ArrayList in Java

ArrayList is one of the most commonly used classes in the Java Collections Framework.

It provides a resizable array that can grow or shrink dynamically.

1. What is ArrayList?

Definition:
ArrayList is a resizable-array implementation of the List interface.

It:

• Maintains insertion order

• Allows duplicate elements

• Allows null values

• Provides index-based access

• Grows dynamically

Syntax:

ArrayList<Type> list = new ArrayList<>();

Example:

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

names.add("Mani");
names.add("Arun");
names.add("Kumar");

System.out.println(names);

Output:

[Mani, Arun, Kumar]

2. Creating an ArrayList

Syntax:

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

Example:

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

numbers.add(10);
numbers.add(20);
numbers.add(30);

3. ArrayList with Initial Capacity

Definition:
We can specify an initial capacity when creating an ArrayList.

Syntax:

ArrayList<Type> list = new ArrayList<>(capacity);

Example:

ArrayList<Integer> numbers = new ArrayList<>(20);

This creates an ArrayList with an initial capacity of 20.

Capacity and size are different. Capacity is the allocated storage; size is the number of elements currently stored.

4. Adding Elements

Definition:
add() adds an element to the end of the list.

Example:

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

names.add("Java");
names.add("Spring");

System.out.println(names);

Output:

[Java, Spring]

5. Adding Element at an Index

Definition:
add(index, element) inserts an element at a specific position.

Example:

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

names.add("Java");
names.add("Spring");

names.add(1, "Python");

System.out.println(names);

Output:

[Java, Python, Spring]

6. Accessing Elements

Definition:
get(index) returns the element at a specific index.

Example:

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

names.add("Java");
names.add("Spring");

System.out.println(names.get(0));

Output:

Java

7. Updating Elements

Definition:
set(index, element) replaces an existing element.

Example:

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

names.add("Java");
names.add("Spring");

names.set(1, "React");

System.out.println(names);

Output:

[Java, React]

8. Removing Elements

Definition:
remove() removes an element by index or object.

Example:

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

names.add("Java");
names.add("Spring");

names.remove(0);

System.out.println(names);

Output:

[Spring]

9. Removing by Object

Example:

names.remove("Spring");

This removes the first matching element.

10. Finding the Size

Definition:
size() returns the number of elements currently stored.

Example:

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

names.add("Java");
names.add("Spring");

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

Output:

2

11. Checking an Element

Definition:
contains() checks whether an element exists.

Example:

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

names.add("Java");

System.out.println(names.contains("Java"));

Output:

true

12. Finding Index

Definition:
indexOf() returns the first index of an element.

Example:

ArrayList<String> names =
        new ArrayList<>(Arrays.asList("Java", "Spring", "Java"));

System.out.println(names.indexOf("Java"));

Output:

0

13. Last Index

Definition:
lastIndexOf() returns the last index of an element.

Example:

System.out.println(names.lastIndexOf("Java"));

Output:

2

14. Checking Empty ArrayList

Definition:
isEmpty() checks whether the list contains no elements.

Example:

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

System.out.println(names.isEmpty());

Output:

true

15. Clearing ArrayList

Definition:
clear() removes all elements.

Example:

names.clear();

System.out.println(names);

Output:

[]

16. Iterating Using for Loop

Example:

ArrayList<String> names =
        new ArrayList<>(Arrays.asList("Java", "Spring", "React"));

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

17. Iterating Using Enhanced for Loop

Example:

for (String name : names) {
    System.out.println(name);
}

18. Iterating Using forEach

Example:

names.forEach(name -> System.out.println(name));

19. Sorting ArrayList

Definition:
An ArrayList can be sorted using Collections.sort() or sort().

Example:

ArrayList<Integer> numbers =
        new ArrayList<>(Arrays.asList(30, 10, 20));

Collections.sort(numbers);

System.out.println(numbers);

Output:

[10, 20, 30]

20. Reverse Sorting

Example:

numbers.sort(Comparator.reverseOrder());

System.out.println(numbers);

Output:

[30, 20, 10]

21. ArrayList with Duplicates

Definition:
ArrayList allows duplicate elements.

Example:

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

names.add("Java");
names.add("Java");

System.out.println(names);

Output:

[Java, Java]

22. ArrayList with null

Definition:
ArrayList allows null elements.

Example:

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

names.add("Java");
names.add(null);

System.out.println(names);

Output:

[Java, null]

23. Converting ArrayList to Array

Definition:
toArray() converts the list into an array.

Example:

ArrayList<String> names =
        new ArrayList<>(Arrays.asList("Java", "Spring"));

String[] array = names.toArray(new String[0]);

24. Copying an ArrayList

Example:

ArrayList<String> original =
        new ArrayList<>(Arrays.asList("Java", "Spring"));

ArrayList<String> copy =
        new ArrayList<>(original);

25. ArrayList vs Array

26. ArrayList Time Complexity

27. Important ArrayList Methods

add()
add(index, element)
get()
set()
remove()
size()
contains()
indexOf()
lastIndexOf()
isEmpty()
clear()
sort()
forEach()
toArray()

Complete Example

import java.util.*;

public class Main {
    public static void main(String[] args) {

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

        languages.add("Java");
        languages.add("Python");
        languages.add("Java");

        System.out.println(languages);

        System.out.println(languages.get(0));

        languages.set(1, "Spring");

        languages.remove("Java");

        System.out.println(languages);
        System.out.println(languages.size());
    }
}

Output:

[Java, Python, Java]
Java
[Python, Java]
2

Conclusion

ArrayList is a commonly used Java collection when we need a dynamic, ordered list with index-based access.

The important points to remember are:

ArrayList
→ Dynamic size
→ Maintains insertion order
→ Allows duplicates
→ Allows null
→ Fast index-based access
→ Slower insertion/removal in the middle

It is one of the most important Java Collections to understand for Java development, Spring Boot, DSA, and interviews.