ArrayList in Java

Reading Time: 10 Minutes
Difficulty: Beginner

Topic Summary

ArrayList is a resizable array implementation of the List interface. Unlike regular Java arrays whose size is fixed at creation, ArrayList can grow and shrink automatically as you add or remove elements. It is the most commonly used collection in Java and is backed by an internal Object[] array.

What You'll Learn

What ArrayList is and how it differs from a regular array
How to create and use an ArrayList
Key methods: add(), remove(), get(), size(), contains(), sort()
How to iterate over an ArrayList
A real-world example: managing a student list

Prerequisites

Java Arrays basics
Collections Framework Overview (Lesson 01)
Generics (<E> type parameters)

Explanation

ArrayList vs Regular Array

Feature	Array	ArrayList
Size	Fixed at creation	Dynamic — grows/shrinks
Syntax	`int[] arr = new int[5]`	`ArrayList<Integer> list = new ArrayList<>()`
Primitives	Supports `int`, `double`, etc.	Stores only objects (`Integer`, `Double`)
Built-in methods	None	`add()`, `remove()`, `sort()`, many more
Performance	Slightly faster	Slight overhead from resizing
Part of	Language	`java.util` package

When to use ArrayList over arrays:

When you don't know the final size in advance
When you need to add/remove elements frequently
When you need built-in methods like sort(), contains(), indexOf()

How ArrayList Works Internally

ArrayList internally uses an Object[] array. Its default initial capacity is 10. When you add the 11th element:

ArrayList creates a new array with capacity = old capacity × 1.5 (approximately)
Copies all existing elements to the new array
Adds the new element

This resizing process is called "growing" and takes O(n) time when it happens, but it happens rarely enough that add() is amortized O(1).

Creating an ArrayList

// Empty ArrayList — can hold Strings
List<String> names = new ArrayList<>();

// With initial capacity (avoid resizing if you know roughly how many)
List<String> names2 = new ArrayList<>(100);

// Initialize from another collection
List<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));

Key Methods

`add(element)` — Add to End

Adds an element to the end. O(1) amortized.

`add(index, element)` — Insert at Position

Inserts at the given index, shifting all elements after it right. O(n).

`get(index)` — Access by Index

Returns the element at the given index. O(1) — this is ArrayList's strength.

`set(index, element)` — Replace at Position

Replaces the element at the given index with a new value.

`remove(index)` — Remove by Index

Removes and returns the element at the given index. Elements after shift left.

`remove(Object)` — Remove by Value

Removes the FIRST occurrence of the given value. Returns true if removed.

`size()` — Count Elements

Returns the current number of elements.

`isEmpty()` — Check if Empty

Returns true if the list has no elements.

`contains(element)` — Check Membership

Returns true if the list contains the given element. O(n).

`indexOf(element)` — Find Position

Returns the index of the first occurrence, or -1 if not found.

`clear()` — Remove All

Removes all elements from the list.

`Collections.sort(list)` — Sort

Sorts the list in natural order (ascending for numbers, alphabetical for strings).

`iterator()` / enhanced for-loop — Iteration

Iterates through all elements one by one.

Time Complexity Summary

Operation	Complexity
`get(index)`	O(1)
`add(end)`	O(1) amortized
`add(index)`	O(n)
`remove(index)`	O(n)
`contains()`	O(n)
`size()`	O(1)

Real-World Analogy

Think of ArrayList as a shopping list on a piece of paper.

A regular Java array is like a fixed-size grid on the paper — you draw 5 boxes and can only write 5 things. If you need a 6th item, you have to start over on a new piece of paper with 6 boxes.

An ArrayList is like a scrollable shopping list on your phone — you can keep adding items as long as you want. The app automatically gives you more space. You can insert an item anywhere, delete any item, and the list automatically adjusts.

Code Example

Example 1: ArrayList Basics

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

public class ArrayListBasics {
    public static void main(String[] args) {

        // Create an ArrayList
        List<String> fruits = new ArrayList<>();

        // add() — add elements
        fruits.add("Mango");
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");

        System.out.println("Fruits: " + fruits);           // [Mango, Apple, Banana, Cherry]
        System.out.println("Size: " + fruits.size());      // 4

        // get() — access by index
        System.out.println("Index 1: " + fruits.get(1));   // Apple

        // set() — update at index
        fruits.set(1, "Kiwi");
        System.out.println("After set: " + fruits);        // [Mango, Kiwi, Banana, Cherry]

        // add(index, element) — insert at position
        fruits.add(2, "Grapes");
        System.out.println("After insert: " + fruits);     // [Mango, Kiwi, Grapes, Banana, Cherry]

