Java ArrayList

Java Collections Framework

The ArrayList class is a dynamically resizable array, differing from regular arrays in that it has no fixed-size limit; elements can be added or removed.

ArrayList inherits from AbstractList and implements the List interface.

The ArrayList class is located in the java.util package and needs to be imported before use. The syntax is as follows:

import java.util.ArrayList; // Import the ArrayList class

ArrayList<E> objectName = new ArrayList<>(); // Initialization

• E: Generic data type, used to set the data type of objectName, only reference data types are allowed.
• objectName: Object name.

ArrayList is an array queue that provides functionalities for adding, removing, modifying, and traversing elements.

Adding Elements

The ArrayList class provides useful methods; elements can be added to an ArrayList using the add() method:

Example

import java.util.ArrayList;

public class tutorialproTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("tutorialpro");
        sites.add("Taobao");
        sites.add("Weibo");
        System.out.println(sites);
    }
}

The output of the above example is:

[Google, tutorialpro, Taobao, Weibo]

Accessing Elements

Elements in an ArrayList can be accessed using the get() method:

Example

import java.util.ArrayList;

public class tutorialproTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("tutorialpro");
        sites.add("Taobao");
        sites.add("Weibo");
        System.out.println(sites.get(1)); // Access the second element
    }
}

Note: Array indices start from 0.

The output of the above example is:

tutorialpro

Modifying Elements

To modify elements in an ArrayList, use the set() method:

Example

import java.util.ArrayList;

public class tutorialproTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("tutorialpro");
        sites.add("Taobao");
        sites.add("Weibo");
        sites.set(2, "Wiki"); // The first parameter is the index position, the second is the value to be modified
        System.out.println(sites);
    }
}

The output of the above example is:

[Google, tutorialpro, Wiki, Weibo]

Deleting Elements

To delete elements in an ArrayList, use the remove() method:

Example

import java.util.ArrayList;

public class tutorialproTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("tutorialpro");
        sites.add("Taobao");
        sites.add("Weibo");
        sites.remove(2); // Remove the third element
        System.out.println(sites);
    }
}

The output of the above example is:

[Google, tutorialpro, Weibo]

sites.add("Google");
sites.add("tutorialpro");
sites.add("Taobao");
sites.add("Weibo");
sites.remove(3); // Removes the fourth element
System.out.println(sites);
}
}

The output of the above example is:

[Google, tutorialpro, Taobao]

Calculating Size

To calculate the number of elements in an ArrayList, you can use the size() method:

Example

import java.util.ArrayList;

public class tutorialproTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("tutorialpro");
        sites.add("Taobao");
        sites.add("Weibo");
        System.out.println(sites.size());
    }
}

The output of the above example is:

4

Iterating Over an ArrayList

We can use a for loop to iterate over the elements in an ArrayList:

Example

import java.util.ArrayList;

public class tutorialproTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("tutorialpro");
        sites.add("Taobao");
        sites.add("Weibo");
        for (int i = 0; i < sites.size(); i++) {
            System.out.println(sites.get(i));
        }
    }
}

The output of the above example is:

Google
tutorialpro
Taobao
Weibo

You can also use a for-each loop to iterate over the elements:

Example

import java.util.ArrayList;

public class tutorialproTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("tutorialpro");
        sites.add("Taobao");
        sites.add("Weibo");
        for (String i : sites) {
            System.out.println(i);
        }
    }
}

The output of the above example is:

Google
tutorialpro
Taobao
Weibo

Other Reference Types

The elements in an ArrayList are actually objects. In the above examples, the elements are of type String.

If we want to store other types, we need to use the wrapper classes of the primitive types.

Here is a table of primitive types and their corresponding wrapper classes:

Additionally, BigInteger and BigDecimal are used for high-precision operations. BigInteger supports arbitrary-precision integers and is also a reference type, but they do not have corresponding primitive types.

ArrayList<Integer> li = new ArrayList<>();     // Stores integer elements

ArrayList<Character> li = new ArrayList<>();   // Store character elements

The following example uses ArrayList to store numbers (using Integer type):

Example

import java.util.ArrayList;

public class tutorialproTest {
    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);
        }
    }
}

The output of the above example is:

10
15
20
25

Sorting ArrayList

The Collections class is also a very useful class located in the java.util package, providing the sort() method to sort lists of characters or numbers.

The following example sorts letters:

Example

import java.util.ArrayList;
import java.util.Collections;  // Import the Collections class

public class tutorialproTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Taobao");
        sites.add("Wiki");
        sites.add("tutorialpro");
        sites.add("Weibo");
        sites.add("Google");
        Collections.sort(sites);  // Sort alphabetically
        for (String i : sites) {
            System.out.println(i);
        }
    }
}

The output of the above example is:

Google
tutorialpro
Taobao
Weibo
Wiki

The following example sorts numbers:

Example

import java.util.ArrayList;
import java.util.Collections;  // Import the Collections class

public class tutorialproTest {
    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 numerically

        for (int i : myNumbers) {
            System.out.println(i);
        }
    }
}

The output of the above example is:

8
12
15
20
33
34

Java ArrayList Methods

The commonly used methods in Java ArrayList are listed below:

For more API methods, see: https://www.tutorialpro.org/manual/jdk11api/java.base/java/util/ArrayList.html

Java Collections Framework