //declare an object which we can use to generate random numbers
    //this object probably uses the system time to generate numbers that appear random
    //but at the end of the day, java does it for us so
    //we don't really need to know or care how it generates random numbers
    Random randomGenerator = new Random();

    //loop over each element in our array
    for (int i = 0; i < a.length; i++){
          //for each element, set that element to a random between 1 and 100 inclusive
          //nextInt(x) gets a number between 0 (inclusive) and x (not inclusive)
          //so to translate that to 1 to x inclusive, we need to add 1 to the result
          a[i] = 1 + randomGenerator.nextInt(100); 

    //everything below here does literally nothing to solve the problem
    //everything you need to fill the array with random numbers is above

    for (int i = 0; i < a.length; i++) { 
          int number = 1 + randomGenerator.nextInt(100); 
          int count = 0; 
          for (int j = 0; j < i; j++) {
            if (a[j] == number) {
              count += 1; 
          if (count > 0) i -= 1;
          else a[i] = number; 

Please note that you should use 1 + randomGenerator.nextInt(10); to fill the array with numbers between 1 and 10, not 1 + randomGenerator.nextInt(100);.

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