想对后台线程多些了解...

后台线程和前台线程相伴相随....当前台线程运行结束后，假如后台线程还在运行，java虚拟机就会终止后台线程....教科书上一般对后台线程作上述的这样的描述...对后台线程有点不太理解，如果只有一个cpu，某一时刻只有一个线程占用cpu，那么前台和后台线程是否都符合这个原则，如果符合，那么前台线程和后台线程切换运行时，会用到一些方法，比如sleep，yield等，下面有段程序package withdaemon;
public class Machine extends Thread{
  private int a;
  private static int count;  public void start(){
    super.start();
    Thread deamon=new Thread(){  //匿名线程类
      public void run(){
        while(true){  //无限循环
          //每隔1秒把实例变量a设为0。
          reset();
          try{
            sleep(100);
          }catch(InterruptedException e){throw new RuntimeException(e);}
        }
      }
    };
    deamon.setDaemon(true);
    deamon.start();
  }
  public void reset(){a=0;}
  public void run(){
    while(true){
      System.out.println(getName()+":"+a++);
      if(count++==100)break;
      yield();
    }
  }
  public static void main(String args[])throws Exception{
    Machine machine=new Machine();
    machine.start();
  }
}
应该是主线程先启动后台线程，后台线程sleep后，前台线程再开始运行？不太明白后台线程的运行机制上面程序的结果为Thread-0:0
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Press any key to continue...
我将前台线程的yield()注释掉，结果为Thread-0:0
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前后台线程是一起运行的吗，后台线程的用途在哪？