Java Tutorial · Collections · Lesson 25 of 37

Java ArrayList

A resizable list: add, get, set, remove, loop and sort.

ArrayList

ArrayList is a list that grows as you add items. Declare variables with the List interface and choose the implementation on the right:

Example 1

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> todo = new ArrayList<>();
        todo.add("Write code");
        todo.add("Test");
        todo.add(0, "Plan");          // insert at index 0
        todo.set(2, "Write tests");
        System.out.println(todo + " size=" + todo.size());
        System.out.println(todo.get(1) + " " + todo.contains("Test") + " " + todo.indexOf("Write tests"));
        todo.remove("Plan");
        for (String item : todo) System.out.println("- " + item);
        List<Integer> nums = new ArrayList<>(List.of(5, 3, 9, 1));
        Collections.sort(nums);
        System.out.println(nums);
        nums.removeIf(n -> n > 4);
        System.out.println(nums);
    }
}

Output

[Plan, Write code, Write tests] size=3
Write code false 2
- Write code
- Write tests
[1, 3, 5, 9]
[1, 3]

Run this on your computer — see “Get Started” for setup.

Collections hold objects, so use wrapper types: List<Integer>, not List<int>.

Immutable lists

List.of(...) creates an unmodifiable list — adding to it throws UnsupportedOperationException. Use it for fixed data:

Example 2

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> days = List.of("Mon", "Tue", "Wed");
        System.out.println(days);
        try {
            days.add("Thu");
        } catch (UnsupportedOperationException e) {
            System.out.println("List.of() lists are immutable");
        }
        List<String> copy = new ArrayList<>(days);
        copy.add("Thu");
        System.out.println(copy);
    }
}

Output

[Mon, Tue, Wed]
List.of() lists are immutable
[Mon, Tue, Wed, Thu]

Run this on your computer — see “Get Started” for setup.

LinkedList also implements List, but ArrayList is faster for almost every real use case.

Test yourself

Why can't you write List<int>?

int is too small
Generics only work with objects, so use List<Integer>
Lists can't hold numbers
You can — it's allowed

Show answer

Answer: Generics only work with objects, so use List<Integer>