Cheking/Printing a List
i created a deck of cards and then made a method which creates a CardList(a list) out of a couple of those cards. I wanna check if thisa works. i try to print it out in my driver(ill post it) and im not sure if im printing it incorrectly or if my list has a problem. can anyone help?
Deck Class
publicclass Deck{
//variables
private Card deck[];
privateint deckMarker = 0;
private CardList someCards;
//accesors
public Deck(){
makeDeck();
}
//methods
public Card[] makeDeck(){
int count = 0;
deck =new Card[40];
for(int suit = 1; suit <= 4; suit++){
for(int value = 1; value <= 10; value++){
deck[count] =new Card(value, suit);
count++;
}
}
return deck;
}
publicvoid shuffle(){
for (int i = 39; i > 0; i-- ){
int rand = (int)(Math.random()*(i+1));
Card temp = deck[i];
deck[i] = deck[rand];
deck[rand] = temp;
}
}
public CardList draw (int n){
deckMarker += n;
System.out.println(deckMarker);
CardList someCards = CardList.NIL;
for(int i=0;i < deckMarker;i++){
System.out.println(i);
someCards = someCards.push(deck[i]);
}
return someCards;
}
public String toString(){
String s ="Deck ";
for (int i = 40 - 1; i >= 0; i--)
s += deck[i] +" ";
return s;
}
}
heres where i try to print it
publicclass CardTest{
publicstaticvoid main(String ars[]){
Deck aDeck =new Deck();
//aDeck.shuffle();
System.out.println(aDeck);
System.out.println(aDeck.draw(3));
}
}
i getthis error
Exception in thread"main" java.lang.NullPointerException
at List.toString(List.java:57)
at List.toString(List.java:57)
at List.toString(List.java:57)
at List.toString(List.java:57)
at java.lang.String.valueOf(String.java:2827)
at java.io.PrintStream.println(PrintStream.java:771)
at CardTest.main(CardTest.java:11)
[b]heres line 57[/b]
[code]public String toString (){
return this.isEmpty() ?"()" :"(" + this.head().toString() +
(this.tail().isEmpty() ?"" :" ") + this.tail().toString().substring(1);
}
thanks guys
If your List class is not too big, could you post it as well?
here is the List class(basically CardList just fOr all objects), btw CardList implements List, so its basicaly just same methods just for Cards:
public class List implements Comparable {
protected Object data;
protected List next;
public static final List NIL = new List();
public List (Object d, List n) { data = d; next = n; }
public List (Object d) { this(d, NIL); }
protected List () { }
public List nil () {
return NIL;
}
public List push (Object d) {
return new List(d, this);
}
public Object head () { return data; }
public List tail () { return next; }
public boolean isEmpty () {
return this == nil();
}
public int length () {
return this.isEmpty() ? 0 : 1 + this.tail().length();
}
public List find (Object d) {
return this.isEmpty() || d.equals(this.head()) ? this : this.tail().find(d);
}
public List append (List a) {
return this.isEmpty() ? a : this.tail().append(a).push(this.head());
}
public List reverse () {
return this.isEmpty() ? this : this.tail().reverse().append(nil().push(this.head()));
}
public Object nth (int n) {
if (this.isEmpty() || n < 0) return null;
return n == 0 ? this.head() : this.tail().nth(n - 1);
}
public List delete (Object d) {
if (this.isEmpty()) return this;
if (d.equals(this.head())) return this.tail().delete(d);
return this.tail().delete(d).push(this.head());
}
public List purge () {
return this.isEmpty() ? this : this.tail().delete(this.head()).purge().push(this.head());
}
public String toString () {
return this.isEmpty() ? "()" : "(" + this.head().toString() +
(this.tail().isEmpty() ? "" : " ") + this.tail().toString().substring(1);
}
public static List parseIntList (String s) {
IllegalArgumentException badArg = new IllegalArgumentException(s);
int openBracket = s.indexOf('(');
int closeBracket = s.indexOf(')');
if (openBracket != 0 || closeBracket != s.length() - 1) throw badArg;
String[] intStrings = s.substring(openBracket + 1, closeBracket).split(" ");
List result = NIL;
for (int i = intStrings.length - 1; i >= 0; i--)
result = result.push(Integer.parseInt(intStrings[i]));
return result;
}
private List firstN (int n) {
return n == 0 ? nil() : this.tail().firstN(n - 1).push(this.head());
}
public List firstHalf () {
return firstN(this.length() / 2);
}
private List remFirstN (int n) {
return n == 0 ? this : this.tail().remFirstN(n - 1);
}
public List lastHalf () {
return remFirstN(this.length() / 2);
}
public List merge (List m) {
if (this.isEmpty()) return m;
if (m.isEmpty()) return this;
if (((Comparable) this.head()).compareTo(m.head()) < 0)
return this.tail().merge(m).push(this.head());
return this.merge(m.tail()).push(m.head());
}
public List sort () {
if (this.isEmpty() || this.tail().isEmpty()) return this;
return (this.firstHalf().sort()).merge(this.lastHalf().sort());
}
public List insert (Comparable d) {
return this.isEmpty() || d.compareTo(this.head()) < 0 ?
