One list, two names

Module 10 showed that a method gets a copy of an int argument, so changing the parameter doesn't change the caller's variable. Lists behave differently, and here's why.

A list variable doesn't hold the list's values. It holds a reference: a note of where the list is in the computer's memory. new ArrayList<>() makes the list, and the variable remembers where it is. Copying the variable copies the note, not the list:

ArrayList<String> mine = new ArrayList<>();
mine.add("tea");
ArrayList<String> yours = mine;
yours.add("coffee");
System.out.println(mine);
System.out.println(yours);

[tea, coffee]
[tea, coffee]

There's only one list here, with two names. Adding through yours changes the list mine refers to, because it's the same list.

A method can change the caller's list

A list parameter works the same way. The method gets a copy of the reference, and it leads to the caller's own list. Whatever the method does to that list, the caller sees:

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> prices = new ArrayList<>();
        prices.add(10);
        prices.add(25);
        raiseAll(prices, 2);
        System.out.println(prices);
    }

    public static void raiseAll(ArrayList<Integer> list, int amount) {
        for (int i = 0; i < list.size(); i++) {
            list.set(i, list.get(i) + amount);
        }
    }
}

[12, 27]

list.set(index, value) puts a new value at an index, in place of the old one. raiseAll returns nothing, and still main's list changed. That's handy when changing the list is the method's job. When it isn't, it's a nasty surprise, so a method that only needs to read a list (like groupPrice in the last step) should leave it alone.

A real copy

For a second, separate list, make a new list from the old one: new ArrayList<>(original) creates a new list with the same values in the same order.

ArrayList<String> original = new ArrayList<>();
original.add("tea");
ArrayList<String> copy = new ArrayList<>(original);
copy.add("coffee");
System.out.println(original);
System.out.println(copy);

[tea]
[tea, coffee]

Now there are two lists, and changing one leaves the other as it was.