Arraylist

Introduction

ArrayList is a collection similar to array but with dynamic size.

Simply it's size is not defined during initialization and changes when element is added / removed.

In order to use it the following line should be added:

import java.util.ArrayList;

Syntax

ArrayList<type> arrName = new ArrayList<type>();

type specifies the values type - Integer, Double, String etc...

Initialization

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

Adding element to the back

This can be done via the add function. Like in the following code:

ArrayList<String > arrList = new ArrayList<String>(); arrList.add("Alex"); arrList.add("John");

Accessing specific index

Accessing element at specific index is done via the get function.

Let's add elements to ArrayList and access the element at index 1.

import java.util.ArrayList; public class Main { public static void main(String[] args) { ArrayList<Integer> numbers = new ArrayList<Integer>(); numbers.add(5); numbers.add(6); numbers.add(7); System.out.print(numbers.get(1)); } }

Output:

6

Outside of bounds

Accessing element outside of ArrayList's bounds results in exception.

Let's look at the following example

import java.util.ArrayList; public class Main { public static void main(String[] args) { ArrayList<Integer> numbers = new ArrayList<Integer>(); numbers.add(15); numbers.add(26); numbers.add(37); System.out.print(numbers.get(3)); } }

numbers has 3 elements (indexes from 0 to 2) but at line 11 the 3rd index is accessed.

Size

Size is the count of elements in collection.

In order to get it the size function is used.

Example:

import java.util.ArrayList; public class Main { public static void main(String[] args) { ArrayList<Integer> numbers = new ArrayList<Integer>(); numbers.add(15); numbers.add(26); numbers.add(37); System.out.print(numbers.size()); } }

Output:

3

Looping elements

Let's have an ArrayList of ages and print only those who are greater than 20.

import java.util.ArrayList; public class Main { public static void main(String[] args) { ArrayList<Integer> ages = new ArrayList<Integer>(); ages.add(18); ages.add(32); ages.add(22); ages.add(45); ages.add(19); for (int age : ages) { if (age > 20) { System.out.print(age + " "); } } } }

Output:

32 22 45