Pls suggest a reason for a compile time error

Question

the following program gives a compile time error bcoz of the statement
((State)this).val=this.val. if u could suggest why?
public class Mainc
public static void main(String args[])
State st=new State();
System.out.println(st.getvalue());
State.Memento mem= st.memfunc();
st.altervalue();
System.out.println(st.getvalue());
mem.restore();
System.out.println(st.getvalue());
public static class State
protected int val=11;
int getvalue()
     return val;
void altervalue()
     val=(val+7)%31;
Memento memfunc()
     return new Memento();
class Memento
     int val;
Memento()
     this.val=state.this.val;
     void restore()
          ((State)this).val=this.val;
}

Answer 1

The statement :
((State)this).val
tries to cast Memento into its super class State which is inorrect [former is not latter's subclass]
restore method should be as follows:
void restore()
State.this.val=this.val;
}