How do I create a unique ID in Java? [duplicate]

Question

Create a UUID.

String uniqueID = UUID.randomUUID().toString();

If you want short, human-readable IDs and only need them to be unique per JVM run:

private static long idCounter = 0;

public static synchronized String createID()
{
    return String.valueOf(idCounter++);
}    

Edit: Alternative suggested in the comments - this relies on under-the-hood "magic" for thread safety, but is more scalable and just as safe:

private static AtomicLong idCounter = new AtomicLong();

public static String createID()
{
    return String.valueOf(idCounter.getAndIncrement());
}

java.util.UUID : toString() method

Here's my two cent's worth: I've previously implemented an IdFactory class that created IDs in the format [host name]-[application start time]-[current time]-[discriminator]. This largely guaranteed that IDs were unique across JVM instances whilst keeping the IDs readable (albeit quite long). Here's the code in case it's of any use:

public class IdFactoryImpl implements IdFactory {
  private final String hostName;
  private final long creationTimeMillis;
  private long lastTimeMillis;
  private long discriminator;

  public IdFactoryImpl() throws UnknownHostException {
    this.hostName = InetAddress.getLocalHost().getHostAddress();
    this.creationTimeMillis = System.currentTimeMillis();
    this.lastTimeMillis = creationTimeMillis;
  }

  public synchronized Serializable createId() {
    String id;
    long now = System.currentTimeMillis();

    if (now == lastTimeMillis) {
      ++discriminator;
    } else {
      discriminator = 0;
    }

    // creationTimeMillis used to prevent multiple instances of the JVM
    // running on the same host returning clashing IDs.
    // The only way a clash could occur is if the applications started at
    // exactly the same time.
    id = String.format("%s-%d-%d-%d", hostName, creationTimeMillis, now, discriminator);
    lastTimeMillis = now;

    return id;
  }

  public static void main(String[] args) throws UnknownHostException {
    IdFactory fact = new IdFactoryImpl();

    for (int i=0; i<1000; ++i) {
      System.err.println(fact.createId());
    }
  }
}

Generate Unique ID Using Java

UUID is the fastest and easiest way to generate unique ID in Java.

import java.util.UUID;

public class UniqueIDTest {
  public static void main(String[] args) {
    UUID uniqueKey = UUID.randomUUID();
    System.out.println (uniqueKey);
  }
}