Java: Unique 10 digit ID

Question

I need to generate a unique 10 digit ID in Java. These are the restrictions for this ID:

• Only Numeric
• Maximum 10 digits
• Possible to create up to 10 different IDs per second
• Has to be unique (even if the application re-starts)
• Not possible to save a number in the Database
• As fast as possible NOT to add much lattency to the system

The best solution I found so far is the following:

private static int inc = 0;

private static long getId(){

    long id = Long.parseLong(String.valueOf(System.currentTimeMillis())
            .substring(1,10)
            .concat(String.valueOf(inc)));
    inc = (inc+1)%10;
    return id;
}

This solution has the following problems:

• If for any reason there is a need to create more than 10 IDs per seccond, this solution won't work.
• In about 32 years this ID could be repeated (This is probably acceptable)

Any other solution to create this ID?

Any other problem I haven't thought of with mine?

Thanks for your help,

Answer 1

This is a small enhancement to yours but should be resilient.

Essentially, we use the current time in milliseconds unless it hasn't ticked since the last id, in which case we just return last + 1.

private static final long LIMIT = 10000000000L;
private static long last = 0;

public static long getID() {
  // 10 digits.
  long id = System.currentTimeMillis() % LIMIT;
  if ( id <= last ) {
    id = (last + 1) % LIMIT;
  }
  return last = id;
}

As it is it should manage up to 1000 per second with a comparatively short cycle rate. To extend the cycle rate (but shorten the resolution) you could use (System.currentTimeMillis() / 10) % 10000000000L or (System.currentTimeMillis() / 100) % 10000000000L.

Answer 2

This may be a crazy idea but its an idea :).

• First generate UUID and get a string representation of it with java.util.UUID.randomUUID().toString()

• Second convert generated string to byte array (byte[])

• Then convert it to long buffer: java.nio.ByteBuffer.wrap( byte digest[] ).asLongBuffer().get()

• Truncate to 10 digits

Not sure about uniqueness of that approach tho, I know that you can rely on uniqueness of UUIDs but haven't checked how unique are they converted and truncated to 10 digits long number.

Example was taken from JavaRanch, maybe there is more.

Edit: As you are limited to 10 digits maybe simple random generator would be enough for you, have a look into that quesion/answers on SO: Java: random long number in 0 <= x < n range