Exception in thread "main"

Question

package ch7;
import  java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;class Student  implements Serializable{
  private static final long SerialVersionUID=1L;
  int id;
  String name;
  int age;
  String depart;
  public Student( int id,String name,int age,String depart)
  { this.id=id;
    this.name=name;
    this.age=age;
    this.depart=depart;
    }
  }public class SerialDemo{
  public static void main(String args[]) throws Exception
   {  Student stu = new Student(20051064, "zhang shan", 20, "CQUCS");      FileOutputStream fout = new FileOutputStream("d:data1.txt");     ObjectOutputStream oout = new ObjectOutputStream(fout);     oout.writeObject(stu); // 输出对象
     oout.close();
     stu = null;
     FileInputStream fin = new FileInputStream("d:data1.txt");     ObjectInputStream oin = new ObjectInputStream(fin);
     stu = (Student) oin.readObject(); // 读入对象
     oin.close();
                 System.out.println("学生信息：");
                 System.out.println("ID：        " + stu.id);
                 System.out.println("name：      " + stu.name);
                 System.out.println("age：       " + stu.age);
                 System.out.println("depart：    " + stu.depart);         }
 }运行结果是 java.long.NoSuchMethodError:main
Exception in thread "main"
我有写main呀，怎么有错误？怎么解决？