Java ArrayList

The ArrayList class is a resizable array, which can be found in the java.util package.

The difference between a built-in array and an ArrayList in Java, is that the size of an array cannot be modified (if you want to add or remove elements to/from an array, you have to create a new one). While elements can be added and removed from an ArrayList whenever you want. The syntax is also slightly different:

Create an ArrayList object called cars that will store strings:

import java.util.ArrayList; // import the ArrayList class

ArrayList cars = new ArrayList(); // Create an ArrayList object   

Add Items

The ArrayList class has many useful methods. For example, to add elements to the ArrayList, use the add() method:

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList cars = new ArrayList();
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");
    cars.add("Mazda");
    System.out.println(cars);
  }
}                 
       

Access an Item

To access an element in the ArrayList, use the get() method and refer to the index number:

cars.get(0);      

Change an Item

To modify an element, use the set() method and refer to the index number:

cars.set(0, "Opel");    

Remove an Item

To remove an element, use the remove() method and refer to the index number:

cars.remove(0);                 
       

ArrayList Size

To find out how many elements an ArrayList have, use the size method:

cars.size();      

Loop Through an ArrayList

Loop through the elements of an ArrayList with a for loop, and use the size() method to specify how many times the loop should run:

public class Main {
  public static void main(String[] args) {
    ArrayList cars = new ArrayList();
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");
    cars.add("Mazda");
    for (int i = 0; i < cars.size(); i++) {
      System.out.println(cars.get(i));
    }
  }
}                 
       

Sort an ArrayList

Another useful class in the java.util package is the Collections class, which include the sort() method for sorting lists alphabetically or numerically:

        import java.util.ArrayList;
        import java.util.Collections;  // Import the Collections class
        
        public class Main {
          public static void main(String[] args) {
            ArrayList cars = new ArrayList();
            cars.add("Volvo");
            cars.add("BMW");
            cars.add("Ford");
            cars.add("Mazda");
            Collections.sort(cars);  // Sort cars
            for (String i : cars) {
              System.out.println(i);
            }
          }
        }