The solution is pretty easy.
It should be enough to iterate through the stack.
public String toString() {
String result = "[ Top = " + size +"]" + "[Stack = [";
if (top == null) {
return result + "]]";
Node temp = top;
while (temp != null) {
result += temp + ', '
temp = temp.next;
}
return result += temp.element + "]]";
}
Of course you should add at least getter methods to Node class, i.e. getElement() and getNext();
PS: the code isn't tested, but it should be fine.