this.push(d) :
this.tail().insert(d).push(this.head());
}
public int compareTo (Object o) {
if (!this.getClass().equals(o.getClass())) throw new ClassCastException("List compared to non-List");
List ol = (List) o;
if (this.isEmpty() && ol.isEmpty()) return 0;
if (this.isEmpty()) return -1;
if (ol.isEmpty()) return 1;
switch (((Comparable) this.head()).compareTo(ol.head())) {
case -1: return -1;
case 1: return 1;
default: return this.tail().compareTo(ol.tail());
}
}
public List powerSet () { return powerSet(nil()); }
public List powerSet (List selected) {
if (this.isEmpty()) return NIL.push(selected); // notice use of NIL as opposed to nil()
// this is because a powerSet is always a List, not
// the potential subclass
return this.tail().powerSet(selected).append(this.tail().powerSet(selected.push(this.head())));
}
}
> here is the List class(basically CardList just fOr
> all objects), btw CardList implements List, so its
> basicaly just same methods just for Cards:
>
You probably mean extends List, that monster can't be implemented since it's no interface.
My advice is to use some sort of collection from the java.util package: this thing is horrible! It seems to be some sort of likned list, but I don't see a Node class.
If you can't use a java.util.List implementation (if it's homework), then I suggest throwing this thing away and implement a proper (linked) list.
Sorry I couldn't be of more help. Good luck though.
your right it does extend it not implement it, sorry for that.Bt what do you mean by node class. do you mean my Card class? becuase i didntpost all the classes to this program
> your right it does extend it not implement it, sorry
> for that.Bt what do you mean by node class. do you
> mean my Card class?
> ...
No, I mean your List class seems to be some sort of a linked list. Linked list normally consist of Node objects (head and tail and everything in between). In your case a List has some sort of head and tail made out of type List: that's not the way to do it.
Here's a small demo of how you'd implement a linked list:class TestMyLinkedList {
public static void main(String[] args) {
MyLinkedList myList = new MyLinkedList();
myList.add("A");
myList.add("B");
myList.add("C");
System.out.println(myList);
}
}
class MyLinkedList {
private Node head, tail;
private int size;
public MyLinkedList() {
head = tail = null;
size = 0;
}
public void add(Object o) {
if(head == null) {
head = tail = new Node(o);
} else {
tail.next = new Node(o);
tail = tail.next;
}
size++;
}
// the rest of your methods like 'get(int index)', 'size()', etc.
public String toString() {
StringBuffer strb = new StringBuffer();
Node pointer = head;
while(pointer != null) {
strb.append(pointer);
strb.append(" --> ");
pointer = pointer.next;
}
strb.append("null");
return strb.toString();
}
}
class Node {
protected Object data;
protected Node next;
protected Node(Object d){ this.data = d; }
public boolean equals(Object o) { return data.equals(o); }
public int hashCode(){ return data.hashCode(); }
public String toString(){ return "["+data+"]";}
}
Everything in thisprogram now works, and according to the assignment it need to use that List class so as to the way you showed me how to make a linked list i dont think i can use that. the only prob i see now is my toString method i think. i run the CardTest class and i get this error:
Exception in thread "main" java.lang.NullPointerException
at List.toString(List.java:57)
at List.toString(List.java:57)
at List.toString(List.java:57)
at List.toString(List.java:57)
at java.lang.String.valueOf(String.java:2827)
at java.io.PrintStream.println(PrintStream.java:771)
at Deck.draw(Deck.java:43)
at CardTest.main(CardTest.java:11)
i showed my TA and he couldnt figfure it out he thnks it has to do with my toString method but i just dont know. anyone?
Could you please post the toString() method of your List class?
sorry, its in one of theprevious posts in he List class
> Everything in thisprogram now works, and according to
> the assignment it need to use that List class so as
> to the way you showed me how to make a linked list i
> dont think i can use that.
You don't need to use it: you need to see how a linked list works! Your own implementation of your List class has an attribute 'next' which is of type List, which is plain wrong!
public class List implements Comparable {
protected Object data;
protected List next; // <- !!!!!
// ...
}
Best of luck with your assignment.
