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

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🧲 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

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🔑 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.

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🧪 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

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📦 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()

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🔁 Looping Through an ArrayList

🧩 Using for loop

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

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🧩 Using for-each loop

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

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🧩 Using forEach() method (Java 8+)

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

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🧠 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

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🧰 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

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🚀 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.

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✅ 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

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❓ 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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