List Java Help *Please*

[acash229]

Hey Guys,

This is my assignment:
1. Rename SLListPartial to SLList and complete it by writing code for the methods that are stubs. Do NOT use private methods addFirst, addAfter, getNode shown in the text

2. Write a SLLIstIter inner class for SLList. This class should implement Iterator, not ListIterator. Write methods hasNext, next, and remove for this class.

Test your code by

* reading the tokens in the file data.txt and adding them to an SLList of String
* traversing the list printing its content
* Removing from the list all tokens of length greater than 5
* traversing again the list printing its content


This my code that i have done:

package Lab_4;

import java.util.*;
import java.io.*;
import java.io.File;

/**
 *
 * @author Owner
 */
public class SLList<E> extends AbstractList<E> implements List<E>, Iterable<E> {

    /** data fields */
    private Node<E> head = null;
    private int size = 0;

    public static void main(String[] args) throws IOException {
        List<String> list = new SLList<String>();
        Scanner file = new Scanner(new File("data.txt"));

        while (file.hasNext()) {
            list.add(file.next());
        }
        System.out.println(list);
        Iterator<String> it = list.iterator();

        while (it.hasNext()) {
            String next = it.next();

            if (next.length() > 5) {
                it.remove();
            }
        }




        System.out.println(list);
    }

    private static class Node<E> {

        private E data;
        private Node<E> next;

        private Node(E data, Node<E> node) {
            this.data = data;
            this.next = node;
        }

        private Node(E data) {
            this(data, (Node<E>) null);
        }
    }

    @Override
    public String toString() {
        String output = "[";

        Node<E> curr = head;

        while (curr != null) {
            output += curr.data + ", ";
            curr = curr.next;
        }

        output = output.substring(0, output.length() - 2) + "]";

        return output;
    }

    @Override
    public int size() {
        return size;
    }

    @Override
    public int indexOf(Object obj) {
        /** do this one. Hint: To compare obj with the item referenced by
         * temp, use the following code:
         * if (temp.data.getClass() == obj.getClass() &&
        temp.data.equals( (E) obj)) {
        return cursor;
         */
        Node<E> curr = head;

        int idx = 0;

        while (curr != null) {
            if (curr.data.getClass() == obj.getClass() && curr.data.equals((E) obj)) {
                return idx;
            }
            curr = curr.next;
            idx++;
        }


        return -1;
    }

    @Override
    public void add(int index, E item) {

        // This is the following hints i have recieved but doesnt help me
        // Do this without using getNode, addFirst, or addLast
        /** algorithm
        1. if index is not valid (too small or too big) throw an exception
        2. Set temp to head and i to 0
        3. while i is less than index-1
        Advance temp down the list and increment i
        4. if index is 0
        insert at list head so head points to a new node
        else
        Insert a new node between temp and temp's successor
        5. Increment size
         */
        // size check
        if (index < 0 || index > size) {
            throw new IndexOutOfBoundsException("Index: " + index);
        }

        // case index = 0
        if (index == 0) {
            head = new Node<E>(item, head);
        } // case index > 0
        else {

            Node<E> curr = head;

            while (index > 1) // searching for node before location
            {
                index--;
                curr = curr.next;
            }

            curr.next = new Node<E>(item, curr.next);
        }

        // update size
        size++;
    }

    @Override
    public boolean add(E item) {
        add(size, item);
        return true;
    }

    @Override
    public E get(int index) {
        Node<E> curr = head;

        while (index > 0 && curr != null) {
            index--;
            curr = curr.next;
        }

        if (curr == null) {
            return null;
        } else {
            return curr.data;
        }
    }

    @Override
    public Iterator<E> iterator() {
        return new SLListIter<E>(this);
    }

    private class SLListIter<E> implements Iterator<E> {

        private Node<E> prevItem = null; // prev item is node before last item returned for remove()
        private Node<E> nextItem = null;
        private Node<E> lastItemReturned = null;
        // link to list to be able to remove head
        private SLList<E> list;

        public SLListIter(SLList<E> list) {
            // I need to fill this in
            nextItem = list.head;
            this.list = list;
            prevItem = list.head;
        }

        @Override
        public boolean hasNext() {
            return nextItem != null;
        }

        @Override
        public E next() {
            if (nextItem == null) {
                return null;
            } else {

                lastItemReturned = nextItem;
                nextItem = nextItem.next;

                // prev item must stay 2 behind next item
                if (nextItem != list.head && lastItemReturned != list.head) {
                    prevItem = prevItem.next;
                }

                return nextItem.data;

