线程礼让是指当前正在运行的线程退出运行状态，暂时将运行权让给优先级相同或更高的线程。
调用yield（）方法实现礼让，他会将当前程序转到就绪状态。
yield（）方法不抛出任何异常。
class Test1 implements Runnable{
public void run(){
for(int i = 1 ;i <= 5 ;i++){
System.out .println("running thread: " + Thread.currentThread().getName() + ", i = " + i);
Thread.currentThread ().yield();
}
}
}
class Test2 implements Runnable{
public void run(){
for(int i = 1 ;i <= 5 ;i++){
System.out .println("running thread: " + Thread.currentThread().getName() + ", i = " + i);
}
}
}
public class ThreadDemo11{
public static void main (String[] args){
Test1 t1 = new Test1();
Thread th1 = new Thread(t1 ,"Thread-1");
Test2 t2 = new Test2();
Thread th2 = new Thread(t2 ,"Thread-2");
th1.start ();
th2.start ();
}
}
从理论上，t1线程运行的几率要小于t2线程，因为t1总是让其它线程先执行
