// 主线程
public class Father extends Thread {
  public void run() {
    Son s = new Son();
    s.start();
    s.join();
    ...
  }
}
// 子线程
public class Son extends Thread {
  public void run() {
    ...
  }
}

public final void join() throws InterruptedException {
  join(0);
}
public final synchronized void join(long millis)
throws InterruptedException {
  long base = System.currentTimeMillis();
  long now = 0;
  if (millis < 0) {
    throw new IllegalArgumentException("timeout value is negative");
  }
  if (millis == 0) {
    while (isAlive()) {
      wait(0);
    }
  } else {
    while (isAlive()) {
      long delay = millis - now;
      if (delay <= 0) {
        break;
      }
      wait(delay);
      now = System.currentTimeMillis() - base;
    }
  }
}

// JoinTest.java的源码
public class JoinTest{ 
  public static void main(String[] args){ 
    try {
      ThreadA t1 = new ThreadA("t1"); // 新建“线程t1”
      t1.start();           // 启动“线程t1”
      t1.join();            // 将“线程t1”加入到“主线程main”中，并且“主线程main()会等待它的完成”
      System.out.printf("%s finish\n", Thread.currentThread().getName()); 
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  } 
  static class ThreadA extends Thread{
    public ThreadA(String name){ 
      super(name); 
    } 
    public void run(){ 
      System.out.printf("%s start\n", this.getName()); 
      // 延时操作
      for(int i=0; i <1000000; i++)
        ;
      System.out.printf("%s finish\n", this.getName()); 
    } 
  } 
}