Java ArrayList Explained: Usage, Key Methods, and Practical Examples

What is ArrayList in Java?

ArrayList is a dynamic array that automatically expands and contracts as elements are added or removed. It’s essential when you need a flexible collection that can grow or shrink during runtime.

Unlike a fixed‑size array (think of a rigid rope), an ArrayList behaves like a stretchable rubber band, allowing you to store more items without having to predefine a maximum size.

When you add elements, ArrayList grows automatically; when elements are removed, it shrinks, ensuring efficient memory usage.

ArrayList Methods

• add(Object o) – Appends an element to the end of the list or inserts it at a specified index.
• remove(Object o) – Removes the first occurrence of the specified element.
• remove(int index) – Deletes the element at the given index.
• size() – Returns the current number of elements in the list.
• contains(Object o) – Checks whether the list contains the specified element.

Java ArrayList Example

Below is a concise demonstration of the most common operations on an ArrayList.

import java.util.ArrayList;

public class TestArrayList {
    public static void main(String[] args) {
        // Create a generic ArrayList
        ArrayList<String> arlTest = new ArrayList<>();

        // Initial size
        System.out.println("Size at creation: " + arlTest.size());

        // Add elements
        arlTest.add("D");
        arlTest.add("U");
        arlTest.add("K");
        arlTest.add("E");

        // Size after additions
        System.out.println("Size after adding: " + arlTest.size());
        System.out.println("Elements: " + arlTest);

        // Remove by value
        arlTest.remove("D");
        System.out.println("After removing " + "D": " + arlTest);

        // Remove by index
        arlTest.remove(2); // removes "K"
        System.out.println("After removing index 2: " + arlTest);

        // Final size
        System.out.println("Final size: " + arlTest.size());
        System.out.println("Final list: " + arlTest);

        // Contains check
        System.out.println("Contains " + "K" + ": " + arlTest.contains("K"));
    }
}

Output:

Size at creation: 0
Size after adding: 4
Elements: [D, U, K, E]
After removing D: [U, K, E]
After removing index 2: [U, K]
Final size: 2
Final list: [U, K]
Contains K: true

Note: While the example uses single characters, ArrayList can hold any object type—strings, integers, custom classes, and more.