ArrayList & Dynamic Arrays

ArrayList is a resizable array implementation. Unlike regular arrays, ArrayLists can grow and shrink dynamically. They are the most commonly used collection in Java. This is a foundational concept in enterprise application development that professional developers rely on daily. The explanations below are written to be beginner-friendly while covering the depth and nuance that comes from real-world Java experience. Take your time with each section and practice the examples

40 min•By Priygop Team•Last updated: Feb 2026

ArrayList Operations

Example

import java.util.ArrayList;
import java.util.Collections;

public class ArrayListDemo {
    public static void main(String[] args) {
        // Creating an ArrayList
        ArrayList<String> fruits = new ArrayList<>();
        
        // Adding elements
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");
        fruits.add("Date");
        fruits.add("Elderberry");
        System.out.println("Fruits: " + fruits);
        
        // Accessing elements
        System.out.println("First: " + fruits.get(0));
        System.out.println("Size: " + fruits.size());
        
        // Modifying
        fruits.set(1, "Blueberry");  // Replace Banana
        System.out.println("After set: " + fruits);
        
        // Removing
        fruits.remove("Date");        // By value
        fruits.remove(0);             // By index
        System.out.println("After remove: " + fruits);
        
        // Searching
        System.out.println("Contains Cherry? " + fruits.contains("Cherry"));
        System.out.println("Index of Cherry: " + fruits.indexOf("Cherry"));
        
        // Sorting
        Collections.sort(fruits);
        System.out.println("Sorted: " + fruits);
        
        // Iterating
        System.out.println("\nAll fruits:");
        for (String fruit : fruits) {
            System.out.println("  - " + fruit);
        }
        
        // ArrayList of numbers
        ArrayList<Integer> numbers = new ArrayList<>();
        for (int i = 1; i <= 10; i++) {
            numbers.add(i * i);  // Squares
        }
        System.out.println("\nSquares: " + numbers);
    }
}

Try It Yourself: To-Do List Manager

Try It Yourself: To-Do List ManagerJava

Java Editor

✓ ValidTab = 2 spaces

import java.util.ArrayList;

public class TodoList {
    public static void main(String[] args) {
        ArrayList<String> tasks = new ArrayList<>();
        ArrayList<Boolean> completed = new ArrayList<>();
        
        // Add tasks
        addTask(tasks, completed, "Learn Java Collections");
        addTask(tasks, completed, "Build a project");
        addTask(tasks, completed, "Practice coding");
        addTask(tasks, completed, "Read documentation");
        addTask(tasks, completed, "Submit assignment");
        
        // Mark some as done
        completeTask(tasks, completed, 0);
        completeTask(tasks, completed, 2);
        
        // Display
        printTasks(tasks, completed);
        
        // Remove completed tasks
        System.out.println("\nRemoving completed tasks...");
        for (int i = tasks.size() - 1; i >= 0; i--) {
            if (completed.get(i)) {
                tasks.remove(i);
                completed.remove(i);
            }
        }
        printTasks(tasks, completed);
    }
    
    static void addTask(ArrayList<String> tasks, ArrayList<Boolean> completed, String task) {
        tasks.add(task);
        completed.add(false);
    }
    
    static void completeTask(ArrayList<String> tasks, ArrayList<Boolean> completed, int index) {
        completed.set(index, true);
        System.out.println("Completed: " + tasks.get(index));
    }
    
    static void printTasks(ArrayList<String> tasks, ArrayList<Boolean> completed) {
        System.out.println("\n=== To-Do List ===");
        for (int i = 0; i < tasks.size(); i++) {
            String status = completed.get(i) ? "[X]" : "[ ]";
            System.out.println(status + " " + tasks.get(i));
        }
        System.out.println("Total: " + tasks.size() + " tasks");
    }
}

import java.util.ArrayList;

public class TodoList {
    public static void main(String[] args) {
        ArrayList<String> tasks = new ArrayList<>();
        ArrayList<Boolean> completed = new ArrayList<>();
        
        // Add tasks
        addTask(tasks, completed, "Learn Java Collections");
        addTask(tasks, completed, "Build a project");
        addTask(tasks, completed, "Practice coding");
        addTask(tasks, completed, "Read documentation");
        addTask(tasks, completed, "Submit assignment");
        
        // Mark some as done
        completeTask(tasks, completed, 0);
        completeTask(tasks, completed, 2);
        
        // Display
        printTasks(tasks, completed);
        