        // remove by index
        fruits.remove(0);
        System.out.println("After remove index 0: " + fruits); // [Kiwi, Grapes, Banana, Cherry]

        // remove by value
        fruits.remove("Grapes");
        System.out.println("After remove 'Grapes': " + fruits); // [Kiwi, Banana, Cherry]

        // contains() and indexOf()
        System.out.println("Contains 'Banana': " + fruits.contains("Banana")); // true
        System.out.println("Index of 'Cherry': " + fruits.indexOf("Cherry"));  // 2

        // Sort
        Collections.sort(fruits);
        System.out.println("Sorted: " + fruits);           // [Banana, Cherry, Kiwi]

        // isEmpty() and clear()
        System.out.println("Is empty: " + fruits.isEmpty()); // false
        fruits.clear();
        System.out.println("After clear: " + fruits);      // []
        System.out.println("Is empty: " + fruits.isEmpty()); // true
    }
}

Output

Fruits: [Mango, Apple, Banana, Cherry]
Size: 4
Index 1: Apple
After set: [Mango, Kiwi, Banana, Cherry]
After insert: [Mango, Kiwi, Grapes, Banana, Cherry]
After remove index 0: [Kiwi, Grapes, Banana, Cherry]
After remove 'Grapes': [Kiwi, Banana, Cherry]
Contains 'Banana': true
Index of 'Cherry': 2
Sorted: [Banana, Cherry, Kiwi]
Is empty: false
After clear: []
Is empty: true

Example 2: Real-World — Student List Manager

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

public class StudentListManager {

    public static void main(String[] args) {

        List<String> students = new ArrayList<>();

        // Enroll students
        students.add("Alice Johnson");
        students.add("Bob Smith");
        students.add("Charlie Brown");
        students.add("Diana Prince");
        students.add("Eve Wilson");

        System.out.println("=== Student List ===");
        printRoster(students);

        // Add a late enrollment
        students.add(2, "Frank Castle");  // Insert at position 2
        System.out.println("\n=== After Late Enrollment ===");
        printRoster(students);

        // Remove a student who withdrew
        students.remove("Charlie Brown");
        System.out.println("\n=== After Withdrawal ===");
        printRoster(students);

        // Search
        String searchStudent = "Diana Prince";
        if (students.contains(searchStudent)) {
            System.out.println("\n" + searchStudent + " is enrolled at position: "
                + (students.indexOf(searchStudent) + 1));
        }

        // Sort alphabetically
        Collections.sort(students);
        System.out.println("\n=== Alphabetical Order ===");
        printRoster(students);

        System.out.println("\nTotal students: " + students.size());
    }

    static void printRoster(List<String> students) {
        for (int i = 0; i < students.size(); i++) {
            System.out.println("  " + (i + 1) + ". " + students.get(i));
        }
    }
}

Output

=== Student List ===
Alice Johnson
Bob Smith
Charlie Brown
Diana Prince
Eve Wilson

=== After Late Enrollment ===
Alice Johnson
Bob Smith
Frank Castle
Charlie Brown
Diana Prince
Eve Wilson

=== After Withdrawal ===
Alice Johnson
Bob Smith
Frank Castle
Diana Prince
Eve Wilson

Diana Prince is enrolled at position: 4

=== Alphabetical Order ===
Alice Johnson
Bob Smith
Diana Prince
Eve Wilson
Frank Castle

Total students: 5

Example 3: Iterating an ArrayList

import java.util.*;

public class IterationDemo {
    public static void main(String[] args) {

        List<Integer> numbers = new ArrayList<>(Arrays.asList(10, 20, 30, 40, 50));

        // Method 1: for loop with index
        System.out.println("For loop:");
        for (int i = 0; i < numbers.size(); i++) {
            System.out.print(numbers.get(i) + " ");
        }

        // Method 2: enhanced for-each loop (most common)
        System.out.println("\nFor-each loop:");
        for (int num : numbers) {
            System.out.print(num + " ");
        }

        // Method 3: Iterator
        System.out.println("\nIterator:");
        Iterator<Integer> it = numbers.iterator();
        while (it.hasNext()) {
            System.out.print(it.next() + " ");
        }

        // Method 4: forEach with lambda (Java 8+)
        System.out.println("\nforEach lambda:");
        numbers.forEach(n -> System.out.print(n + " "));
        System.out.println();
    }
}

Output

For loop:
10 20 30 40 50 
For-each loop:
10 20 30 40 50 
Iterator:
10 20 30 40 50 
forEach lambda:
10 20 30 40 50 

