Generating a 64 bit unique integer in Java

Question

I need to generate a 64 bit unique integer in Java. I need to make sure that there is very few or no collisions if possible.

I came up with the below code which works fine:

public class TestUniqueness {

    private static final AtomicLong TS = new AtomicLong();

    public static void main(String[] args) {
        // for testing, just added the for loop
        for (int i = 1; i <= 100000; i++) {
            System.out.println(getUniqueTimestamp());
        }
    }

    public static long getUniqueTimestamp() {
        long micros = System.currentTimeMillis() * 1000;
        for (;;) {
            long value = TS.get();
            if (micros <= value)
                micros = value + 1;
            if (TS.compareAndSet(value, micros))
                return micros;
        }
    }
}

I will be running the above code in production.

Answer 1

My specification is only to generate the 64 bit unique integer, that's it.

In this case, there's no need for anything more complicated than atomically incrementing a counter:

public class Counter {

    private static final AtomicLong counter = new AtomicLong(0);

    public static long getNextNumber(){
        return counter.incrementAndGet();
    }
}

To offer a more specific critique of your code, it's unnecessarily complicated and inefficient. I don't see anything incorrect about it (i.e. it seems like it meets your criteria), but there's the old saying about obviously no bugs vs no obvious bugs (paraphrased).

Answer 2

Just some points:

for (;;) {
    long value = TS.get();
    if (micros <= value)
        micros = value + 1;
    if (TS.compareAndSet(value, micros))
        return micros;
}

Here, I suggest you do while(true) instead of for(;;). It is a matter of preference, but I think while(true) is easier to understand.

---

Also, always put braces for if statements. If you don't, horrible bugs may occur. Here is an example:

Say you have this if statement:

if(isSomething())
    doSomething();

And then you decide that the code has to do something else in the if statement:

if(isSomething())
    doSomething();
    doSomethingElse();

Now you have a horrible bug. It looks fine, but when you run it, doSomethingElse() will execute no matter what isSomething() returns. If you actually had braces:

if(isSomething()) {
    doSomething();
}

Then:

if(isSomething()) {
    doSomething();
    doSomethingElse();
}

That should work fine.

Answer 3

Is there any reason why you are not using the Apache library?

The RandomStringsUtils library provides a method for creating a random number.

The best part: you specify the range.

Library

By the way, the method is randomNumber()