Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
        at ShoppingCart.add(ShoppingCart.java:25)
        at ShoppingCart.main(ShoppingCart.java:99)

import java.util.Scanner; //Used to read user input from keyboard
import java.text.DecimalFormat; //Used to format output of decimal values

public class IceCreamOrder
{
//Private instance variable declarations
private String flavor;
private String vessel;
private String amount;
private double unitPrice;
private int quantity;

//Constructor declarations
public IceCreamOrder(String flavor, String vessel, String amount, double unitPrice, int quantity)
{
    this.flavor = flavor;
    this.vessel = vessel;
    this.amount = amount;
    this.unitPrice = unitPrice;
    this.quantity = quantity;
}

public IceCreamOrder(String flavor, String vessel, String amount, double unitPrice)
{
    this.flavor = flavor;
    this.vessel = vessel;
    this.amount = amount;
    this.unitPrice = unitPrice;
    this.quantity = 1;
}

public IceCreamOrder()
{
    this.flavor = "";
    this.vessel = "";
    this.amount = "";
    this.unitPrice = 0.0;
    this.quantity = 0;
}

//Calculates the total price of order
public double price()
{
    return quantity * unitPrice;
}

//Accessor method declarations
public String getFlavor()
{
    return flavor;
}

public String getVessel()
{
    return vessel;
}

public String getAmount()
{
    return amount;
}

public double getUnitPrice()
{
    return unitPrice;
}

public int getQuantity()
{
    return quantity;
}

//Mutator method declarations
public void setFlavor(String flavor)
{
    this.flavor = flavor;
}

public void setVessel(String vessel)
{
    this.vessel = vessel;
}

public void setAmount(String amount)
{
    this.amount = amount;
}

public void setUnitPrice(double unitPrice)
{
    this.unitPrice = unitPrice;
}

public void setQuantity(int quantity)
{
    this.quantity = quantity;
}

//toString method declaration
public String toString()
{
    DecimalFormat pattern0dot00 = new DecimalFormat("$0.00");

    return (((getQuantity() == 1) ? (getQuantity() + " order") : (getQuantity() + " orders")) + " of " +
            getAmount() + " of " + getFlavor() + " ice cream in a " + getVessel() + " for " +
            pattern0dot00.format(price()) + " = " + getQuantity() + " x " + getUnitPrice());
}

public static void main(String[] args)
{
    //Object declarations
    IceCreamOrder newOrder = new IceCreamOrder();
    Scanner keyboard = new Scanner(System.in);

    //Array declarations
    String[] flavorList = {"Avocado", "Banana", "Chocolate", "Hazelnut", "Lemon", "Mango", "Mocha", "Vanilla"};
    String[] vesselList = {"Cone", "Cup", "Sundae"};
    String[] amountList = {"Single Scoop", "Double Scoop", "Triple Scoop"};
    String[] quantityList = {"One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten"};

    System.out.println("Placing an order is as easy as ABC, and D.");
    System.out.println("Step A: Select your favorite flavour");
    String outputFlavor = "";
    for (int i = 1; i <= flavorList.length; i++)
    {
        outputFlavor = "    (" + i + ") " + flavorList[i-1];
        System.out.println(outputFlavor);
    }

    System.out.print("?-> Enter an option number: ");

    int inputFlavor = keyboard.nextInt();
    String flavorString = "";

    switch (inputFlavor)
    {
        case 1:
            flavorString = flavorList[0];
            break;
        case 2:
            flavorString = flavorList[1];
            break;
        case 3:
            flavorString = flavorList[2];
            break;
        case 4:
            flavorString = flavorList[3];
            break;
        case 5:
            flavorString = flavorList[4];
            break;
        case 6:
            flavorString = flavorList[5];
            break;
        case 7:
            flavorString = flavorList[6];
            break;
        case 8:
            flavorString = flavorList[7];
            break;
    }

    newOrder.setFlavor(flavorString);
    System.out.println();

    System.out.println("Step B: Select a vessel for your ice cream:");
    String outputVessel = "";
    for (int i = 1; i <= vesselList.length; i++)
    {
        outputVessel = "   (" + i + ") " + vesselList[i-1];
        System.out.println(outputVessel);
    }

    System.out.print("?-> Enter an option number: ");

    int inputVessel = keyboard.nextInt();
    String vesselString = "";

    switch (inputVessel)
    {
        case 1:
            vesselString = vesselList[0];
            break;
        case 2:
            vesselString = vesselList[1];
            break;
        case 3:
            vesselString = vesselList[2];
            break;
    }

    newOrder.setVessel(vesselString);
    System.out.println();

    System.out.println("Step C: How much ice cream?");
    String outputAmount = "";
    for (int i = 1; i <= amountList.length; i++)
    {
        outputAmount = "   (" + i + ") " + amountList[i-1];
        System.out.println(outputAmount);
    }

