Operations that may seem safe are not, when threads are present : Lock Synchronize « Threads « Java

Operations that may seem safe are not, when threads are present

Operations that may seem safe are not, when threads are present

// : c13:SerialNumberChecker.java
// Operations that may seem safe are not,
// when threads are present.
// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// www.BruceEckel.com. See copyright notice in CopyRight.txt.

// 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 SerialNumberGenerator:
    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 CircularSet serials = new CircularSet(1000);

  static class SerialChecker extends Thread {
    SerialChecker() {

    public void run() {
      while (true) {
        int serial = SerialNumberGenerator.nextSerialNumber();
        if (serials.contains(serial)) {
          System.out.println("Duplicate: " + serial);

  public static void main(String[] args) {
    for (int i = 0; i < 10; i++)
      new SerialChecker();
    // Stop after 4 seconds:
    new Timeout(4000, "No duplicates detected");
} ///:~
 class SerialNumberGenerator {
  private static volatile int serialNumber = 0;
  public static int nextSerialNumber() {
    return serialNumber++;
} ///:~
class Timeout extends java.util.Timer {
  public Timeout(int delay, final String msg) {
    super(true); // Daemon thread
    schedule(new TimerTask() {
      public void run() {
    }, delay);
} ///:~


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