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Java ArrayList

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Java ArrayList

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

Nội dung chính

Java ArrayListJava ArrayListAccess an ItemChange an ItemRemove an ItemArrayList SizeLoop Through an ArrayListOther TypesSort an ArrayListVideo liên quan

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:

Example

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:

Example

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);

Try it Yourself »

Access an Item

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

Example

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:

Example

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:

Example

cars.remove(0);

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

Example

cars.clear();

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ArrayList Size

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

Example

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:

Example

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));

Try it Yourself »

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

Example

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);

Try it Yourself »

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 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:

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 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 »

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 myNumbers = new ArrayList();
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);

Try it Yourself »

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