Java ArrayList and exception handling

I’d like to share my experience from a recent project that heavily utilized Java’s ArrayList. At first glance, ArrayList seems simple—dynamic, flexible, and easy to use—but in reality, it’s easy to encounter some tricky runtime exceptions if you’re not careful. Below are the problems I encountered, my debugging methods, and the lessons I learned.

We often need to store collections of objects that may grow or shrink. ArrayList is clearly the best choice because, unlike arrays, it doesn’t require a predefined size. My commonly used operations include:

• Adding elements: add()
• Removing elements: remove()
• Accessing elements: get()
• Modifying elements: set()

soon enough, I ran into runtime exceptions that forced me to rethink how I handled ArrayList. These included:

• IndexOutOfBoundsException
• NullPointerException
• ConcurrentModificationException

Below, I’ll walk through how each of these tripped me up and what actually worked.

IndexOutOfBoundsException

This one hit me the first week I was working with a list of user inputs. I naively tried to get an element without checking the size, and… boom:

import java.util.ArrayList;

public class ArrayListExceptionExample {
    public static void main(String[] args) {
        ArrayList<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");

        try {
            // Attempting to access an invalid index
            System.out.println(fruits.get(5));
        } catch (IndexOutOfBoundsException e) {
            System.out.println("Error: " + e.getMessage());
        }

        System.out.println("Program continues after exception handling.");
    }
}

When I ran this, the console spat out:

Error: Index 5 out of bounds for length 3
Program continues after exception handling.

I remember thinking at the time, “Right, this list only has three elements… I need to check the index before retrieving the elements.”

Debugging Insight: I added checks like if(index < list.size()) before accessing. It seems trivial, but in a project where lists are dynamically updated, this saved me hours of IndexOutOfBoundsException chaos.

2. NullPointerException

This one came as a surprise during testing. I had declared an ArrayList but forgot to initialize it in one branch of the code:

import java.util.ArrayList;

public class NullPointerExample {
    public static void main(String[] args) {
        ArrayList<String> fruits = null;

        try {
            // Attempting to add an element to a null ArrayList
            fruits.add("Apple");
        } catch (NullPointerException e) {
            System.out.println("Caught NullPointerException: Initialize your ArrayList first!");
        }

        // Proper initialization
        fruits = new ArrayList<>();
        fruits.add("Apple");
        System.out.println("ArrayList after initialization: " + fruits);
    }
}

Output:

Caught NullPointerException: Initialize your ArrayList first!
ArrayList after initialization: [Apple]

I was incredibly annoyed—how could I forget to use new ArrayList<>()? However, after several hours of debugging, I realized there were multiple initialization paths in the project, and some of them didn’t initialize the list. The lesson learned is: always initialize before use.

3. ConcurrentModificationException

This issue nearly caused a feature to crash. We were iterating through a list of items to remove some invalid entries:

import java.util.ArrayList;
import java.util.Iterator;

public class ConcurrentModificationExample {
    public static void main(String[] args) {
        ArrayList<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");

        try {
            for (String fruit : fruits) {
                if (fruit.equals("Banana")) {
                    fruits.remove(fruit); // Unsafe modification
                }
            }
        } catch (Exception e) {
            System.out.println("Exception caught: " + e);
        }

        // Correct approach using Iterator
        Iterator<String> iterator = fruits.iterator();
        while (iterator.hasNext()) {
            if (iterator.next().equals("Banana")) {
                iterator.remove();
            }
        }

        System.out.println("ArrayList after safe removal: " + fruits);
    }
}

Running the unsafe loop threw:

Exception caught: java.util.ConcurrentModificationException
ArrayList after safe removal: [Apple, Orange]

Modifying a list within a for-each loop is not advisable. Using an iterator is safer. I later timed it specifically—for large lists, using an iterator is not only correct, but also faster than creating a copy of the list for safe deletion. This was a pleasant surprise.

I have developed some practical habits:

• Always check list boundaries before accessing elements.
• Initialize lists before using them, even if it feels redundant.
• Use try-catch judiciously—don’t just silence exceptions; log meaningful messages.
• Prefer Iterator for modifications during iteration; avoid the sneaky ConcurrentModificationException.

I realized that handling exceptions is not just about keeping the program running—it’s about anticipating failure points and writing code that won’t crash unexpectedly.

My experience using ArrayList has taught me that even such a common feature in Java can encounter a variety of complexities in real-world projects. Seemingly trivial errors—such as incorrect indexing, null references, and unsafe iteration—can lead to hours of debugging if ignored. Now, I keep these lessons in mind whenever I develop functions involving large numbers of lists.