class MyThread extends Thread{ 
  public void run(){
    ...
  } 
};
MyThread mythread = new MyThread();

 // Demo.java 的源码
 class MyThread extends Thread{ 
   public MyThread(String name) {
     super(name);
   }
   public void run(){
     System.out.println(Thread.currentThread().getName()+" is running");
   } 
 }; 
 public class Demo { 
   public static void main(String[] args) { 
     Thread mythread=new MyThread("mythread");
     System.out.println(Thread.currentThread().getName()+" call mythread.run()");
     mythread.run();
     System.out.println(Thread.currentThread().getName()+" call mythread.start()");
     mythread.start();
   } 
 }

main call mythread.run()
main is running
main call mythread.start()
mythread is running

public synchronized void start() {
  // 如果线程不是"就绪状态"，则抛出异常！
  if (threadStatus != 0)
    throw new IllegalThreadStateException();
  // 将线程添加到ThreadGroup中
  group.add(this);
  boolean started = false;
  try {
    // 通过start0()启动线程
    start0();
    // 设置started标记
    started = true;
  } finally {
    try {
      if (!started) {
        group.threadStartFailed(this);
      }
    } catch (Throwable ignore) {
    }
  }
}

private native void start0(); 

public void run() {
  if (target != null) {
    target.run();
  }
}