ArrayList: a list that grows

A list that can grow

• An array has a fixed size. Once you make it, the length cannot change.
• An ArrayList is a list that can grow and shrink while the program runs.
• You must import it at the top of the file: import java.util.ArrayList;.

Making an ArrayList

• Write the type of the items inside <...>. This is the AP CSA way.
• ArrayList<String> holds text. ArrayList<Integer> holds whole numbers.
• A new list starts empty, with size 0.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<String>();
        System.out.println(names.size());   // 0
        names.add("Ann");
        names.add("Bo");
        System.out.println(names.size());   // 2
        System.out.println(names);          // [Ann, Bo]
    }
}

The five methods you need

• list.size() — how many items are in the list.
• list.add(x) — put x at the end of the list.
• list.get(i) — the item at position i (the first position is 0).
• list.set(i, x) — replace the item at position i with x.
• list.remove(i) — take out the item at position i; later items shift left.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> fruit = new ArrayList<String>();
        fruit.add("apple");
        fruit.add("pear");
        fruit.add("plum");
        System.out.println(fruit.get(0));   // apple
        fruit.set(1, "grape");              // pear -> grape
        fruit.remove(2);                    // take out "plum"
        System.out.println(fruit);          // [apple, grape]
    }
}

Numbers need Integer

• An ArrayList can only hold objects, not the simple type int.
• So we write ArrayList<Integer>. Integer is the object form of int.
• Java changes between int and Integer for you. This is called autoboxing.
• You can add(7) an int, and get(0) gives back a value you can use as int.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> nums = new ArrayList<Integer>();
        nums.add(10);          // int 10 is boxed into Integer
        nums.add(20);
        int first = nums.get(0);   // unboxed back to int
        System.out.println(first + nums.get(1));   // 30
    }
}

Two ways to traverse

• An index for-loop uses positions 0 to size() - 1. Use it when you need the index.
• An enhanced for-loop (for (int x : list)) reads each item in turn. Use it when you only need the values.
• Both visit every item once, in order.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> nums = new ArrayList<Integer>();
        nums.add(3);
        nums.add(5);
        nums.add(8);

        // index for-loop
        for (int i = 0; i < nums.size(); i++) {
            System.out.println("at " + i + ": " + nums.get(i));
        }

        // enhanced for-loop
        int total = 0;
        for (int x : nums) {
            total = total + x;
        }
        System.out.println("total " + total);   // total 16
    }
}

Common mistakes

• ArrayList uses .add, .get(i), .size() — not [].
• Store objects: use ArrayList<Integer>, not ArrayList<int>.

Now you try

• Each task pre-fills the class skeleton — write your code inside main, or complete the method shown.
• Press Run to compile and run, then Check answer.
• Your code compiles and runs on the server, so even the first run is fast.