单继承具有局限性，推荐使用Runnable接口，启动线程必须借用Thread类对象

new Thread(Run).start()

public void run(){for(int i=0;i<5;i++){ System.out.println("aa");}}public static void main(String[]args){ThreadDownload ra=new ThreadDownload();Thread it=new Thread(ra);it.start();//或者new Thread(new ThreadDownload()).start();for(int i=0;i<5;i++){ System.out.println("ff");}}