关于Java多线程中启动线程问题

public class Example15_1 {
public static void main(String[] args) {
    // TODO Auto-generated method stub
     SpeakHello speakHello;
     SpeakNinhao speakNinhao;
     speakHello = new SpeakHello();
     speakNinhao= new SpeakNinhao();
     speakHello.start();
     speakNinhao.start();
     for(int i=1;i<=5;i++){
      System.out.println("大家好"+i+"");
     }
     }


}
 public class SpeakHello extends Thread {
 public void run(){
 for(int i=1;i<=7;i++){
 System.out.println("hello"+i+"");
 }
 }
 }
 public class SpeakNinhao {
 public void run(){
 for(int i=1;i<=7;i++){
 System.out.println("ninhao"+i+"");
 }
 }
 }

speakNinhao.start();这个语句Myeclipse里面报错:The method start()is undefined for the type 'SpeakNinhao'.我发现即使在SpeakNinhao类里面写一个start()也不能通过编译。谁知道如何改