Wrapper Classes: Integer, Double and Friends

Collections hold objects, not primitives

You can't write ArrayList<int>. Collections can only hold objects, so every primitive type has a wrapper class that wraps it in an object:

Java converts between them automatically. This is called autoboxing (and unboxing back):

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> nums = new ArrayList<>();
        nums.add(40);
        nums.add(15);
        nums.add(25);
        int total = 0;
        for (int n : nums) {
            total += n;
        }
        System.out.println(nums + " add up to " + total);
        System.out.println(Integer.MAX_VALUE);
    }
}

Output

[40, 15, 25] add up to 80
2147483647

The remove trap

With a list of Integers, remove(1) removes position 1, not the number 1. To remove the value, wrap it: remove(Integer.valueOf(1)):

import java.util.ArrayList;
import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> a = new ArrayList<>(Arrays.asList(5, 9, 1));
        a.remove(1);
        System.out.println(a);

        ArrayList<Integer> b = new ArrayList<>(Arrays.asList(5, 9, 1));
        b.remove(Integer.valueOf(1));
        System.out.println(b);
    }
}

Output

[5, 1]
[5, 9]

Tip: new ArrayList<>(Arrays.asList(5, 9, 1)) is a quick way to make a list that already has values.

Common mistake: Compare two Integer objects with .equals(), not == – just like Strings.