An ArrayList in Java is part of the java.util package. It is a resizable array-like data structure that allows you to dynamically manage a collection of elements.

Importing the ArrayList Class

import java.util.ArrayList;

Declaring and Initializing

// ArrayList listName = new ArrayList<>();
ArrayList<String> names = new ArrayList<>();

Adding Elements

Use the add() method to add elements.

ArrayList<String> names = new ArrayList<>();

names.add("Alice");
names.add("Bob");

System.out.println(names);  // Output: [Alice, Bob]

Accessing Elements

Use the get(index) method to access elements.

System.out.println(names.get(0));  // Output: Alice

Modifying Elements

Use the set(index, element) method to update an element.

ArrayList<String> names = new ArrayList<>();

names.set(0, "Alice");
names.set(1, "Charlie");

System.out.println(names);  // Output: [Alice, Charlie]

Removing Elements

1. By Index

names.remove(0);  // Removes the first element

2. By Value

names.remove("Charlie");  // Removes "Charlie"

Checking the Size

Use size() to get the number of elements.

System.out.println("Size: " + names.size());  // Output: Size: 1

Iterating Over

1. Using a for Loop

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

2. Using an Enhanced for Loop (for-each)

for (String name : names) {
    System.out.println(name);
}

3. Using an Iterator

import java.util.Iterator;

Iterator<String> iterator = names.iterator();
while (iterator.hasNext()) {
    System.out.println(iterator.next());
}

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Common Methods in ArrayList

Method Description

• add(element): Adds an element to the end of the list.

• add(index, element): Adds an element at the specified index.

• get(index): Returns the element at the specified index.

• set(index, element): Updates the element at the specified index.

• remove(index): Removes the element at the specified index.

• remove(element): Removes the first occurrence of the specified element.

• size(): Returns the number of elements in the list.

• clear(): Removes all elements from the list.

• contains(element): Checks if the list contains the specified element.

• isEmpty(): Returns true if the list is empty.

• indexOf(element): Returns the index of the element.

Example: Complete Usage

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> colors = new ArrayList<>();

        // Adding elements
        colors.add("Red");
        colors.add("Blue");
        colors.add("Green");

        // Accessing elements
        System.out.println("First color: " + colors.get(0));  // Output: First color: Red

        // Modifying elements
        colors.set(1, "Yellow");

        // Iterating using a for-each loop
        for (String color : colors) {
            System.out.println(color);
        }

        // Checking size
        System.out.println("Total colors: " + colors.size());  // Output: Total colors: 3

        // Removing an element
        colors.remove("Green");
        System.out.println(colors);  // Output: [Red, Yellow]
    }
}

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Difference Between Arrays and ArrayLists

Array

• Fixed size (cannot change after creation).

• Can store primitives and objects.

• No built-in methods for operations.

ArrayList

• Dynamic size (can grow and shrink).

• Stores objects (Wrapper classes for primitives).

• Provides built-in methods like add(), remove(), etc.