Common Mistakes

❌ Mistake: Calling remove(1) thinking it removes the value 1 from an ArrayList<Integer> → ✅ Fix: remove(1) removes the element at index 1. Use remove(Integer.valueOf(1)) to remove the value 1
❌ Mistake: Modifying an ArrayList while iterating with for-each → ✅ Fix: Use Iterator.remove() or collect items to remove then do list.removeAll(toRemove)
❌ Mistake: Using ArrayList when frequent insertion/deletion in the middle is needed → ✅ Fix: Use LinkedList for frequent middle insertions
❌ Mistake: Comparing ArrayList elements with == → ✅ Fix: Use .equals() for content comparison; == only checks reference equality

Best Practices

Declare with the List interface: List<String> list = new ArrayList<>() for flexibility
Provide an initial capacity when you know approximate size: new ArrayList<>(500)
Use isEmpty() instead of size() == 0 — more readable
Use enhanced for-each loop for simple iteration; use index-based loop only when index is needed
Prefer Collections.sort() or list.sort(Comparator) for sorting

Interview Questions

Q: How does ArrayList grow internally?
A: ArrayList starts with a default capacity of 10. When the 11th element is added, it creates a new array with approximately 1.5x the old capacity (oldCapacity + (oldCapacity >> 1)), copies all elements to the new array, and adds the new element. This is called resizing or growing.

Q: What is the difference between ArrayList and LinkedList?
A: ArrayList uses a dynamic array — O(1) for index-based access but O(n) for insertion/deletion in the middle. LinkedList uses a doubly-linked list — O(n) for index-based access but O(1) for insertion/deletion at known positions. Use ArrayList for most cases; LinkedList when you frequently insert/delete at the beginning or middle.

Q: How do you remove an element by value (not index) from ArrayList<Integer>?
A: You must use remove(Integer.valueOf(value)) or cast: list.remove((Integer) value). If you call list.remove(5), Java treats 5 as an index and removes the element at position 5, not the value 5.

Q: Is ArrayList thread-safe?
A: No. ArrayList is not synchronized. If multiple threads access and modify an ArrayList simultaneously, it can cause data corruption. Use Collections.synchronizedList(new ArrayList<>()) or CopyOnWriteArrayList for thread-safe operations.

Quick Revision

✔ ArrayList = resizable array; grows automatically (default capacity 10, grows 1.5x)
✔ get() is O(1); add(index) and remove() are O(n) due to shifting
✔ remove(int) removes by INDEX; remove(Object) removes by VALUE
✔ Allows duplicates and null values; maintains insertion order
✔ Not thread-safe — use Collections.synchronizedList() if needed

LinkedList (Lesson 03)
Collections Overview (Lesson 01)
Comparable and Comparator for sorting (Lesson 08)

Next Lesson

03 - LinkedList

Topic Summary​

What You'll Learn​

Prerequisites​

Explanation​

ArrayList vs Regular Array​

How ArrayList Works Internally​

Creating an ArrayList​

Key Methods​

add(element) — Add to End​

add(index, element) — Insert at Position​

get(index) — Access by Index​

set(index, element) — Replace at Position​

remove(index) — Remove by Index​

remove(Object) — Remove by Value​

size() — Count Elements​

isEmpty() — Check if Empty​

contains(element) — Check Membership​

indexOf(element) — Find Position​

clear() — Remove All​

Collections.sort(list) — Sort​

iterator() / enhanced for-loop — Iteration​

Time Complexity Summary​

Real-World Analogy​

Code Example​

Example 1: ArrayList Basics​

Output​

Example 2: Real-World — Student List Manager​

Output​

Example 3: Iterating an ArrayList​

Output​

Common Mistakes​

Best Practices​

Interview Questions​

Quick Revision​

Related Topics​

Next Lesson​

Topic Summary

What You'll Learn

Prerequisites

Explanation

ArrayList vs Regular Array

How ArrayList Works Internally

Creating an ArrayList

Key Methods

`add(element)` — Add to End

`add(index, element)` — Insert at Position

`get(index)` — Access by Index

`set(index, element)` — Replace at Position

`remove(index)` — Remove by Index

`remove(Object)` — Remove by Value

`size()` — Count Elements

`isEmpty()` — Check if Empty

`contains(element)` — Check Membership

`indexOf(element)` — Find Position

`clear()` — Remove All

`Collections.sort(list)` — Sort

`iterator()` / enhanced for-loop — Iteration

Time Complexity Summary

Real-World Analogy

Code Example

Example 1: ArrayList Basics

Output

Example 2: Real-World — Student List Manager

Output

Example 3: Iterating an ArrayList

Output

Common Mistakes

Best Practices

Interview Questions

Quick Revision

Related Topics

Next Lesson