银行窗口取号系统，共享数据的线程安全性

对共享数据的访问和修改要在同步代码块里

同步代码块的代码是顺序执行的，所以能够保证共享数据的安全性

public class Demo {

    public static void main(String[] args) {

        Runnable runnable = new WindowRunnable();
        new Thread(runnable, "窗口1").start();
        new Thread(runnable, "窗口2").start();
        new Thread(runnable, "窗口3").start();

    }

    static class WindowRunnable implements Runnable {
        final Object monitor = new Object();
        
        int index = 0; // index是共享数据，对共享数据的访问和修改要在同步代码块里
        static final int MAX = 50;

        @Override
        public void run() {
            while (true) {
                synchronized (monitor) {
                    if (index < MAX) {
                        printMsg();
                        try {
                            Thread.sleep(100);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        }

        public void printMsg() {
            System.out.println(Thread.currentThread().getName() + ": " + (++index));
        }
    }
}