ArrayList

The ArrayList Class in Java

The ArrayList class is one of the most commonly used implementations of the List interface in the Java Collection Framework. It represents a resizable array that dynamically grows or shrinks as elements are added or removed. Unlike arrays in Java, which have a fixed size, ArrayList provides flexibility and convenience for managing collections of objects.

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Key Characteristics of ArrayList

• Dynamic Resizing: Automatically resizes when elements are added or removed.
• Random Access: Fast index-based access (O(1) for get() and set()).
• Duplicates Allowed
• Maintains Insertion Order
• Not Thread-Safe

For thread safety you can use:

• Collections.synchronizedList()
• CopyOnWriteArrayList

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Common Use Cases

• Maintaining ordered collections
• Frequent index-based access
• Dynamically changing list sizes

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Important Methods

Method	Description
boolean add(E e)	Adds element at end
void add(int index, E element)	Adds element at index
E get(int index)	Returns element
E set(int index, E element)	Replaces element
E remove(int index)	Removes element
boolean remove(Object o)	Removes first occurrence
int size()	Returns size
boolean isEmpty()	Checks if empty
boolean contains(Object o)	Checks presence
Object[] toArray()	Converts to array

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Example 1: Basic Operations

import java.util.ArrayList;
import java.util.List;

public class ArrayListExample {

    public static void main(String[] args) {
        // Create an ArrayList
        List<String> fruits = new ArrayList<>();

        // Add elements
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");

        System.out.println("Fruits: " + fruits);
        
        // Access elements by index
        System.out.println("First fruit: " + fruits.get(0));

        fruits.set(1, "Mango");

        System.out.println("After replacement: " + fruits);

        fruits.remove("Cherry");

        System.out.println("After removal: " + fruits);

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

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Example 2: Dynamic Resizing

import java.util.ArrayList;
import java.util.List;

public class DynamicResizingExample {

    public static void main(String[] args) {

        List<Integer> numbers = new ArrayList<>();

        for (int i = 1; i <= 10; i++) {
            numbers.add(i);
        }

        System.out.println(numbers);

        numbers.removeIf(n -> n % 2 == 0);

        System.out.println("After removing evens: " + numbers);
    }
}

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Example 3: Converting to Array

import java.util.ArrayList;
import java.util.List;

public class ArrayListToArrayExample {

    public static void main(String[] args) {

        List<String> colors = new ArrayList<>();

        colors.add("Red");
        colors.add("Green");
        colors.add("Blue");

        String[] colorArray = colors.toArray(new String[0]);

        System.out.println(String.join(", ", colorArray));
    }
}

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Performance Characteristics

Operation	Complexity
get()	O(1)
set()	O(1)
add() (end)	O(1) amortized
add() (middle)	O(n)
remove() (middle)	O(n)

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When to Use ArrayList

Use ArrayList when:

• Random access is frequent
• Insertions mostly happen at the end
• You need ordered collections

Avoid when:

• Many insertions/deletions occur in the middle of the list
• Prefer LinkedList in those cases.

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ArrayList vs LinkedList

Feature	ArrayList	LinkedList
Structure	Dynamic Array	Doubly Linked List
Random Access	Fast O(1)	Slow O(n)
Insert/Delete Middle	Slow	Faster
Memory	Lower	Higher