为什么我的for循环不向我的列表添加多个随机值？

Question

I'm trying to create a bubble sort algorithm in java and I'm trying to create a list with random integers in it with a for loop, however it only ever has one value. This is the code:

public class bubbleSort {

public static void main(String[] args) 
{

    int length = getLength();

    System.out.println(length);

    List<Integer> randomList = createArray(length);

    System.out.println(randomList);

}

public static int getLength() 
{
    System.out.println("Please enter how long you want the array to be");

    Scanner reader = new Scanner(System.in);

    int length = Integer.parseInt(reader.nextLine());

    return length;
}

public static List<Integer> createArray(int length) 
{
    Random rand = new Random();

    List<Integer> randomList = new ArrayList<Integer>();

    for(int x = 0 ; x < (length + 1) ; x++);
    {
        int randomnumber = rand.nextInt((100 - 1) + 1) + 1;
        randomList.add(randomnumber);
    }

    return randomList;  
}

}

Answer 1

public static List<Integer> createArray(int length) 
{
   Random rand = new Random();

   List<Integer> randomList = new ArrayList<Integer>();

   for(int x = 0 ; x < (length + 1) ; x++)
   {
        int randomnumber = rand.nextInt((100 - 1) + 1) + 1;
        randomList.add(randomnumber);
   }

   return randomList;  
}

Your problem is here.

You should only create a single Random instance that you use to generate your random numbers.

This is because the random numbers are from a seed value, and when you don't provide a seed for the random number number generator generator, it will be created with a seed value from epoch time, so all your numbers are the same because of this.

A simple solution would be to update your createArray(int length) to take a random instance, like so createArray(int length, Random random) and use this instance to generate the numbers.

Answer 2

There should be no semicolon (;) after your for loop declaration .That is why the loop keeps exiting after the first loop, It should be

for(int x = 0 ; x < (length + 1) ; x++)
{
    int randomnumber = rand.nextInt((100 - 1) + 1) + 1;
    randomList.add(randomnumber);
}

And why do you have a (length +1)..if you want the size of the random list to be the size of the length ,you should remove the +1.