    System.out.print("?-> Enter an option number: ");

    int inputAmount = keyboard.nextInt();
    String amountString = "";

    switch (inputAmount)
    {
        case 1:
            amountString = amountList[0];
            break;
        case 2:
            amountString = amountList[1];
            break;
        case 3:
            amountString = amountList[2];
            break;
    }

    newOrder.setAmount(amountString);
    System.out.println();

    System.out.println("Step D: How many orders of your current selection?");
    String outputQuantity = "";
    for (int i = 1; i <= quantityList.length; i++)
    {
        outputQuantity = "   (" + i + ") " + quantityList[i-1];
        System.out.println(outputQuantity);
    }

    System.out.print("?-> Enter how many orders: ");

    int inputQuantity = keyboard.nextInt();
    newOrder.setQuantity(inputQuantity);
    System.out.println();

    if (newOrder.getAmount() == amountList[0])
    {
        if (newOrder.getVessel() == vesselList[0])
        {
            newOrder.setUnitPrice(2.99);
        }

        else if (newOrder.getVessel() == vesselList[1])
        {
            newOrder.setUnitPrice(3.49);
        }

        else
        {
            newOrder.setUnitPrice(4.25);
        }
    }

    else if (newOrder.getAmount() == amountList[1])
    {
        if (newOrder.getVessel() == vesselList[0])
        {
            newOrder.setUnitPrice(3.99);
        }

        else if (newOrder.getVessel() == vesselList[1])
        {
            newOrder.setUnitPrice(4.49);
        }

        else
        {
            newOrder.setUnitPrice(5.25);
        }
    }

    else
    {
        if (newOrder.getVessel() == vesselList[0])
        {
            newOrder.setUnitPrice(4.99);
        }

        else if (newOrder.getVessel() == vesselList[1])
        {
            newOrder.setUnitPrice(5.49);
        }

        else
        {
            newOrder.setUnitPrice(6.25);
        }
    }

    System.out.println(newOrder);
}

public class ShoppingCart
{
private IceCreamOrder[] shoppingCart;
private int maxQuantity;
private int orderTracker;

//Constructor declarations
public ShoppingCart()
{
    this.shoppingCart = new IceCreamOrder[maxQuantity];
    this.maxQuantity = 5;
    this.orderTracker = 1;
}

public void add(IceCreamOrder order)
{
    if (orderTracker > maxQuantity)
    {
        System.out.println("Shopping cart is full.");
    }

    else
    {
        shoppingCart[orderTracker - 1] = order;
        orderTracker++;
    }
}

//Method determines if shopping cart is empty
public boolean isEmpty()
{
    int orderCount = 0;

    for (int i = 0; i < shoppingCart.length; i++)
    {
        if (shoppingCart[i] != null)
        {
            orderCount++;
        }
    }

    return ((orderCount == 0) ? true : false);
}

//Method determines if shopping cart is full
public boolean isFull()
{
    int orderCount = 0;

    for (int i = 0; i < shoppingCart.length; i++)
    {
        if (shoppingCart[i] != null)
        {
            orderCount++;
        }
    }

    return ((orderCount == maxQuantity) ? true : false);
}

public IceCreamOrder get(int position)
{
    return shoppingCart[position-1];
}

//Method determines the number of orders currently in shopping cart
public int size()
{
    int orderCount = 0;

    for (int i = 0; i < shoppingCart.length; i++)
    {
        if (shoppingCart[i] != null)
        {
            orderCount++;
        }
    }

    return orderCount;
}

public static void main(String[] args)
{
    ShoppingCart newCart = new ShoppingCart();
    IceCreamOrder firstOrder = new IceCreamOrder("Vanilla", "Cone", "Single Scoop", 2.99);
    IceCreamOrder secondOrder = new IceCreamOrder("Vanilla", "Cone", "Single Scoop", 2.99);
    IceCreamOrder thirdOrder = new IceCreamOrder("Vanilla", "Cone", "Single Scoop", 2.99);
    IceCreamOrder fourthOrder = new IceCreamOrder("Vanilla", "Cone", "Single Scoop", 2.99);
    IceCreamOrder fifthOrder = new IceCreamOrder("Vanilla", "Cone", "Single Scoop", 2.99);
    //IceCreamOrder sixthOrder = new IceCreamOrder("Vanilla", "Cone", "Single Scoop", 2.99);

    newCart.add(firstOrder);
    newCart.add(secondOrder);
    newCart.add(thirdOrder);
    newCart.add(fourthOrder);
    newCart.add(fifthOrder);
    //newCart.add(sixthOrder);*/

    System.out.println(newCart.size());
    System.out.println(firstOrder);
    System.out.println(newCart.isEmpty());
    System.out.println(newCart.isFull());
}