// © Copyright 1996-1999, Joseph Bergin.  All rights reserved. 

package cs1;

public class SumVisitor implements LispwVisitor.Visitor
{	// Precondition: the list contains Integer objects only and no nulls.
	// Usage:
	//	SumVisitor visitor = new SumVisitor();
	//	 aLispwVisitor.accept(visitor);
	// Postcondition: visitor.sum() will be the sum of the elements in the list. 
	// Caution: Intended to be used once. There is no way to reinitialize the count
	// to zero after usage. 

	public Object apply(Node aList)
	{	sum += ((Integer) aList).intValue();
		aList.tail().accept(this); // This is the recursion step. 
	}
	
	public Object apply(NIL empty)
	{ // nothing for the nil object
	}
		
	public int sum(){ return sum;}
	
	private int sum = 0;
}