Unstacking can’t return an empty element?

Question

Code taken from a Data Structure booklet.

public class PilhaContig {
        private Item [] info;
        private int topo;

        public PilhaContig(int qte){
            this.topo = 0;
            this.info = new Item [qte];
        }
        public Item getInfo(){
            return this.info[this.topo-1];
        }
        public int getTopo(){
            return this.topo;
        }
        public boolean eVazia(){
            return (this.topo == 0);
        }
        public boolean eCheia(){
            return (this.topo == this.info.length);
        }
        public boolean empilhar (Item elem){
            if (this.eCheia()) 
                return false;
            else {
                this.info[this.topo]= elem;
                this.topo++;
                return true;
            }   
        }
        public Item desempilhar(){
            if (this.eVazia())
                return null;
            else{
                this.topo--;
                return this.info[this.topo];
            }
        }   

In the stack class the top variable points to the empty memory space above the last item inserted into the stack. If I pop the last item with this method (decreasing 1 at the top variable this.topo--; and returning the vector info with the index at the diminished top return this.info[this.topo];) I wouldn’t end up returning the last item from the stack instead of unpacking that item?

Answer 1

It’s a logical question, just read what you wrote.

The pointer topo is always pointing to the next free position on the stack. When you do the -1 it passes point to the last item and returns it. From then on this item is disregarded because soon after this number is no longer accessible by the algorithm written except to write a new item up there (function empilhar()).