JAVA不允许一个类继承自多个类，为了解决这个问题，JAVA提供了java.lang.Runnable接口，它有一个run()方法：

1 package runimpl; 2 public class Machine implements Runnable{ 3   private int a=0; 4   public void run(){ 5     for(a=0;a<50;a++){ 6       System.out.println(Thread.currentThread().getName()+":"+a); 7       try{ 8         Thread.sleep(100); 9       }catch(InterruptedException e){
   throw new RuntimeException(e);}10     }11   }12   public static void main(String args[]){13     Machine machine=new Machine();14     Thread t1=new Thread(machine);15     Thread t2=new Thread(machine);16     t1.start();17     t2.start();18   }19 }