// threads/SerialNumberChecker.java
// (c)2016 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://mindviewinc.com/Books/OnJava/ for more book information.
// Operations that might seem safe are not,
// when threads are present
// {java SerialNumberChecker 4}
import java.util.concurrent.*;

// Reuses storage so we don't run out of memory:
class CircularSet {
  private int[] array;
  private int len;
  private int index = 0;
  public CircularSet(int size) {
    array = new int[size];
    len = size;
    // Initialize to a value not produced
    // by the SerialNumberSupplier:
    for(int i = 0; i < size; i++)
      array[i] = -1;
  }
  public synchronized void add(int i) {
    array[index] = i;
    // Wrap index and write over old elements:
    index = ++index % len;
  }
  public synchronized boolean contains(int val) {
    for(int i = 0; i < len; i++)
      if(array[i] == val) return true;
    return false;
  }
}

public class SerialNumberChecker {
  private static final int SIZE = 10;
  private static CircularSet serials =
    new CircularSet(1000);
  private static ExecutorService exec =
    Executors.newCachedThreadPool();
  static class SerialChecker implements Runnable {
    @Override
    public void run() {
      while(true) {
        int serial =
          SerialNumberSupplier.nextSerialNumber();
        if(serials.contains(serial)) {
          System.out.println("Duplicate: " + serial);
          System.exit(0);
        }
        serials.add(serial);
      }
    }
  }
  public static void
  main(String[] args) throws Exception {
    for(int i = 0; i < SIZE; i++)
      exec.execute(new SerialChecker());
    // Stop after n seconds if there's an argument:
    if(args.length > 0) {
      TimeUnit.SECONDS.sleep(new Integer(args[0]));
      System.out.println("No duplicates detected");
      System.exit(0);
    }
  }
}
/* Output:
Duplicate: 4119
*/