Generate unique random number on each call

Question

In a interview I was asked to wrtie a method which will generate unique 5 digit random number everytime when it is called.For ex: if I call the method and get 22222 then in next call i should not get 22222.

I wrote a code as below:

import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;

public class RandomNumberGen {


    private static ArrayList arr=new ArrayList();
    private static int k=-1;
    public RandomNumberGen(){
        for (int i=10000;i<99999;i++){
            arr.add(i);
        }
        Collections.shuffle(arr);
    }
    public static void main(String[] args) {
        for(int m=0;m<10;m++){
            try {
                System.out.println(new RandomNumberGen().randomNumbermethod());
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }


    }
    public Integer randomNumbermethod() throws Exception{
        k++;
        if(k>=arr.size()){
            throw new Exception("No more number available");
        }else return (Integer) arr.get(k);
    }

}

Answer got accepted but I was asked to avoid memory wastage now. My question is here as you can see I am using only 10 numbers.So rest of the space occupied by arraylist is a memory-wastage.Is there a way I can achieve same thing without using extra memory. What I mean is there someway using which unique number can be generated on each call so that this much memory do not get wasted.

Answer 1

private static int number = 10000;
public int getNextUniqueRandomNumber() {
   return number++;
}