ArrayList in Java

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An ArrayList in Java is a resizable (or dynamic) array from the java.util package that can grow or shrink automatically as elements are added or removed, unlike regular arrays with a fixed size.

• Indexed Access: Elements can be accessed using their index, just like arrays.
• Allows Duplicates: Duplicate elements are allowed.
• Maintains Insertion Order: Elements are stored in the order they are inserted.
• Not Synchronized: ArrayList is not thread-safe. To make it thread-safe, you must wrap it manually using Collections.synchronizedList().

import java.util.ArrayList;

class Main {
    public static void main (String[] args) {
        
      	// Creating an ArrayList
      	ArrayList<Integer> a = new ArrayList<Integer>();
      	
      	// Adding Element in ArrayList
      	a.add(1);
      	a.add(2);
      	a.add(3);
      
      	// Printing ArrayList
      	System.out.println(a);
    }
}

[1, 2, 3]

Hierarchy of ArrayList

It implements List Interface which is a sub-interface of Collection Interface.

ArrayList Constructors in Java

Java provides multiple constructors to create an ArrayList based on different requirements:

1. ArrayList()

Creates an empty ArrayList with default initial capacity.

ArrayList<Integer> arr = new ArrayList<>();

2. ArrayList(Collection<? extends E> c)

Creates an ArrayList initialized with elements from the specified collection.

ArrayList<String> arr = new ArrayList<>(collection);

3. ArrayList(int initialCapacity)

This constructor is used to build an array list with the initial capacity being specified.

ArrayList<Double> arr = new ArrayList<>(20);

Operations of ArrayList

Now, Using the constructors we have got ArrayList for further operations like Insertion,Deletion and Updation of the elements in ArrayList.

import java.util.*;

class GFG{
    
    public static void main(String args[]){
      
        // Creating an Array of string type
        ArrayList<String> al = new ArrayList<>();

        // 1. Adding elements to ArrayList at the end
        al.add("Geeks");
        al.add("Geeks");

      	System.out.println("Original List : "+al);
      	
      	// Adding Elements at the specific index
        al.add(1, "For");

      	System.out.println("After Adding element at index 1 : "+ al);
      	
      	// 2. Removing Element using index
      	al.remove(0);
      
      	System.out.println("Element removed from index 0 : "+ al);
      	
      	// Removing Element using the value
      	al.remove("Geeks");
      
      	System.out.println("Element Geeks removed : "+ al);
      	
      	// 3. Updating value at index 0
      	al.set(0, "GFG");
      
        System.out.println("List after updation of value : "+al);
    }
}

Original List : [Geeks, Geeks]
After Adding element at index 1 : [Geeks, For, Geeks]
Element removed from index 0 : [For, Geeks]
Element Geeks removed : [For]
List after updation of value : [GFG]

Complexity of Java ArrayList

Java ArrayList Methods

ArrayList in Java

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