Updating quantity
- Java course
- Beginner
- Lesson 51/116
- 6 min
Manage cart with updateQuantity and removeItem.
Updating quantity
In this lesson we'll learn to update and remove an item in the cart. cart.set(i, newValue) replaces the i-th item; cart.remove(i) deletes it:
cart.set(0, "Apple x2"); // replaces first item
cart.remove(1); // deletes second itemIndexes start at 0. set changes the item but keeps the size; remove deletes the item and shrinks the size by one. After removal, indexes shift; so pay attention to the removal order.
Index and ordering
set and remove take an index. Indexes start at 0: cart.get(0) is the first, cart.get(1) the second item.
ArrayList<String> cart = new ArrayList<>();
cart.add("Apple");
cart.add("Bread");
cart.set(0, "Apple x2"); // [Apple x2, Bread]
cart.remove(1); // [Apple x2]Use set(0, ...) to replace the first item, remove(1) to delete the second. After removal, the remaining items' indexes shift.
Practice Update and remove items
Add to the previous code: update the first item with cart.set(0, "Apple x2"), delete the second with cart.remove(1). Print the updated cart with System.out.println(cart). (Keep the price and total lines.)
Starter code
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<String> cart = new ArrayList<>();
cart.add("Apple");
cart.add("Bread");
System.out.println(cart);
System.out.println(cart.size());
ArrayList<Double> prices = new ArrayList<>();
prices.add(10.0);
prices.add(20.0);
double subtotal = 0;
for (double price : prices) {
subtotal += price;
}
double tax = subtotal * 0.10;
double total = subtotal + tax;
System.out.println("Subtotal: " + subtotal);
System.out.println("Tax: " + tax);
System.out.println("Total: " + total);
// Update the cart: change one item's quantity and remove one
}
}Sample solution
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<String> cart = new ArrayList<>();
cart.add("Apple");
cart.add("Bread");
cart.set(0, "Apple x2");
cart.remove(1);
ArrayList<Double> prices = new ArrayList<>();
prices.add(10.0);
prices.add(20.0);
double subtotal = 0;
for (double price : prices) {
subtotal += price;
}
double tax = subtotal * 0.10;
double total = subtotal + tax;
System.out.println(cart);
System.out.println("Subtotal: " + subtotal);
System.out.println("Total: " + total);
}
}Expected output
[Apple x2] Subtotal: 30.0 Total: 33.0
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the update methods
| Term | Meaning |
|---|---|
set |
Replace item |
remove |
Delete item |
add |
Add item |
size() |
Item count |
2 What does cart.set(0, "Apple x2") do?
- Adds Apple x2 to the cart
- Removes Apple x2 from the cart
- Sorts the cart
- Makes the first item Apple x2 Correct answer
Explanation: set(0, ...) replaces the first item.
3 Complete the remove call
cart.____(1);cart.remove(1);Explanation: cart.remove(1) deletes the second item.