No enclosing instance of type ...
小弟新学java，请教这段代码哪里出问题了，谢谢啊

public class College {

public class Student{
String sname=null;
String sid=null;
Student(){}

void Setname(String name){
this.sname=name;
}
void nprint(){
System.out.println( "sname "+ sname);

}
}

public class Mstudent extends Student{
String mname=null;
void setmname(String name){
this.mname=name;

}
void mprint(){
System.out.println( "Mname "+mname);

}
}
public class Ustudent extends Student{
String fname=null;
void setfname (String name){
this.fname=name;
}
void fprint(){
System.out.println( "Fname "+ fname);
}
}


public static void main(String[] args ){

College s=new College();
Student s1= new Student();
s1.Setname( "zhou ");
s1.nprint();

}
}



[解决办法]
public class College {

public static class Student{
String sname=null;
String sid=null;
Student(){}

void Setname(String name){
this.sname=name;
}
void nprint(){
System.out.println( "sname "+ sname);

}
}

public class Mstudent extends Student{
String mname=null;
void setmname(String name){
this.mname=name;

}
void mprint(){
System.out.println( "Mname "+mname);

}
}
public class Ustudent extends Student{
String fname=null;
void setfname (String name){
this.fname=name;
}
void fprint(){
System.out.println( "Fname "+ fname);
}
}


public static void main(String[] args ){

College s=new College();
Student s1= new College.Student();
s1.Setname( "zhou ");
s1.nprint();

}
}

红色是我加上的,这样应该可以了,呵呵