meet it

Lists hold objects, so numbers travel in a wrapper.

Angle brackets only take classes, and int is not a class. Put int between them and Java stops with unexpected type. Each plain type has a to use instead: for int, Double for double and Boolean for boolean.

You will hardly notice the wrapper. scores.add(7) wraps the 7 for you, and int first = scores.get(0); unwraps it again. This is called : Java pops the number in a little box and takes it out when you need it.

A for-each loop works on a list the same way it works on an array, so a running total looks the same as before.

import java.util.ArrayList;
public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> scores = new ArrayList<>();
        scores.add(7);
        scores.add(12);
        scores.add(5);
        int total = 0;
        for (int score : scores) {
            total = total + score;
        }
        System.out.println(scores + " add up to " + total);
    }
}

Integer and Double start with a capital letter, because they are classes.

guess first

Decimal prices in a list. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

import java.util.ArrayList;
public class Main {
    public static void main(String[] args) {
        ArrayList<Double> prices = new ArrayList<>();
        prices.add(1.5);
        prices.add(2.25);
        prices.add(3.0);
        double total = 0;
        for (double price : prices) {
            total = total + price;
        }
        System.out.println(prices.size() + " items cost " + total);
    }
}

your turn

Your turn. Find your best day.

Add three daily step counts, as whole numbers, on lines 5 to 7. Then press run to find your best day.

Your code

output

press run to see what happens

fix it

This list of ages will not even start. Fix it!

Broken code to fix

output

press run to see what happens

Java says unexpected type on line 4. Look between the angle brackets.
Angle brackets only take classes. int is a plain type, so it needs its wrapper class.
Change the int between the angle brackets to Integer on line 4, then press run.

nailed it

you wrapped numbers and kept them in a list. :D

Angle brackets take classes, so write Integer, not int.
Autoboxing wraps and unwraps numbers for you.
A for-each loop works on lists too.

new words in your collection

wrapper class
Integer
autoboxing

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