Java : How do I use add in Arraylist?

In your switch statement, you refer to a variable called cart. But no such variable exists at that point.

The only cart you have is defined within your for loop. So each time through the loop, you instantiate a new cart (a new ArrayList object), and immediately discard it. At loop’s end, any variables defined within the loop, and never shared outside the loop, instantly become candidates for garbage-collection, become unreachable to you, and will eventually be deleted by the garbage collector.

At the point the flow-of-control reaches your switch, no cart object remains available to your code.

👉 Move the definition of the cart list to outside the for loop.

By the way, following the naming conventions (initial uppercase for class, initial lowercase for methods & vars) and indenting conventions of Java will make your code easier to read and comprehend.

cart.add(new item(quantity, name, price)); //gets error in "cart" 

This will not be accessible as you have defined that inside a local scope.

Check below out , if this caters your requirement :

import java.util.ArrayList;
import java.util.Scanner;

public class Item {


    private int quantity;
    private String name;
    private double price;

    public Item(){

    }
    public Item(int quantity, String name, double price) {
        this.quantity = quantity;
        this.name = name;
        this.price = price;
    }

    public static void main(String[] args)
    {
        item();

    }//end main


    public static void item()
    {
        Scanner myinput = new Scanner(System.in);
        int total ;
        int quantity;
        String name = null;
        double price;
        String option;
        ArrayList<Item> cart = new ArrayList<Item>();

        System.out.println("How many kind of items do you have?");
        total = myinput.nextInt();

        for (int i = 0; i<=total; i++)
        {
            System.out.println("Please type the quantity of your item(s) (ex : If you have 2 shirts, please type 2)");
            Item item=new Item();
            item.quantity = myinput.nextInt();
            if (item.quantity== 0)
            {
                break;
            }

            System.out.println("Please type the name of your item(s)");
            item.name = myinput.next();

            System.out.println("Please type the price of your item(s)");
            item.price = myinput.nextDouble();

            cart.add(item);
            //ArrayList<Item> cart = new ArrayList<Item>(i);

        }


        System.out.println("Please choose one option below \n1) Add item(s) \n2) Remove item(s) \n3) View Cart \n4) Checkout \n5) Exit");
        option = myinput.next();

        switch(option)
        {
            case "1" :
                System.out.print("Please type the name of your item(s)");
                name = myinput.next();

                System.out.print("Please type the price of your item(s)");
                price = myinput.nextDouble();

                System.out.print("Please type the quantity of your item(s)");
                quantity = myinput.nextInt();

                cart.add(new Item(quantity, name, price)); //gets error in "cart"
                for(int i=0;i<cart.size();i++){
                    System.out.println(cart.get(i));
                }
                break;
        }


    }//end item

    @Override
    public String toString() {
        return "Item{" +
                "quantity=" + quantity +
                ", name='" + name + '\'' +
                ", price=" + price +
                '}';
    }
}