import *;
import java.awt.*;
import java.awt.event.*;
public class Student
{
   
       public static void main(String args[])
       {
           Stu1 a=new Stu1(29065001L,"一","小明 ","男",22);

          System.out.println(a.toString());
       }
}

 class Stu1
{
                  
           long number;
           String classname;
           String name;
           String sex;
           int age;
        

public String toString()
{
   String s="学号：  "+number+"\n班级名： "+classname+"\n名字：  "+name+"\n性别：  "+sex+"\n年龄：  "+age;
   return s;
}     

}   
   

呵呵，你没写Stu1的构造方法。

同楼上

Stu1 a=new Stu1(29065001L,"一","小明 ","男",22);//你直接new的

class Stu1 {                  
           long number;
           String classname;
           String name;
           String sex;
           int age;
public void Stu1(Long l,String s1,String s2,String s3,int i){
    this.number = l;
    this.classname = s1;
    this.name = s2;
    this.sex = s3;
    this.age = i;
}
public String toString()
{
   String s="学号：  "+number+"\n班级名： "+classname+"\n名字：  "+name+"\n性别：  "+sex+"\n年龄：  "+age;
   return s;
}     

}    //可以了

同上！             Stu1 a=new Stu1(29065001L,"一","小明 ","男",22);必须有Stu1构造方法！ public Stu1(long l,String s1,String s2,String s3,int age)

class lampakhang{
public static void main(String[] args){
   System.out.println("我是一个新手，尝试学习Java。今天路过，收藏来学习，谢谢分享");
   }
}