Generate random numbers without repeating

Question

I have a doubt if I leave my index++ inside ifn as it is in the code below, the program does not execute, however if I put it at the beginning the program never generates new numbers if the ones already generated are the same, I really don't know where my mistake is Note: I'm doing it in Windows Builder

    private int[] number = new int[] { -1 , -1 , -1 , -1 , -1 , -1 , -1 , -1 , -1 , -1};
            private int index;
            private int randomNumber;
            private boolean numberRepeated;

                Random random = new Random();
        index = 0;
        while(index < 10) {
            randomNumber = random.nextInt(9);
            numberRepeated = false;  
            for(int i = 0 ; i < index; i++) {
                if(number[i] == randomNumber) {
                    numberRepeated = true;
                    break;
                }
            }
            if(!numberRepeated) {
                number[index] = randomNumber;
                index++;
            }
            
        }

I've already tried changing index++, but when I do this the program doesn't generate new values ​​if the same values ​​have already been entered.

Answer 1

The first problem is that Random.nextInt(9) gives values from 0 upto 8.

The numberRepeated will happen ever more, the larger is index. There even is no 100% guarantee that the last digit will ever by found by Random.

private int[] number = new int[10];


    Random random = new Random();
    for (int index = 0; index < 10; ++index) {
        randomNumber = random.nextInt(10); // 0 .. 9
        boolean numberRepeated = false;  
        for (int i = 0 ; i < index; i++) {
            if (number[i] == randomNumber) {
                numberRepeated = true;
                break;
            }
        }
        if (!numberRepeated) {
            number[index] = randomNumber;
            index++;
        }
    }

Better and much faster would be

    // Start sorted:
    for (int index = 0; index < 10; ++index) {
        number[index] = index;
    }
    // Randomly swap elements:
    Random random = new Random();
    for (int index = 0; index < 10; ++index) {
        int index2 = randomNumber = random.nextInt(10); // 0 .. 9
        // Swap values at index and index2;
        int temp = number[index];
        number[index] = number[index2];
        number[index2] = temp;
    }

This will swap at least every element once, sometimes with itself (good!).

This might be done otherwise with newer java constructs, like for 0 .. 9:

    number = IntStream.range(0, 10).toArray();

or:

    Arrays.setAll(number, IntUnaryOperator::identity);

or;

    Arrays.setAll(number, i -> i);