卖出的票为负数的情况：

class MyThread22 implements Runnable {
    private int ticket = 10;

    public void run() {
        for (int i = 0; i < 100; i++) {
            if (ticket > 0) {
                try {
                   Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Saling: ticket= " + ticket--);
            }
        }
    }
}

public class SyncDemo01 {
    public static void main(String[] args) {
        MyThread22 mt = new MyThread22();
        Thread t1 = new Thread(mt);
        Thread t2 = new Thread(mt);
        Thread t3 = new Thread(mt);
        t1.start();
        t2.start();
        t3.start();
    }
}

使用同步方法解决此问题：

class MyThread30 implements Runnable {
    private int ticket = 20;

    public void run() {
        for (int i = 0; i < 100; i++) {
            this.sale();
        }
    }

    public synchronized void sale() {
        if (ticket > 0) {
            try {
                Thread.sleep(300);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("saling: ticket=" + ticket--);
        }
    }
}

public class SyncDemo03 {
    public static void main(String[] args) {
        MyThread30 mt = new MyThread30();
        Thread t1 = new Thread(mt);
        Thread t2 = new Thread(mt);
        Thread t3 = new Thread(mt);
        t1.start();
        t2.start();
        t3.start();
    }
}