了解 join（） 方法示例

Java thread join（） 方法让我有点困惑。我有以下示例

class MyThread extends Thread {
    private String name;
    private int sleepTime;
    private Thread waitsFor;

    MyThread(String name, int stime, Thread wa) { … }

    public void run() {
        System.out.print("["+name+" ");

        try { Thread.sleep(sleepTime); }
        catch(InterruptedException ie) { }

        System.out.print(name+"? ");

        if (!(waitsFor == null))
        try { waitsFor.join(); }
        catch(InterruptedException ie) { }

        System.out.print(name+"] ");

和

public class JoinTest2 {
    public static void main (String [] args) {
        Thread t1 = new MyThread("1",1000,null);
        Thread t2 = new MyThread("2",4000,t1);
        Thread t3 = new MyThread("3",600,t2);
        Thread t4 = new MyThread("4",500,t3);
        t1.start();
        t2.start();
        t3.start();
        t4.start();
    }
}

线程以什么顺序终止？