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Java ArrayList – Complete Guide with Syntax, Examples & Best Practices

Introduction – Why Java ArrayList Is a Must-Know Collection

When building Java apps, you often need to store, access, and manipulate dynamic collections of data. That’s where ArrayList comes in — a powerful, resizable array from Java’s Collection Framework.

The ArrayList class is widely used because of its simplicity, flexibility, and random-access performance.

By the end of this article, you’ll understand:

What Java ArrayList is and how it works
How to declare, initialize, and manipulate ArrayLists
Key methods like add(), remove(), get(), contains()
Best practices, performance tips, and real-world use cases

What Is ArrayList in Java?

An ArrayList in Java is a resizable array, part of java.util package, which allows duplicate elements and preserves insertion order.

It implements the List interface and supports generics for type safety.

Basic Syntax – Creating an ArrayList

import java.util.ArrayList;

public class Demo {
    public static void main(String[] args) {
        ArrayList<String> fruits = new ArrayList<>(); // Using Generics
        fruits.add("Apple");
        fruits.add("Banana");
        System.out.println(fruits); // Output: [Apple, Banana]
    }
}

Explanation:

ArrayList<String>: Declares a list of Strings
add(): Adds elements to the list
Insertion order is preserved

Common ArrayList Methods

Method	Description	Example
`add(E e)`	Adds element at the end	`list.add("Mango")`
`add(int index, E e)`	Inserts at specific position	`list.add(1, "Kiwi")`
`get(int index)`	Returns element at index	`list.get(0)`
`set(int index, E e)`	Replaces element at index	`list.set(1, "Orange")`
`remove(int index)`	Removes element at index	`list.remove(0)`
`contains(Object o)`	Checks if element exists	`list.contains("Apple")`
`indexOf(Object o)`	Returns index of element	`list.indexOf("Apple")`
`clear()`	Removes all elements	`list.clear()`
`size()`	Returns number of elements	`list.size()`
`isEmpty()`	Checks if list is empty	`list.isEmpty()`

Looping Through an ArrayList

Using `for` loop

for (int i = 0; i < fruits.size(); i++) {
    System.out.println(fruits.get(i));
}

Using `for-each` loop

for (String fruit : fruits) {
    System.out.println(fruit);
}

Using `forEach()` method (Java 8+)

fruits.forEach(System.out::println);

ArrayList vs Array

Feature	`Array`	`ArrayList`
Size	Fixed	Dynamic/resizable
Data types	Supports primitives	Supports objects only
Length	`array.length`	`list.size()`
Performance	Slightly faster	More flexible
Memory	Less overhead	Uses extra memory internally

Real-World Example – Student List

import java.util.ArrayList;

public class StudentApp {
    public static void main(String[] args) {
        ArrayList<String> students = new ArrayList<>();
        students.add("Aman");
        students.add("Sneha");
        students.add("Ravi");

        if (students.contains("Sneha")) {
            System.out.println("Sneha is enrolled.");
        }
    }
}

Useful in apps like:

E-commerce carts
To-do lists
Dynamic forms
Real-time search suggestions

Performance Tips

Use ArrayList when:

You need fast access (get() is O(1))
You rarely insert/remove in the middle (those are O(n))

If frequent inserts/removals at the beginning/middle are needed, consider using LinkedList.

Summary

ArrayList is a dynamic array structure in Java that supports ordered and duplicate elements
Ideal for fast random access and flexible list operations
Comes with powerful built-in methods for management and iteration
Widely used in real-world Java applications for storing object collections

FAQs – Java ArrayList

Can ArrayList hold primitive types?

No. Use wrapper classes like Integer, Double, etc. (autoboxing handles this automatically).

Is ArrayList synchronized?

No. Use Collections.synchronizedList() or CopyOnWriteArrayList for thread safety.

Can ArrayList store `null`?

Yes, ArrayList allows null values.

What is the default size of an ArrayList?

Internally, the capacity starts at 10 (but size is 0 until elements are added).

How to convert ArrayList to array?

String[] arr = fruits.toArray(new String[0]);

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