Java如何实现多线程同步？

现在主线程到了某个位置时创建并启动了3个子线程：t1、t2和t3，仅当t1、t2、t3都完成自己任务（比如它们都要计算1到100之和）之后主线程才能执行其下面的工作（比如将t1、t2和t3的运算结果加起来并输出）。请问该如何实现这个功能？

听说好像要用到wait()、notify()，但不知道如何用到上面的例子上。

t1.join()
t2.join()
t3.join()

[quote]
仅当t1、t2、t3都完成自己任务（比如它们都要计算1到100之和）之后主线程才能执行其下面的工作
[/quote]

[code="java"]package org.demo.tmp;

/**

线程同步测试
@author
@date 2010-12-5
@file org.demo.tmp.Test.java
*/
public class Test1 {
/**
- @param args / public static void main(String[] args)throws Exception { / init 3 task [这里可直接用CountDownLatch]*/ CountDown count = new CountDown(3); Task1 t1 = new Task1(count); Task1 t2 = new Task1(count); Task1 t3 = new Task1(count); /* init thread and start / new Thread(t1, "Thread one").start(); new Thread(t2, "Thread two").start(); new Thread(t3, "Thread three").start(); / wait all threads done / synchronized (count) { count.wait(); } / print the result / int result = t1.getTotal() + t2.getTotal() + t3.getTotal(); System.out.println(">> result = " + result); } } /*
用来计数的类
@author
/
class CountDown {
/ count */
private int count;
public CountDown(int value){
this.count = value;
}
public void sub(int number){
count -= number;
}
public int getValue(){
return count;
}
}
/**
线程需要执行的任务
@author
*/
class Task1 implements Runnable{
/* total */
private int total = 0;
/* count */
private CountDown count;
/**
- constructor
- @param count */ public Task1(CountDown count){ this.count = count; }
@Override
public void run() {
int sum = 0;
for (int i=1; i<=100; i++){
sum += i;
}
this.total = sum;
System.out.println(Thread.currentThread().getName() + " done.");
synchronized (count) {
count.sub(1);
if (count.getValue() <= 0){
count.notify();
}
}

}
/**
- get total value
- @return */ public int getTotal(){ return total; } }[/code]