            }
        }

        @Override
        public void remove() {

            // if last item was the head, remove it
            if (lastItemReturned == list.head) {
                list.head = list.head.next;
                prevItem = list.head;
            } // otherwise, do normal remove
            else {
                prevItem.next = nextItem;
            }
            lastItemReturned.next = null;

            // update size
            list.size--;
        }
    }
}

This is the error i get:
run:
Exception in thread "main" java.io.FileNotFoundException: data.txt (The system cannot find the file specified)
at java.io.FileInputStream.open(Native Method)
at java.io.FileInputStream.<init>(FileInputStream.jav a:106)
at java.util.Scanner.<init>(Scanner.java:636)
at Lab_4.SLList.main(SLList.java:19)
Java Result: 1


Can Someone please help me

Thanks alot,

[Iman Reihanian]

in this issue you can see this message (The system cannot find the file specified), it means the file can not find via program then you have to use absolute path in this case, for example stead of data.txt you have to use D:\foo\bar\data.txt

[acash229]

I tried and still dont work

[Iman Reihanian]

if same you get older message, you have to use forward slash (/) stead of back slash (\) in path, it's better that you use a IDE like eclipse.

[acash229]

Okay i got it to read the file, but NOW i am getting this error:

Exception in thread "main" java.lang.NullPointerException
[joe, josephine, samuel, sue, ann, sid, cameron, daniel, john]
at Lab_4.SLList$Node.access$102(SLList.java:42)
at Lab_4.SLList$SLListIter.remove(SLList.java:219)
at Lab_4.SLList.main(SLList.java:32)

[Tolls]

Which line is line 42?

[acash229]

private static class Node<E> {

private E data;
private Node<E> next;

private Node(E data, Node<E> node) {
this.data = data;
this.next = node;
}

private Node(E data) {
this(data, (Node<E>) null);
}
}

Its that part^^^

[Norm]

Try debugging your code by adding some print outs where there can be undetected null variables. For example add these at the appropriate points:

System.out.println("next=" + next);

System.out.println("nextItem=" + nextItem);

[acash229]

What do those codes do
and what do you mean by appropriate spots?

[Norm]

appropriate spots

Just before the variable in the print out is used, where it could cause a NPE.

You have holes in your logic where you need to see if variables are null.
Printing them out will show you where

[acash229]

Does it go right here:

package Lab_4;

import java.util.*;
import java.io.*;
import java.io.File;

/**
 *
 * @author Owner
 */
public class SLList<E> extends AbstractList<E> implements List<E>, Iterable<E> {

    /** data fields */
    private Node<E> head = null;
    private int size = 0;

    public static void main(String[] args) throws IOException {
        List<String> list = new SLList<String>();
        Scanner file = new Scanner(new File("C:\\Users\\Akash Patel\\Desktop\\data.txt"));
        System.out.println("next=" + next);

System.out.println("nextItem=" + nextItem); 

        while (file.hasNext()) {
            list.add(file.next());
        }
        System.out.println(list);
        Iterator<String> it = list.iterator();

        while (it.hasNext()) {
            String next = it.next();

            if (next.length() > 5) {
                it.remove();
            }
        }




        System.out.println(list);
    }

    private static class Node<E> {

        private E data;
        private Node<E> next;

        private Node(E data, Node<E> node) {
            this.data = data;
            this.next = node;
        }

        private Node(E data) {
            this(data, (Node<E>) null);
        }
    }

    @Override
    public String toString() {
        String output = "[";

        Node<E> curr = head;

        while (curr != null) {
            output += curr.data + ", ";
            curr = curr.next;
        }

        output = output.substring(0, output.length() - 2) + "]";

        return output;
    }

    @Override
    public int size() {
        return size;
    }

    @Override
    public int indexOf(Object obj) {
        /** do this one. Hint: To compare obj with the item referenced by
         * temp, use the following code:
         * if (temp.data.getClass() == obj.getClass() &&
        temp.data.equals( (E) obj)) {
        return cursor;
         */
        Node<E> curr = head;

        int idx = 0;

        while (curr != null) {
            if (curr.data.getClass() == obj.getClass() && curr.data.equals((E) obj)) {
                return idx;
            }
            curr = curr.next;
            idx++;
        }


        return -1;
    }

    @Override
    public void add(int index, E item) {

        // This is the following hints i have recieved but doesnt help me
        // Do this without using getNode, addFirst, or addLast
        /** algorithm
        1. if index is not valid (too small or too big) throw an exception
        2. Set temp to head and i to 0
        3. while i is less than index-1
        Advance temp down the list and increment i
        4. if index is 0
        insert at list head so head points to a new node
        else
        Insert a new node between temp and temp's successor
        5. Increment size
         */
        // size check
        if (index < 0 || index > size) {
            throw new IndexOutOfBoundsException("Index: " + index);
        }

