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

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🧲 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.

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🔑 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<>();

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📋 Common Java ArrayList Methods

🧩 Method	📘 Purpose
add()	Adds element to list
add(index, element)	Inserts at specified position
get(index)	Gets element at index
set(index, element)	Replaces element at index
remove(index/object)	Removes by index or object
clear()	Removes all elements
contains()	Checks if element exists
isEmpty()	Checks if list is empty
size()	Returns number of elements
indexOf() / lastIndexOf()	Gets position of element
toArray()	Converts to array
sort()	Sorts elements (with Collections.sort())
forEach()	Iterates using lambda
retainAll() / removeAll()	Bulk operations

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✅ Java ArrayList Method Examples

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✅ 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.

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

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

Output:

[Apple, Mango, Banana]

✅ Inserts element at index 1.

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✅ 3. get(index) – Access Element

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

✅ Fetches the element at index 0.

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✅ 4. set(index, element) – Replace Value

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

Output:

[Apple, Orange, Banana]

✅ Replaces “Mango” with “Orange”.

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✅ 5. remove() – Delete by Index or Value

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

Output:

[Apple, Orange]

✅ Removes the first occurrence of “Banana”.

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✅ 6. clear() – Empty the List

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

✅ Clears all elements from the list.

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✅ 7. contains() – Check for Element

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

✅ Checks if the list has “Kiwi”.

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✅ 8. size() – List Length

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

✅ Returns the number of elements.

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✅ 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.

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✅ 10. toArray() – Convert to Array

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

✅ Converts list to array format.

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✅ 11. forEach() – Iterate with Lambda

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

✅ Uses Java 8+ lambda for cleaner iteration.

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✅ 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.

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📌 Summary Table of Methods

🔧 Operation	✅ Methods
Add	add(), add(index, element)
Get/Set	get(), set()
Delete	remove(), clear()
Check	contains(), isEmpty()
Size/Info	size(), indexOf(), lastIndexOf()
Convert	toArray()
Sort/Iterate	Collections.sort(), forEach()
Bulk	retainAll(), removeAll()

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💡 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.

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🧠 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.

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❓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.

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