Module 7: Arrays and ArrayList

ArrayList Basics

Use `ArrayList` when you need a list that can grow and shrink as values are added or removed.

Author

Java Learner Editorial Team

Reviewer

Technical review by Java Learner

Last reviewed

2026-04-16

Java version

Java 25 LTS

How this lesson was prepared: AI-assisted draft, edited by hand, and checked against current Java 25 documentation and runnable examples.

Learning goals

Create an `ArrayList` with generics
Add, read, update, and remove elements
Understand why `ArrayList` is more flexible than a fixed array

ArrayList is a resizable list from the collections library: It solves the main limit of arrays, which is fixed length.

Generics make the element type clear: ArrayList<String> means the list should hold strings.

Common methods are simple and practical: add, get, set, remove, and size are the core ones to learn first.

Beginner rule: Use arrays when the size is truly fixed. Use ArrayList when the list may change.

Runnable examples

Add and remove items from an ArrayList

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> tasks = new ArrayList<>();
        tasks.add("Read");
        tasks.add("Practice");
        tasks.remove("Read");

        System.out.println(tasks.get(0));
        System.out.println(tasks.size());
    }
}

Expected output

Practice
1

Common mistakes

Forgetting to specify the element type

Use generics such as `ArrayList<String>` so the list stays type-safe.

Mini exercise

Create an `ArrayList<Integer>`, add three values, then print the second value.

Summary

`ArrayList` is a resizable alternative to arrays.
Generics show what type the list holds.
Core methods are `add`, `get`, `set`, `remove`, and `size`.

Next step

Finish the module by deciding when an array is better and when an `ArrayList` is better.

Sources used

ArrayList Basics

Add and remove items from an ArrayList

What is the main advantage of `ArrayList` over an array?