Beginner/Intermediate Java Code - NullPointerException and wrong output.

First off, this IS a homework assignment I have been working on.
I believe I'm close to finishing but can't seem to figure out this small bit.

The input:

Quote:

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buy 100 share(s) at $20 each;buy 20 share(s) at $24 each;buy 200 share(s) at $36 each;sell 150 share(s) at $30 each;buy 50 share(s) at $25 each;sell 200 share(s) at $35 each;

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The output should be $1070 (using FIFO protocol).

I can see the problem being something to do with my sell() method when trying to perform Stocklist.peek()...

I've tried different variations of code and seem to be stuck..

Any hints/help would be greatly appreciated.

Code :

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import java.util.*;
import java.text.*;
 
public class Stocks {
 
    private int shares;
    private int price;
    private int temp;
    private int finalPrice;
    private int finalShares;
    private static int total;
    private Queue<Stocks> StockList = new LinkedList<Stocks>();
 
    private static NumberFormat nf = NumberFormat.getCurrencyInstance();
 
    public Stocks()
    {
        shares      = 0;
        price       = 0;
 
    }
 
    public Stocks(int shares, int price)
    {
        this.shares     = shares;
        this.price      = price;
    }
 
    public int getShares()
    {
        return this.shares;
    }
 
    public int getPrice()
    {
        return this.price;
    }
 
    public void setShares(int shares)
    {
        this.shares = shares;
    }
 
    public void setPrice(int price)
    {
        this.price = price;
    }    
 
    public void sell() {
        int sharesToSell = this.getShares();
        int priceToSell = this.getPrice();
 
        while (!StockList.isEmpty() == false) { // **Something wrong starting here.
 
             int numShares = StockList.peek().getShares(); // **Throws NullPointerException
             int sharePrice = StockList.peek().getPrice();
 
            while (numShares < sharesToSell || numShares == sharesToSell) {
                temp = sharesToSell - numShares; // Remaining shares to sell
                finalShares = sharesToSell - temp; // # Shares selling at price
                finalPrice = priceToSell - sharePrice; // Shares sold at adjusted price                
                total += (finalPrice * finalShares); // Calculates total
                System.out.println(total);
 
            } sharesToSell = temp;
 
            if (numShares > sharesToSell) {
                temp = numShares - sharesToSell; // Remaining shares that were bought
                finalPrice = priceToSell - sharePrice; // Shares sold at adjusted price
                total += (finalPrice * sharesToSell); // Adds to running total
                StockList.peek().setShares(temp);
 
            }
        }
    }
 
    public void buy() { 
        int numShares = this.getShares();
        int priceToBuy = this.getPrice();
 
        Stocks newStock = new Stocks(numShares,priceToBuy);
        StockList.add(newStock); // adds stock to list
 
        total += (numShares * priceToBuy);
    }
 
    public static int getTotal() { // gets total profit (or loss)
        return total;
    }
 
    // *****MAIN METHOD*****
    public static void main(String[] args){
 
 
        Scanner scan = new Scanner(System.in);
 
        System.out.println("Enter transaction sequence:");
 
        String input = scan.nextLine().trim();
        String[] inputArray = new String[50];
        String[] inputArray2 = new String[50];
        String[] inputArray3 = new String[50];
        int numShares, sharePrice;
 
        inputArray = input.split(";"); // Splits input at ";"
                                      // and enters into inputArray
        for (String i : inputArray) { // Checks for BUY or SELL
            if (i.toUpperCase().contains("BUY")) { 
                inputArray2 = i.split(" ");
                inputArray2[4] = inputArray2[4].substring(1); // Removes $ sign
 
                try {
                    numShares = Integer.parseInt(inputArray2[1]);
                    sharePrice = Integer.parseInt(inputArray2[4]);
 
                    Stocks newStock = new Stocks(numShares,sharePrice);
                    newStock.buy();
 
                } catch (NumberFormatException e) {
                    System.out.println("Number Format Error");
                    return;
                }
 
            }
 
            else if (i.toUpperCase().contains("SELL")) {
                inputArray3 = i.split(" ");
                inputArray3[4] = inputArray3[4].substring(1);
 
                try {
                    numShares = Integer.parseInt(inputArray3[1]);
                    sharePrice = Integer.parseInt(inputArray3[4]);
 
                    Stocks newStock = new Stocks(numShares,sharePrice);
                    newStock.sell(); // **Must be problem with sell() method.
 
                } catch (NumberFormatException e) {
                    System.out.println("Number Format Error");
                    return;
                }
            } else {
                System.out.println("An error has been found.");
            }
        } 
        System.out.println();
        System.out.println("Total:"+nf.format(getTotal()));
    }
}

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Code :

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int numShares = StockList.peek().getShares();

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Most probably, this StockList.peek() is returning a null and you want a null to call getShares(). That is actually the problem. Good Luck

I think what I'm not understanding is why the the StockList.peek() returns null?

Shouldn't the list contain Stocks from the buy() method?

Quote:

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Originally Posted by tt52

I think what I'm not understanding is why the the StockList.peek() returns null?

Shouldn't the list contain Stocks from the buy() method?

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Did you try debugging your program?
If you don't know debugging, place print statements to find the value of your queue(list) at different locations.