Java application grocery ordering logic with OOP

I am doing a Groceries Ordering System and have a problem regarding the ordering logic.

Currently, the program will ask customers how many groceries they want to order but I am finding is that a way to ordering without specifying the number of how many orders want to make at the beginning.

Does anyone have any idea? Thank you.

Below is the driver class Shop.java

package groceries.ordering.system;
import java.util.*;
public class Shop {
public static void main(String[] args){
Scanner myScanner = new Scanner(System.in);
Category[] groceries = {new Category("Apple 1pcs", 1.5, "Fruit"),
new Category("Orange 1pcs", 1.4, "Fruit"),
new Category("Egg 10pcs", 8, "Dairy"),
new Category("Full Cream Milk", 6.5, "Dairy"),
new Category("Olive Oil 1L", 30, "Cooking Oil"),
new Category("Palm Oil 1L", 20, "Cooking Oil")};
displayMenu(groceries);
System.out.print("How many groceries you would like to order: ");
int totalGroceries = myScanner.nextInt();
OrderItem[] items = new OrderItem[totalGroceries];
for(int i=0;i<items.length;i++){
System.out.println("Enter your choice of groceries (1 - 6) : ");
int choice = myScanner.nextInt()-1;
System.out.println("Enter quantity ordered: ");
int quantity = myScanner.nextInt();
items[i] = new OrderItem(groceries[choice],quantity);
}
Order order = new Order(items,totalGroceries);
displayOrderItem(items);
System.out.printf("%65s %9.2f\n", "Grand Total: ", order.getGrandTotalPrice());
} 
public static void displayMenu(Groceries[] groceries){
System.out.printf("\t========================================\n");
System.out.printf("\tWelcome to the Groceries Ordering System\n");
System.out.printf("\t========================================\n");
System.out.printf("%-10s %-25s %-11s %s \n\n", "No", "Groceries", "Price(RM)", "Category");
for(Groceries grocery : groceries){
System.out.println(grocery.displayMenu());
}
System.out.println("\n");
}
public static void displayOrderItem(OrderItem[] items){
System.out.printf("\n\t\t\t\t Ordered Item\n");
System.out.printf("\t\t\t\t===============\n");
System.out.printf("%-10s %-20s %-14s %-13s %-13s\n", "No","Groceries", "Price(RM)","Quantity","Total Price(RM)");
System.out.printf("--------------------------------------------------------------------------------------\n");
for(OrderItem item : items){
System.out.println(item.displayOrder());
}
System.out.printf("--------------------------------------------------------------------------------------\n");
}     
}

And this the Groceries class (will not posting the subclass(category) for this class as I think do not have any related)

package groceries.ordering.system;
public abstract class Groceries {
private String name;
private double price;
public Groceries() {
this("",0);
}
public Groceries(String prodName, double prodPrice){
super();
name = prodName;
price = prodPrice;
}
public String getName() {
return name;
}
public double getPrice() {
return price;
}
public String displayMenu(){
return String.format("%-25s %-11.2f ", name,price);
}
}

This is the OrderItem class

public class OrderItem {
private Groceries groceries;
private int quantity;
private static int prodCount = 1;
public OrderItem(Groceries groceries, int quantity) {
this.groceries = groceries;
this.quantity = quantity;
}
public double getUnitPrice(){
return groceries.getPrice();
}
public double getSubTotalPrice(){
return getUnitPrice() * quantity;
}
public String displayOrder() {
return String.format("%-10d %-20s %9.2f %13d %19.2f",prodCount++, groceries.getName(),
getUnitPrice(),quantity,getSubTotalPrice());
}
}

and lastly the Order class

public class Order {
private OrderItem[] items;
private int totalItem;
public Order(OrderItem[] items, int totalItem) {
this.items = items;
this.totalItem = totalItem;
}
public double getGrandTotalPrice() {
double price = 0.0;
for (OrderItem item : items) {
price += item.getSubTotalPrice();
}
return price;
}
}

Sure. You can do it in two ways.

1. Create a very large array beforehand (Not recommended).
2. Use some List (ArrayList or LinkedList or something else) and keep adding items till the users are done.

<br/>

List<OrderItem> items = new ArrayList<>();
do {
System.out.println("Enter your choice of groceries (1 - 6) : ");
int choice = myScanner.nextInt()-1;
System.out.println("Enter quantity ordered: ");
int quantity = myScanner.nextInt();
items.add(new OrderItem(groceries[choice],quantity));
// compute shouldContinue - something like asking the user to enter -1 to quit.
} while(shouldContinue);

Yes , Use List implementation either ArrayList or LinkedList in place below array and use its functions:

Replace this - OrderItem[] items = new OrderItem[totalGroceries];

with ArrayList- List<OrderItem> items = new ArrayList<>();

You can use CopyOnWriteArrayList as well if you want to run this application in multi threaded environment.