How to Generate Unique ID in Java (Integer)?

How to generate unique 开发者_运维百科ID that is integer in java that not guess next number?

How unique does it need to be?

If it's only unique within a process, then you can use an AtomicInteger and call incrementAndGet() each time you need a new value.

int uniqueId = 0;

int getUniqueId()
{
    return uniqueId++;
}

Add synchronized if you want it to be thread safe.

 import java.util.UUID;

 public class IdGenerator {
    public static int generateUniqueId() {      
        UUID idOne = UUID.randomUUID();
        String str=""+idOne;        
        int uid=str.hashCode();
        String filterStr=""+uid;
        str=filterStr.replaceAll("-", "");
        return Integer.parseInt(str);
    }

    // XXX: replace with java.util.UUID

    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            System.out.println(generateUniqueId());
            //generateUniqueId();
        }
    }

}

Hope this helps you.

It's easy if you are somewhat constrained.

If you have one thread, you just use uniqueID++; Be sure to store the current uniqueID when you exit.

If you have multiple threads, a common synchronized generateUniqueID method works (Implemented the same as above).

The problem is when you have many CPUs--either in a cluster or some distributed setup like a peer-to-peer game.

In that case, you can generally combine two parts to form a single number. For instance, each process that generates a unique ID can have it's own 2-byte ID number assigned and then combine it with a uniqueID++. Something like:

return (myID << 16) & uniqueID++

It can be tricky distributing the "myID" portion, but there are some ways. You can just grab one out of a centralized database, request a unique ID from a centralized server, ...

If you had a Long instead of an Int, one of the common tricks is to take the device id (UUID) of ETH0, that's guaranteed to be unique to a server--then just add on a serial number.

If you really meant integer rather than int:

Integer id = new Integer(42); // will not == any other Integer

If you want something visible outside a JVM to other processes or to the user, persistent, or a host of other considerations, then there are other approaches, but without context you are probably better off using using the built-in uniqueness of object identity within your system.

Just generate ID and check whether it is already present or not in your list of generated IDs.

UUID class

Do you need it to be;

• unique between two JVMs running at the same time.
• unique even if the JVM is restarted.
• thread-safe.
• support null? if not, use int or long.

if only int is required then AtomicInteger can make it possible.

if String is needed then the below code should work by mixing timeStamp and AtomicLong.

AtomicLong idCounter =  new AtomicLong(100);
long timestamp = System.currentTimeMillis();
long nextLong = idCounter.incrementAndGet();
String randomId = String.valueOf(timestamp)+String.valueOf(nextLong); 

Unique at any time:

int uniqueId = (int) (System.currentTimeMillis() & 0xfffffff);