        // Remove completed tasks
        System.out.println("\nRemoving completed tasks...");
        for (int i = tasks.size() - 1; i >= 0; i--) {
            if (completed.get(i)) {
                tasks.remove(i);
                completed.remove(i);
            }
        }
        printTasks(tasks, completed);
    }
    
    static void addTask(ArrayList<String> tasks, ArrayList<Boolean> completed, String task) {
        tasks.add(task);
        completed.add(false);
    }
    
    static void completeTask(ArrayList<String> tasks, ArrayList<Boolean> completed, int index) {
        completed.set(index, true);
        System.out.println("Completed: " + tasks.get(index));
    }
    
    static void printTasks(ArrayList<String> tasks, ArrayList<Boolean> completed) {
        System.out.println("\n=== To-Do List ===");
        for (int i = 0; i < tasks.size(); i++) {
            String status = completed.get(i) ? "[X]" : "[ ]";
            System.out.println(status + " " + tasks.get(i));
        }
        System.out.println("Total: " + tasks.size() + " tasks");
    }
}

Output

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Edit the code on the left, then click Run

Java|51 lines|1804 chars|✓ Valid syntax

UTF-8

Quick Quiz: ArrayList

ArrayList & Dynamic Arrays

40 min•By Priygop Team•Last updated: Feb 2026

ArrayList Operations

Example

import java.util.ArrayList;
import java.util.Collections;

public class ArrayListDemo {
    public static void main(String[] args) {
        // Creating an ArrayList
        ArrayList<String> fruits = new ArrayList<>();
        
        // Adding elements
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");
        fruits.add("Date");
        fruits.add("Elderberry");
        System.out.println("Fruits: " + fruits);
        
        // Accessing elements
        System.out.println("First: " + fruits.get(0));
        System.out.println("Size: " + fruits.size());
        
        // Modifying
        fruits.set(1, "Blueberry");  // Replace Banana
        System.out.println("After set: " + fruits);
        
        // Removing
        fruits.remove("Date");        // By value
        fruits.remove(0);             // By index
        System.out.println("After remove: " + fruits);
        
        // Searching
        System.out.println("Contains Cherry? " + fruits.contains("Cherry"));
        System.out.println("Index of Cherry: " + fruits.indexOf("Cherry"));
        
        // Sorting
        Collections.sort(fruits);
        System.out.println("Sorted: " + fruits);
        
        // Iterating
        System.out.println("\nAll fruits:");
        for (String fruit : fruits) {
            System.out.println("  - " + fruit);
        }
        
        // ArrayList of numbers
        ArrayList<Integer> numbers = new ArrayList<>();
        for (int i = 1; i <= 10; i++) {
            numbers.add(i * i);  // Squares
        }
        System.out.println("\nSquares: " + numbers);
    }
}

Try It Yourself: To-Do List Manager

Try It Yourself: To-Do List ManagerJava

Java Editor

✓ ValidTab = 2 spaces

import java.util.ArrayList;

public class TodoList {
    public static void main(String[] args) {
        ArrayList<String> tasks = new ArrayList<>();
        ArrayList<Boolean> completed = new ArrayList<>();
        
        // Add tasks
        addTask(tasks, completed, "Learn Java Collections");
        addTask(tasks, completed, "Build a project");
        addTask(tasks, completed, "Practice coding");
        addTask(tasks, completed, "Read documentation");
        addTask(tasks, completed, "Submit assignment");
        
        // Mark some as done
        completeTask(tasks, completed, 0);
        completeTask(tasks, completed, 2);
        
        // Display
        printTasks(tasks, completed);
        
        // Remove completed tasks
        System.out.println("\nRemoving completed tasks...");
        for (int i = tasks.size() - 1; i >= 0; i--) {
            if (completed.get(i)) {
                tasks.remove(i);
                completed.remove(i);
            }
        }
        printTasks(tasks, completed);
    }
    
    static void addTask(ArrayList<String> tasks, ArrayList<Boolean> completed, String task) {
        tasks.add(task);
        completed.add(false);
    }
    
    static void completeTask(ArrayList<String> tasks, ArrayList<Boolean> completed, int index) {
        completed.set(index, true);
        System.out.println("Completed: " + tasks.get(index));
    }
    
    static void printTasks(ArrayList<String> tasks, ArrayList<Boolean> completed) {
        System.out.println("\n=== To-Do List ===");
        for (int i = 0; i < tasks.size(); i++) {
            String status = completed.get(i) ? "[X]" : "[ ]";
            System.out.println(status + " " + tasks.get(i));
        }
        System.out.println("Total: " + tasks.size() + " tasks");
    }
}

import java.util.ArrayList;

Output

Click ▶ Run to see the result

Edit the code on the left, then click Run

Java|51 lines|1804 chars|✓ Valid syntax

UTF-8

ArrayList & Dynamic Arrays

ArrayList Operations

Try It Yourself: To-Do List Manager

Quick Quiz: ArrayList

Topics in This Module

ArrayList & Dynamic Arrays

ArrayList Operations

Try It Yourself: To-Do List Manager

Quick Quiz: ArrayList

Topics in This Module