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:
ExampleGet your own Java Server
Create an ArrayList object called cars that will store strings:
import java.util.ArrayList; // Import the ArrayList class
ArrayList<String> cars = new ArrayList<String>(); // Create an ArrayList objectNow you can use methods like add(), get(), set(), and remove() to manage your list of elements.
Add Elements
The ArrayList class has many useful methods. For example, to add elements to the
list, 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);
}
}Remember: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.
Access an Element
To access an element in the ArrayList, use the get() method and refer to the index number:
Change an Element
To modify an element, use the set() method
and refer to the index number:
Remove an Element
To remove an element, use the remove() method
and refer to the index number:
To remove all the elements in the ArrayList, use the clear() method:
ArrayList Size
To find out how many elements an ArrayList have, use the size method:
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
Create an ArrayList to store numbers (add elements of type Integer):
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
Sort an ArrayList of Strings:
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
Sort an ArrayList of Integers:
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 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).
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.
Exercise?What is this?
Test your skills by answering a few questions about the topics of this page
True or False:
An ArrayList is a fixed-size array, while a regular array is resizable.
