Java ArrayList

Java ArrayList

An ArrayList is like a resizable array.

It is part of the java.util package and implements the List interface.

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.

Create an ArrayList

To use an ArrayList, you must first import it from java.util:

Example

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

ArrayList<String> cars = new ArrayList<String>(); // Create an ArrayList object

Now you can use methods like add(), get(), set(), and remove() to manage your list of elements.

Add Elements

To add elements to an ArrayList, use the add() method:

Example

import java.util.ArrayList;

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

You can also add an element at a specified position by referring to the index number:

Example

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> cars = new ArrayList<String>();
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");

cars.add(0, "Mazda"); // Insert element at the beginning of the list (0)

    System.out.println(cars);
  }
}

Note: An ArrayList keeps elements in the same order you add them, so the first item you add will be at index 0, the next at index 1, and so on.

Access an Element

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

Example

cars.get(0);  // Get the first element

Change an Element

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

Example

cars.set(0, "Opel");

Remove an Element

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

Example

cars.remove(0);

To remove all the elements in the ArrayList, use the clear() method:

Example

cars.clear();

ArrayList Size

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

Example

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:

Example

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

You can also loop through an ArrayList with the for-each loop:

Example

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

Other Types

Elements in an ArrayList are actually objects. In the examples above, we created elements (objects) of type "String". Remember that a String in Java is an object (not a primitive type). To use other types, such as int, you must specify an equivalent wrapper class: Integer. For other primitive types, use: Boolean for boolean, Character for char, Double for double, etc:

Example

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> myNumbers = new ArrayList<Integer>();
    myNumbers.add(10);
    myNumbers.add(15);
    myNumbers.add(20);
    myNumbers.add(25);
    for (int i : myNumbers) {
      System.out.println(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:

Example

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

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

Example

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

public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> myNumbers = new ArrayList<Integer>();
    myNumbers.add(33);
    myNumbers.add(15);
    myNumbers.add(20);
    myNumbers.add(34);
    myNumbers.add(8);
    myNumbers.add(12);

    Collections.sort(myNumbers);  // Sort myNumbers

    for (int i : myNumbers) {
      System.out.println(i);
    }
  }
}

The var Keyword

From Java 10, you can use the var keyword to declare an ArrayList variable without writing the type twice. The compiler figures out the type from the value you assign.

This makes code shorter, but many developers still use the full type for clarity. Since var is valid Java, you may see it in other code, so it's good to know that it exists:

Example

// Without var
ArrayList<String> cars = new ArrayList<String>();

// With var
var cars = new ArrayList<String>();

The List Interface

Note: Sometimes you will see both List and ArrayList in Java code, like this:

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

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

This means the variable (cars) is declared as a List (the interface), but it stores an ArrayList object (the actual list). Since ArrayList implements the List interface, this is possible.

It works the same way, but some developers prefer this style because it gives them more flexibility to change the type later.

Complete ArrayList Reference

For a complete reference of ArrayList methods, go to our Java ArrayList Reference.