I would like an efficient utility to generate unique sequences of bytes. UUID is a good candidate but UUID.randomUUID().toString()
generates stuff like 44e128a5-ac7a-4c9a-be4c-224b6bf81b20
which is good, but I would prefer dash-less string.
I'm looking for an efficient way to generate a random strings, only from alphanumeric characters (no dashes or any other special symbols).
Ended up writing something of my own based on UUID.java implementation. Note that I'm not generating a UUID, instead just a random 32 bytes hex string in the most efficient way I could think of.
import java.security.SecureRandom;
import java.util.UUID;
public class RandomUtil {
// Maxim: Copied from UUID implementation :)
private static volatile SecureRandom numberGenerator = null;
private static final long MSB = 0x8000000000000000L;
public static String unique() {
SecureRandom ng = numberGenerator;
if (ng == null) {
numberGenerator = ng = new SecureRandom();
}
return Long.toHexString(MSB | ng.nextLong()) + Long.toHexString(MSB | ng.nextLong());
}
}
RandomUtil.unique()
Some of the inputs I've tested to make sure it's working:
public static void main(String[] args) {
System.out.println(UUID.randomUUID().toString());
System.out.println(RandomUtil.unique());
System.out.println();
System.out.println(Long.toHexString(0x8000000000000000L |21));
System.out.println(Long.toBinaryString(0x8000000000000000L |21));
System.out.println(Long.toHexString(Long.MAX_VALUE + 1));
}
not sure why this is upvoted more, this generated UUID without the "-" in the most efficient method from all the options written here. String replacement is not better then conversion from long to string. It's true that both are O(n), yet at scale where you generate millions of uuid's a minute it becomes meaningful.