java的Thread.Join，是把指定的线程加入到当前线程，也就是如果调用了某个线程的join方法，则后续线程必须等待该线程执行完成后才能执行。

假设有三个线程，同时执行并输出当前线程的名称，输出后让线程sleep100毫秒，由于是并发执行，我们无法控制其输出的顺序，代码如下：

public class ThreadJoinDemo {
	
	class MyThread extends Thread {
		
		private String threadName;
		
		public MyThread(String threadName) {
			this.threadName = threadName;
		}
		
		@Override
		public void run() {
			System.out.println(threadName);
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		
	}
	
	public static void main(String[] args) {
		ThreadJoinDemo.MyThread t1 = new ThreadJoinDemo().new MyThread("t1");
		ThreadJoinDemo.MyThread t2 = new ThreadJoinDemo().new MyThread("t2");
		ThreadJoinDemo.MyThread t3 = new ThreadJoinDemo().new MyThread("t3");
		
		t1.start();
		t2.start();
		t3.start();
	}

}

执行结果：
Thread name: t2
Thread name: t1
Thread name: t3

如果在每个线程启动后调用join方法，则后续线程必须等待该线程执行完成后才可以执行，就可以实现顺序输出的要求了。

public class ThreadJoinDemo {
	
	class MyThread extends Thread {
		
		private String threadName;
		
		public MyThread(String threadName) {
			this.threadName = threadName;
		}
		
		@Override
		public void run() {
			System.out.println("Thread name: " + threadName);
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		
	}
	
	public static void main(String[] args) throws InterruptedException {
		ThreadJoinDemo.MyThread t1 = new ThreadJoinDemo().new MyThread("t1");
		ThreadJoinDemo.MyThread t2 = new ThreadJoinDemo().new MyThread("t2");
		ThreadJoinDemo.MyThread t3 = new ThreadJoinDemo().new MyThread("t3");
		
		t1.start();
		t1.join();
		t2.start();
		t2.join();
		t3.start();
	}

}

执行结果：
Thread name: t1
Thread name: t2
Thread name: t3