How do i impliment this array

Question

Hey everyone, just came across this forum looks quite good so I'll give it a go smile.gif
Just working through a book called 'Objects First with Java' and I'm stuck on exercise 4.31.
This is about a small auction program that has 4 classes - auction, bid, lot and person.
Here's the code:
Auction:
import java.util.ArrayList;
import java.util.Iterator;
* A simple model of an auction.
* The auction maintains a list of lots of arbitrary length.
* @author David J. Barnes and Michael Kolling.
* @version 2008.03.30
public class Auction
    // The list of Lots in this auction.
    private ArrayList<Lot> lots;
    // The number that will be given to the next lot entered
    // into this auction.
    private int nextLotNumber;
    private Lot selectedLot;
     * Create a new auction.
    public Auction()
        lots = new ArrayList<Lot>();
        nextLotNumber = 1;
     * Enter a new lot into the auction.
     * @param description A description of the lot.
    public void enterLot(String description)
        lots.add(new Lot(nextLotNumber, description));
        nextLotNumber++;
     * Show the full list of lots in this auction.
    public void showLots()
        for(Lot lot : lots) {
            System.out.println(lot.toString());
     * Bid for a lot.
     * A message indicating whether the bid is successful or not
     * is printed.
     * @param number The lot number being bid for.
     * @param bidder The person bidding for the lot.
     * @param value  The value of the bid.
    public void bidFor(int lotNumber, Person bidder, long value)
        selectedLot = getLot(lotNumber);
        if(selectedLot != null) {
            boolean successful = selectedLot.bidFor(new Bid(bidder, value));
            if(successful) {
                System.out.println("The bid for lot number " +
                                   lotNumber + " was successful.");
            else {
                // Report which bid is higher.
                Bid highestBid = selectedLot.getHighestBid();
                System.out.println("Lot number: " + lotNumber +
                                   " already has a bid of: " +
                                   highestBid.getValue());
     * Return the lot with the given number. Return null
     * if a lot with this number does not exist.
     * @param lotNumber The number of the lot to return.
    public Lot getLot(int lotNumber){
        if((lotNumber >= 1) && (lotNumber < nextLotNumber)) {
            // The number seems to be reasonable.
            Lot selectedLot = lots.get(lotNumber - 1);
            // Include a confidence check to be sure we have the
            // right lot.
            if(selectedLot.getNumber() != lotNumber) {
                System.out.println("Internal error: Lot number " +
                                   selectedLot.getNumber() +
                                   " was returned instead of " +
                                   lotNumber);
                // Don't return an invalid lot.
                selectedLot = null;
            return selectedLot;
        else {
            System.out.println("Lot number: " + lotNumber +
                               " does not exist.");
            return null;
    public void close() {
        for(Lot lot : lots){
            Bid highestBid=lot.getHighestBid();
            Person person=highestBid.getBidder();
            String personName=person.getName();
            String number = "" + lot.getNumber();
            if(lot.getHighestBid() == null) {
                System.out.println(number + ": " + lot.getDescription()+" has *not* been sold");
            else {
                System.out.println(lot.getDescription()+" has been sold to " );
    // Creates a new array list and stores all unsold items into notSold.
    // Returns all items in the new list.
   /* public ArrayList<Lot> getUnsold;
        ArrayList<String> notSold;
        Integer numberList = 1;
        notSold = new ArrayList<String>(); // Creates new array
        for(Lot lot : lots){ // Although it compiles, when I create an auction object
                             // it returns 'null pointer exception' here, any suggestions?
            if(lot.getHighestBid() == null)
                notSold.add(lot.getDescription());
                for(String unsold : notSold){
                    System.out.println(numberList + ": " + unsold);
                    System.out.println();
                    numberList++;
}        There are a few random things in there, just from me mucking around in other exercises.
Lot:
* A class to model an item (or set of items) in an
* auction: a lot.
* @author David J. Barnes and Michael Kolling.
* @version 2008.03.30
public class Lot
    // A unique identifying number.
    private final int number;
    // A description of the lot.
    private String description;
    // The current highest bid for this lot.
    private Bid highestBid;
     * Construct a Lot, setting its number and description.
     * @param number The lot number.
     * @param description A description of this lot.
    public Lot(int number, String description)
        this.number = number;
        this.description = description;
     * Attempt to bid for this lot. A successful bid
     * must have a value higher than any existing bid.
     * @param bid A new bid.
     * @return true if successful, false otherwise
    public boolean bidFor(Bid bid)
        if((highestBid == null) ||
               (bid.getValue() > highestBid.getValue())) {
            // This bid is the best so far.
            highestBid = bid;
            return true;
        else {
            return false;
     * @return A string representation of this lot's details.
    public String toString()
        String details = number + ": " + description;
        if(highestBid != null) {
            details += "    Bid: " +
                       highestBid.getValue();
        else {
            details += "    (No bid)";
        return details;
     * @return The lot's number.
    public int getNumber()
        return number;
     * @return The lot's description.
    public String getDescription()
        return description;
     * @return The highest bid for this lot.
     *         This could be null if there is
     *         no current bid.
    public Bid getHighestBid()
        return highestBid;
}Bid:
* A class that models an auction bid.
* It contains a reference to the Person bidding and the amount bid.
* @author David J. Barnes and Michael Kolling.
* @version 2008.03.30
public class Bid
    // The person making the bid.
    private final Person bidder;
    // The value of the bid. This could be a large number so
    // the long type has been used.
    private final long value;
     * Create a bid.
     * @param bidder Who is bidding for the lot.
     * @param value The value of the bid.
    public Bid(Person bidder, long value)
        this.bidder = bidder;
        this.value = value;
     * @return The bidder.
    public Person getBidder()
        return bidder;
     * @return The value of the bid.
    public long getValue()
        return value;
}Person:
public class Person
    // The name of this person.
    private final String name;
     * Create a new person with the given name.
     * @param name The person's name.
    public Person(String name)
        this.name = name;
     * @return The person's name.
    public String getName()
        return name;
}Question: Rewrite getLot so that it does not rely on a lot with a particular number being stored at index (number-1) in the collection. For instance, if lot number 2 has been removed, then lot number 3 will have been moved from index 2 to index 1, and all higher lot numbers will also have been moved by one index position. You may assume that lots are always stored in increasing order of their lot number.
So I know it's talking about an arraylist but I can't think how to implement it, I have tried a number of ways but can't seem to get my head round it. After I couldn't work out how to rewrite just the getLot I started spreading out and trying to add new arraylists in different classes and calling them but it ultimately just failed so now I was just wanting to get some ideas from people.

Answer 1

@ The OP....
Word of advice. Posting pages of code that are provided for YOU are not gonna be welcome on these forums. You can however take snippets of code and post them here and in turn ask a more precise question. For example....
I modified this method..
*post code
and get the following error in compiler
*post error