生成单调递增的整数

Question

I need to generate monotonically increasing integers.

Could I use the timestamp to somehow generate such type of integer sequence?

I would request an integer at a time, and I won't be requesting more than an integer in the same second's interval - if I do, I won't mind if it passes me the same integer within that second's interval.

Answer 1

You can use an AtomicInteger object to maintain a thread safe counter. Then use getAndIncrement() when you need the next integer.

Answer 2

Since monotonically increasing integers do not need to be contiguous (ie there can be gaps, as long as the number keeps increasing), and it sounds like you want all calls made in the same second to return the same integer, a method that returns how many seconds the JVM has been up would do nicely.

Here's a simple implementation that does that:

private static long startTime = System.currentTimeMillis();

public static int secondsSinceStart() {
    return (int) TimeUnit.SECONDS.convert(
        System.currentTimeMillis() - startTime, TimeUnit.MILLISECONDS);
}

FYI, this would last 68 years before rolling over.

Answer 3

This is my self made generator...

  public final class IdGenerator {

  private static final int MAGNITUDE = 10000;
  private static long previousTimestamp;

  private static int counter = 0;

  private IdGenerator() {
  }

  public synchronized static long generateId() {
    final long timeMillis = System.currentTimeMillis();
    if (previousTimestamp != timeMillis) {
      counter = 0;
    }
    previousTimestamp = timeMillis;

    final int counterValue = counter++;
    if (counterValue >= MAGNITUDE) {
      //just to be sure
      throw new IllegalStateException("too many id generated for a single timestamp!");
    }

    return timeMillis * MAGNITUDE + counterValue;
  }

}