java 多线程小练

在网上看到这样一道试题,关于多线程的,拿来小练一下
题目:有一个南北向的桥，只能容纳一个人，现桥的两边分别有10人和12人，编制一个多线程序让这些人到达对岸，每个人用一个线程表示，桥为共享资源。在过桥的过程中显示谁在过桥及其走向。

以下是我的代码

package test;

public class MyThread3 extends Thread {

	private String name;
	private String direction;

	public MyThread3(String name, String direction) {
		this.direction = direction;
		this.name = name;
		System.out.println(direction + "方的" + name + "开始过桥...");
	}

	public synchronized void run() {
		try {
			Thread.currentThread().sleep(100);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println(this.direction + "方的" + this.name + "过桥完成!"+"---"+Thread.currentThread().getName());
	}

	public static void main(String[] args) throws InterruptedException {
		MyThread3[] ths = new MyThread3[12];
		int i = 0;
		boolean flag = true;
		while (i <ths.length) {
			if (i < 10)
				ths[i] = new MyThread3("第" + (i+1) + "个人",flag == false ? "北" : "南");
			else
				ths[i] = new MyThread3("第" + (i+1) + "个人", "南");
			ths[i].start();
			ths[i].join();
			ths[i].sleep(1000);
			flag = !flag;
			if (flag == true||i>=10)
				i++;
		}
	}
}

对于synchronized 关键字用法还没完全搞懂,在run中可有可无的