Generate 6 digit random number

Its as simple as that, you can use your code and just do one thing extra here

String.format("%06d", number);

this will return your number in string format, so the "0" will be "000000".

Here is the code.

public static String getRandomNumberString() {
    // It will generate 6 digit random Number.
    // from 0 to 999999
    Random rnd = new Random();
    int number = rnd.nextInt(999999);

    // this will convert any number sequence into 6 character.
    return String.format("%06d", number);
}

If you need a six digit number it has to start at 100000

int i = new Random().nextInt(900000) + 100000;

Leading zeros do not have effect, 000000 is the same as 0. You can further simplify it with ThreadLocalRandom if you are on Java 7+:

int i = ThreadLocalRandom.current().nextInt(100000, 1000000)