Generating 10 digits unique random number in java

Question

I am trying with below code to generate 10 digits unique random number. As per my req i have to create around 5000 unique numbers(ids). This is not working as expected. It a

Answer 1

I don't know why noone realized that but I think the point is to generate "unique" random number which I am also trying to do that. I managed to generate 11 digits random number but I am not sure how to generate unique numbers. Also my approach is a little different. In this method I am appending number chars next to each other with for loop. Then returning long number.

public long generateID() { 
    Random rnd = new Random();
    char [] digits = new char[11];
    digits[0] = (char) (rnd.nextInt(9) + '1');
    for(int i=1; i<digits.length; i++) {
        digits[i] = (char) (rnd.nextInt(10) + '0');
    }
    return Long.parseLong(new String(digits));
}

Answer 2

Hi you can use the following method to generate 10 digit random number

private static int getRndNumber() {
    Random random=new Random();
    int randomNumber=0;
    boolean loop=true;
    while(loop) {
        randomNumber=random.nextInt();
        if(Integer.toString(randomNumber).length()==10 && !Integer.toString(randomNumber).startsWith("-")) {
            loop=false;
        }
        }
    return randomNumber;
}

Answer 3

I think the reason you're getting 8/9 digit values and negative numbers is that you're adding fraction, a long (signed 64-bit value) which may be larger than the positive int range (32-bit value) to aStart.

The value is overflowing such that randomNumber is in the negative 32-bit range or has almost wrapped around to aStart (since int is a signed 32-bit value, fraction would only need to be slightly less than (2^32 - aStart) for you to see 8 or 9 digit values).

You need to use long for all the values.

   private static void createRandomInteger(int aStart, long aEnd, Random aRandom){
    if ( aStart > aEnd ) {
      throw new IllegalArgumentException("Start cannot exceed End.");
    }
    //get the range, casting to long to avoid overflow problems
    long range = aEnd - (long)aStart + 1;
    logger.info("range>>>>>>>>>>>"+range);
    // compute a fraction of the range, 0 <= frac < range
    long fraction = (long)(range * aRandom.nextDouble());
    logger.info("fraction>>>>>>>>>>>>>>>>>>>>"+fraction);
    long randomNumber =  fraction + (long)aStart;    
    logger.info("Generated : " + randomNumber);

  }