UUID to unique integer id?

Answering the How can I have a unique application wide Integer:

If it needs to be unique even after restarts or if you application is clustered you may use a Database sequence.

If it just needs to be unique during the runtime use a static AtomicInteger.

EDIT (example added):

public class Sequence {

  private static final AtomicInteger counter = new AtomicInteger();

  public static int nextValue() {
    return counter.getAndIncrement();
  }
}

Usage:

int nextValue = Sequence.nextValue();

This is thread safe (different threads will always receive distinct values, and no values will be "lost")

You are going to have a problem since the UUID is 128 bits and the int is only 32bit. You'll either have to accept the risk of collisions and try to fudge it to a smaller space (hashCode is probably a good way to do that) or find an alternative (use the UUID directly, map to a BigInteger - difficult to tell without knowing why)

We had a requirement to convert all our UUIDs into serial numbers. Finally, we tested and used the next algorithm:

1. Get CRC64 of uuid(16 bytes) using the ECMA polynomial 0xC96C5795D7870F42. Do not use ISO polynomial because it could cause a lot of collisions for some UUID generation algorithms.

2. Now we have crc64(8 bytes). Take the first N bytes(in our case 5 in yours it will be 4 bytes for int and all bytes for int64)

We tested this method and it works well for several million UUIDs.

Our additional step: convert 5 bytes number into a number with base 36 and finally we have SN: 4YD3SOJB.

A UUID is a 16-Byte number (128 bit). You can't crunch it into an int (32 bit) while preserving it's uniqueness.

Mathematically spoken: 2⁹⁶ UUIDs will share the same Java-int-size hash value (which is ... a lot ;) )

A way out - some real life UUID often have a rather static part. So in isolated scenarios, the real unique portion of UUIDs may be less then 32 bit.