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问：Thread的jion方法什么用？

答：请看Javadoc，明确说明：等待线程终止。假如另一个线程中断了当前线程，就会抛出InterruptedException，并且当前线程的终端状态被清除。

示例代码一：

public class TestThread {
    public static void main(String arg[]) {
        System.out.println("main started.");
        Thread[] ts = new Thread[] { new WaitThread(1000), new WaitThread(2000) };
        for(Thread t : ts)t.start();
        try {
            ts[0].join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("main finished.");
    }
    static class WaitThread extends Thread {
        private int wait;
        public WaitThread(int wait) {
            this.wait = wait;
            setName("Wait" + wait);
        }
        @Override
        public void run() {
            System.out.println(getName() + " started ");
            try {
                Thread.sleep(wait);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(getName() + " finished ");
        }
    }
}

运行结果一：

main started.

Wait1000 started

Wait2000 started

Wait1000 finished

main finished.（注意这里的位置，main线程在Wait2000之前结束了）

Wait2000 finished

把代码一稍作修改：

ts[0].join();

之后添加一行

ts[1].join();

等待两个线程结束。

运行结果二：

main started.

Wait1000 started

Wait2000 started

Wait1000 finished

Wait2000 finished

main finished.（main线程在Wait2000之后结束。）