In this article, we will learn to initialize ArrayList with values in Java.
ArrayList is an implementation class of List interface in Java. It is used to store elements. It is based on a dynamic array concept that grows accordingly.
We can Initialize ArrayList with values in several ways. Let’s see some of them with examples.
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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
If you don't know what a package is, read our Java Packages Tutorial.
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]; } }
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Access an Item
To access an element in the ArrayList, use the get[] method and refer to the index number:
cars.get[0];
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Remember: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.
Change an Item
To modify an element, use the set[] method and refer to the index number:
cars.set[0, "Opel"];
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Remove an Item
To remove an element, use the remove[] method and refer to the index number:
cars.remove[0];
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To remove all the elements in the ArrayList, use the clear[] method:
cars.clear[];
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ArrayList Size
To find out how many elements an ArrayList have, use the size method:
cars.size[];
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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]]; } } }
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You can also loop through an ArrayList with the for-each loop:
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 [String i : cars] { System.out.println[i]; } } }
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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:
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 myNumbers = new ArrayList[]; myNumbers.add[10]; myNumbers.add[15]; myNumbers.add[20]; myNumbers.add[25]; for [int i : myNumbers] { System.out.println[i]; } } }Try it Yourself »
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:
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 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]; } } }Try it Yourself »
Sort an ArrayList of Integers:
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In the last post we discussed about class ArrayList in Java and it’s important methods. Here we are sharing multiple ways to initialize an ArrayList with examples.
Method 1: Initialization using Arrays.asList
Syntax:
ArrayList obj = new ArrayList[ Arrays.asList[Object o1, Object o2, Object o3, ....so on]];Example:
import java.util.*; public class InitializationExample1 { public static void main[String args[]] { ArrayList obj = new ArrayList[ Arrays.asList["Pratap", "Peter", "Harsh"]]; System.out.println["Elements are:"+obj]; } }Output:
Elements are:[Pratap, Peter, Harsh]Method 2: Anonymous inner class method to initialize ArrayList
Syntax:
ArrayList obj = new ArrayList[]{{ add[Object o1]; add[Object o2]; add[Object o3]; ... ... }};Example:
import java.util.*; public class InitializationExample2 { public static void main[String args[]] { ArrayList cities = new ArrayList[]{{ add["Delhi"]; add["Agra"]; add["Chennai"]; }}; System.out.println["Content of Array list cities:"+cities]; } }Output:
Content of Array list cities:[Delhi, Agra, Chennai]Method3: Normal way of ArrayList initialization
Syntax:
ArrayList obj = new ArrayList[]; obj.add["Object o1"]; obj.add["Object o2"]; obj.add["Object o3"]; ... ...Example:
import java.util.*; public class Details { public static void main[String args[]] { ArrayList books = new ArrayList[]; books.add["Java Book1"]; books.add["Java Book2"]; books.add["Java Book3"]; System.out.println["Books stored in array list are: "+books]; } }Output:
Books stored in array list are: [Java Book1, Java Book2, Java Book3]Method 4: Use Collections.ncopies
Collections.ncopies method can be used when we need to initialize the ArrayList with the same value for all of its elements. Syntax: count is number of elements and element is the item value
ArrayList obj = new ArrayList[Collections.nCopies[count, element]];Example:
import java.util.*; public class Details { public static void main[String args[]] { ArrayList intlist = new ArrayList[Collections.nCopies[10, 5]]; System.out.println["ArrayList items: "+intlist]; } }Output:
ArrayList items: [5, 5, 5, 5, 5, 5, 5, 5, 5, 5]