Java ArrayList Methods – Full Guide with Syntax, Examples & Best Practices (2025)

Introduction – Why Use Java ArrayList Methods?

In Java, ArrayList is one of the most popular dynamic data structures. Unlike arrays, it can grow and shrink as needed — and its rich set of built-in methods makes it ideal for modern, flexible applications.

By mastering ArrayList methods, you can:

• Easily add, remove, search, and sort elements
• Write cleaner, more efficient code
• Avoid boilerplate logic with powerful built-in utilities

ArrayList is part of the java.util package and implements the List interface.

What Is an ArrayList in Java?

An ArrayList is a resizable array backed by an internal array. It automatically handles memory allocation, making it ideal for collections where size may vary.

import java.util.ArrayList;
ArrayList<String> list = new ArrayList<>();

Common Java ArrayList Methods

Java ArrayList Method Examples

1. add() – Add Elements to ArrayList

ArrayList<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
System.out.println(fruits);

Output:

[Apple, Banana]

Adds elements to the list.

2. add(index, element) – Insert at Position

fruits.add(1, "Mango");
System.out.println(fruits);

Output:

[Apple, Mango, Banana]

Inserts element at index 1.

3. get(index) – Access Element

System.out.println(fruits.get(0));  // Apple

Fetches the element at index 0.

4. set(index, element) – Replace Value

fruits.set(1, "Orange");
System.out.println(fruits);

Output:

[Apple, Orange, Banana]

Replaces “Mango” with “Orange”.

5. remove() – Delete by Index or Value

fruits.remove("Banana");
System.out.println(fruits);

Output:

[Apple, Orange]

Removes the first occurrence of “Banana”.

6. clear() – Empty the List

fruits.clear();
System.out.println(fruits.isEmpty());  // true

Clears all elements from the list.

7. contains() – Check for Element

fruits.add("Kiwi");
System.out.println(fruits.contains("Kiwi"));  // true

Checks if the list has “Kiwi”.

8. size() – List Length

System.out.println(fruits.size());  // 1

Returns the number of elements.

9. indexOf() and lastIndexOf()

ArrayList<String> colors = new ArrayList<>();
colors.add("Red");
colors.add("Blue");
colors.add("Red");

System.out.println(colors.indexOf("Red"));      // 0
System.out.println(colors.lastIndexOf("Red"));  // 2

Finds first and last index of an element.

10. toArray() – Convert to Array

String[] array = fruits.toArray(new String[0]);
System.out.println(Arrays.toString(array));

Converts list to array format.

11. forEach() – Iterate with Lambda

fruits.forEach(fruit -> System.out.println("Fruit: " + fruit));

Uses Java 8+ lambda for cleaner iteration.

12. retainAll() and removeAll() – Bulk Ops

ArrayList<String> a = new ArrayList<>(List.of("A", "B", "C"));
ArrayList<String> b = new ArrayList<>(List.of("B", "C", "D"));

a.retainAll(b);
System.out.println(a);  // [B, C]

Keeps only common elements between two lists.

Summary Table of Methods

Tips for Using ArrayList Methods

• Use generics like ArrayList<String> to avoid type-casting.
• Prefer List interface when declaring: List<String> list = new ArrayList<>();
• ArrayList is not thread-safe. Use Collections.synchronizedList() for multithreading.
• Avoid frequent add(0, item) calls — it’s slow due to shifting.

Summary – Why Mastering ArrayList Methods Matters

ArrayList methods are versatile, powerful, and essential for working with collections in Java. They help you:

• Add, access, and remove data efficiently
• Perform complex operations like search, filter, and sort
• Build dynamic, user-friendly applications

Mastering these methods gives you a strong foundation in Java collections and object-oriented programming.

FAQs on Java ArrayList Methods

Is ArrayList ordered?

Yes. Elements maintain insertion order.

Can ArrayList contain duplicate elements?

Yes. Duplicates are allowed.

How is ArrayList different from array?

• Array: Fixed size, primitive types
• ArrayList: Resizable, holds objects

How to sort an ArrayList?

Collections.sort(list);

How to iterate over an ArrayList?

Use for, for-each, Iterator, or forEach() with lambda.