java arraylist

Java ArrayList

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:

Example

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

    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:

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<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));

    }

  }

}

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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);

    }

  }

}

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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:

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);

    }

  }

}

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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);

    }

  }

}

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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);

    }

  }

}

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