        // case index = 0
        if (index == 0) {
            head = new Node<E>(item, head);
        } // case index > 0
        else {

            Node<E> curr = head;

            while (index > 1) // searching for node before location
            {
                index--;
                curr = curr.next;
            }

            curr.next = new Node<E>(item, curr.next);
        }

        // update size
        size++;
    }

    @Override
    public boolean add(E item) {
        add(size, item);
        return true;
    }

    @Override
    public E get(int index) {
        Node<E> curr = head;

        while (index > 0 && curr != null) {
            index--;
            curr = curr.next;
        }

        if (curr == null) {
            return null;
        } else {
            return curr.data;
        }
    }

    @Override
    public Iterator<E> iterator() {
        return new SLListIter<E>(this);
    }

    private class SLListIter<E> implements Iterator<E> {

        private Node<E> prevItem = null; // prev item is node before last item returned for remove()
        private Node<E> nextItem = null;
        private Node<E> lastItemReturned = null;
        // link to list to be able to remove head
        private SLList<E> list;

        public SLListIter(SLList<E> list) {
            // I need to fill this in
            nextItem = list.head;
            this.list = list;
            prevItem = list.head;
        }

        @Override
        public boolean hasNext() {
            return nextItem != null;
        }

        @Override
        public E next() {
            if (nextItem == null) {
                return null;
            } else {

                lastItemReturned = nextItem;
                nextItem = nextItem.next;

                // prev item must stay 2 behind next item
                if (nextItem != list.head && lastItemReturned != list.head) {
                    prevItem = prevItem.next;
                }

                return nextItem.data;

            }
        }

        @Override
        public void remove() {

            // if last item was the head, remove it
            if (lastItemReturned == list.head) {
                list.head = list.head.next;
                prevItem = list.head;
            } // otherwise, do normal remove
            else {
                prevItem.next = nextItem;
            }
            lastItemReturned.next = null;

            // update size
            list.size--;
        }
    }
}

[Norm]

Does the code compile the way you show it? Are the variables defined there?

The two print out statements go in different locations.
Put each one just before where the variable being printed is referenced or just after where the variable is being changed.
There is more than one place where they should be placed.

[Fubarable]

For anyone considering helping, you may wish to see the cross-post here: Need Help (Java) Please - Dev Shed

and his previous interesting thread in this forum on the subject: SLLIstIter inner Help

For the OP, please notify us of all cross-posts.

[Norm]

Another idea for having others help you in debugging your code. Replace the Scanner reading an external file with an internal String:

String Data = "data1\ndata2\ndata3\nlastData\n";
Scanner file = new Scanner(Data); //new File("data.txt"));

[acash229]

I get the same error by doing that string.
any solution..its due in 1 hour

[Norm]

Yes that was not to solve the problem. It was a way to allow others to easily work on your problem without needing external files.

Have you tried doing the debugging using the print outs as I suggested earlier?
The print outs will show you where you have null variables so you can fix you code to handle the null values.

[acash229]

Yah i tried debugging but i says it cannot compile all of it.

Doesnt make sense, it prints out my data, but still got an error somewhere

[Norm]

it prints out my data, but still got an error somewhere

The print outs don't fix the problem, you do.
Add more print outs especially around where the NPE was.

Here is the print out with the print outs I added to your code:

list1=[data1, data2, data3, lastData]
nextItem=data=data2, next=data=data3, next=data=lastData, next=null
next=data2
next.length()= 5
nextItem=data=data3, next=data=lastData, next=null
next=data3
next.length()= 5
nextItem=data=lastData, next=null
next=lastData
next.length()= 8
nextItem=null
next=null
list2=[data1, data2, data3]

[Tolls]

private static class Node<E> {

private E data;
private Node<E> next;

private Node(E data, Node<E> node) {
this.data = data;
this.next = node;
}

private Node(E data) {
this(data, (Node<E>) null);
}
}

Its that part^^^

What?
All of that is line 42?
I find that slightly hard to believe...:)

[JosAH]

What?
All of that is line 42?
I find that slightly hard to believe...:)

Yep, some extremely long lines wrap like that; if the parts of the line are properly spaced out they look like the above ;-)

kind regards,

Jos