Java ArrayList Basics

What is an ArrayList?

An ArrayList is a class in Java that stores a list of items. It is similar to an array, but it can grow and shrink as needed.

Why Use an ArrayList?

• You do not need to know the size ahead of time.
• You can easily add and remove items.
• It has built-in methods that make it easy to use.

Importing ArrayList

Before using an ArrayList, you must import it:

import java.util.ArrayList;

Creating an ArrayList

Here is how to make an ArrayList of Strings:

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

Here is how to make an ArrayList of Integers:

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

Adding Items

Use add() to put items into the ArrayList:

fruits.add("Apple");
fruits.add("Banana");
fruits.add("Orange");

Displaying the ArrayList

System.out.println(fruits);

Output:

[Apple, Banana, Orange]

Getting an Item

Use get(index) to access one item:

System.out.println(fruits.get(1));

Output:

Banana

Changing an Item

Use set(index, value) to replace an item:

fruits.set(1, "Grapes");
System.out.println(fruits);

Output:

[Apple, Grapes, Orange]

Removing an Item

Use remove(index) to delete an item:

fruits.remove(0);
System.out.println(fruits);

Output:

[Grapes, Orange]

Finding the Size

Use size() to count how many items are in the ArrayList:

System.out.println(fruits.size());

Looping Through an ArrayList

You can use a regular for loop:

for (int i = 0; i < fruits.size(); i++) {
    System.out.println(fruits.get(i));
}

You can also use a for-each loop:

for (String fruit : fruits) {
    System.out.println(fruit);
}

Common Methods

• add(item) – adds an item
• get(index) – gets an item
• set(index, item) – changes an item
• remove(index) – removes an item
• size() – returns the number of items
• clear() – removes all items

Example Program

import java.util.ArrayList;

public class ArrayListExample {
    public static void main(String[] args) {
        ArrayList<String> students = new ArrayList<String>();

        students.add("Alice");
        students.add("Brian");
        students.add("Carlos");

        System.out.println("Student List: " + students);
        System.out.println("First Student: " + students.get(0));

        students.set(1, "Bella");
        students.remove(2);

        System.out.println("Updated List: " + students);
        System.out.println("Number of Students: " + students.size());
    }
}

Summary

• An ArrayList is a resizable list in Java.
• It is useful when you need a list that can change size.
• You can add, remove, replace, and access items easily.
• Use methods like add(), get(), set(), and remove().