This example demonstrates the usage following ArrayList class methods:

1. add(String e) - Appends the specified element to the end of this list (optional operation).
2. get(int index) - Returns the element at the specified position in this list. 
3. set(int index, String element) - Replaces the element at the specified position in this list with the specified element (optional operation). 

Java ArrayList add(), get() and set() Method Example

package com.javaguides.collections.arraylistexamples;

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

public class AccessElementsFromArrayListExample {
    public static void main(String[] args) {
        List < String > topCompanies = new ArrayList < > ();

        // Check if an ArrayList is empty
        System.out.println("Is the topCompanies list empty? : " + topCompanies.isEmpty());

        topCompanies.add("Google");
        topCompanies.add("Apple");
        topCompanies.add("Microsoft");
        topCompanies.add("Amazon");
        topCompanies.add("Facebook");

        // Find the size of an ArrayList
        System.out.println("Here are the top " + topCompanies.size() + " companies in the world");
        System.out.println(topCompanies);

        // Retrieve the element at a given index
        String bestCompany = topCompanies.get(0);
        String secondBestCompany = topCompanies.get(1);
        String lastCompany = topCompanies.get(topCompanies.size() - 1);

        System.out.println("Best Company: " + bestCompany);
        System.out.println("Second Best Company: " + secondBestCompany);
        System.out.println("Last Company in the list: " + lastCompany);

        // Modify the element at a given index
        topCompanies.set(4, "Walmart");
        System.out.println("Modified top companies list: " + topCompanies);
    }
}

Output

Is the topCompanies list empty? : true
Here are the top 5 companies in the world
[Google, Apple, Microsoft, Amazon, Facebook]
Best Company: Google
Second Best Company: Apple
Last Company in the list: Facebook
Modified top companies list: [Google, Apple, Microsoft, Amazon, Walmart]

Reference

https://www.javaguides.net/2018/06/guide-to-arraylist-class.html