How to generate random ID numbers that can't be reused in java?

Question

For my current java project, I am trying to generate random ID's for registered users. So far I have been using min +(int) (Math.random()*((max-min)+1)) as my formula to generate the random number. The problem that I am facing is that sometimes the numbers repeat themselves and my application wouldn't work with them.

    int min = 1001;
    int max = 1050;
    
    for (int i=1; i<=1; i++)
    {
    int a = min +(int) (Math.random()*((max-min)+1));
    
    

     }
     
     
  

I have tried using and incorporating

    Integer[] arr = new Integer[100];
    for (int i = 1; i < arr.length; i++) {
    arr[i] = i;
    }
    Collections.shuffle(Arrays.asList(arr));

but numbers generated would constantly come out as "null" and it would repeat the loop a few hundred times and flood my txt file.

Answer 1

In general, random generators Random or Math.random() are not the correct ways to generate a unique id. As you mentioned, it could be repeated (and it will definitely be).

I would recommend two ways of generating ID .

The first one is to use AtomicInteger . This is good when your ID should be unique but not random .

private static final AtomicInteger ID = new AtomicInteger(0);

public static String generateUniqueId() {
    return String.valueOf(ID.incrementAndGet());
}

The second one, which is preferable to me, is to use UUID . This is good when your ID should be as unique as random .

public static String generateUniqueId() {
    return String.valueOf(UUID.randomUUID());
}

---

Another one, I can mention is to use System.nanoTime() .

public static String generateUniqueId() {
    return String.valueOf(System.nanoTime());
}

Long ago I had some investigation and find out that this is pretty stable for normal payload. But in general, it could retrieve the same value if you build such a system, that should generate ID so often.

Answer 2

Instead of generating numbers I would recommend to generate UUID. The chance of a is collision is much smaller.

UUID id = UUID.randomUUID();

Otherwise if you want to stick with numbers I would recommend you to implement yourself some Sequence service within your application.

import java.util.concurrent.atomic.AtomicLong;

public class SequenceService {

    private final AtomicLong ids;

    public SequenceService() {

        long initialValue = getInitialValue();
        this.ids = new AtomicLong(initialValue);
    }

    public long generateNextId() {
        return ids.incrementAndGet();
    }

    private long getInitialValue() {
        // this methods reads the last known leased id (e.g. from the file system)
    }
}