I am trying to build a palindrome checker using stacks and a Linked List. I am using generics in order to reuse the nodes and structures to complete two parts of this assignment (to accomplish something different in the next part).
The program is not pushing the letters onto the stack- it is returning nulls. I believe the problem is with the construction of the push method, either in the LinkedStack construction, or the StackDriver implementation, or both. I am just not sure what I am doing wrong; I have tried a multitude of alternatives, and have looked up and tried other methods to construct the push methods but then I get errors and can’t get the program to run at all. (I realize the 2 push methods I have here are different- these are 2 versions I have tried). Should I be looking at some type of boxing for the char c to use the wrapper class?
The program has been set back to the last point at which it ran. The “reversed” popped element seems to be getting the correct amount of characters though- why?
I realize there are other problems with this program as well but I feel like I can’t address them until I get past this stumbling block. Any assistance would be appreciated- thank you!
Mike
The given interface:
public interface Stack<E> {
void push(E data);
E pop();
boolean isEmpty();
int size();
E stackTop();
void clear();
}
The Node and methods:
public class Node<E> {
// create the node structure
private E data;
private Node<E> next;
// getters and setters
public E getData() {
return data;
}
public void setData(E data) {
this.data = data;
}
public Node<E> getNext() {
return next;
}
public void setNext(Node<E> next) {
this.next = next;
}
}
The Stack:
import java.util.EmptyStackException;
public class LinkedStack<E> implements Stack<E>{
// Create the head and nodeCount variables
private Node<E> head;
private int nodeCount;
// also need to be able to convert letters to capitals.
// constructor for the LinkedStack
public LinkedStack()
{
clear();
}
// A method to push the data onto a stack and increment the node count
public void push(E data) {
head = new Node<E>();
nodeCount++;
}
// pop the head off of the stack and decrement the node count
public E pop() {
E item;
if (head == null)
throw new EmptyStackException();
item = head.getData();
head = head.getNext();
nodeCount--;
return item;
}
// Check if the stack is empty
public boolean isEmpty() {
if (head == null);
return true;
}
// check the size of the node
public int size() {
return nodeCount;
}
// this is the peek method
public E stackTop()
{
if (head == null)
throw new EmptyStackException();
return head.getData();
}
// clear the Linked Stack
public void clear() {
head = null;
nodeCount = 0;
}
// convert to text
public String toString() {
String rtn = "";
if (nodeCount == 0) {
rtn += "<empty>";
}
else {
Node<E> t = head;
while (t != null){
/* return the node data on a line with the head node data
at the beginning of the line and the arrow pointing to
each successive node*/
rtn += t.getData() + "->";
// go on to the next
t = t.getNext();
}
rtn += "null";
}
return rtn;
}
}
And the driver:
import java.util.Iterator;
import java.util.Scanner;
public class StackDriver<E> implements Iterator<E>{
/**
* @param args
*/
public static void main(String[] args) {
//Initialize the driver
StackDriver run = new StackDriver();
run.doIt();
}
public void doIt() {
// gather the input
Scanner keyboard = new Scanner(System.in);
System.out.println("Please enter a phrase. This program will verify" +
" if the phrase is a palindrome.");
// holder for the phrase
String phrase;
// holder for the reversed phrase
String reversed = "";
phrase = keyboard.nextLine().toUpperCase();
System.out.println("You entered: "+ phrase);
// create the two stacks for the characters
LinkedStack<E> alpha = new LinkedStack<E>();
LinkedStack<E> punctuation = new LinkedStack<E>();
//------------------------------------------
for(int i=0; i<phrase.length(); i++)
{
// if the character is a letter, push it onto the letters stack
char c = phrase.charAt(i);
if (true == Character.isLetter(c))
{
// (testing purposes only- remove next line)
System.out.println("LETTER");
String A = Character.toString(c);
// push the letter onto the stack
alpha.push((E) new Node<E>());
}
// else push it onto the characters stack
else
{
// (testing purposes only- remove next line)
System.out.println("NOT LETTER");
String B = Character.toString(c);
// push the character onto the stack
punctuation.push((E) new String(B));
}
// then pop the letters stack
while (!alpha.isEmpty());
{
reversed += alpha.pop();
}
}
//------------------------------------------
// if it equals the String phrase
if (reversed == phrase)
// it is a palindrome
System.out.println("The phrase you entered is a palindrome");
else
System.out.println("The phrase you entered is NOT a palindrome");
System.out.println("phrase: " + phrase);
System.out.println("alpha: " + alpha);
System.out.println("reversed: " + reversed);
}
@Override
public boolean hasNext() {
// TODO Auto-generated method stub
return true;
}
@Override
public E next() {
// TODO Auto-generated method stub
return null;
}
@Override
public void remove() {
// TODO Auto-generated method stub
}
}
And the result:
Please enter a phrase. This program will verify if the phrase is a palindrome.
mom
You entered: MOM
LETTER
LETTER
LETTER
The phrase you entered is NOT a palindrome
phrase: MOM
alpha: <empty>
reversed: nullnullnull
