Arrays vs. ArrayLists: A Must-Know Topic for Java Interviews

When preparing for Java interviews, one common question that often trips up candidates is the difference between Arrays and ArrayLists. At first glance, both seem similar—they store elements and let you access them by index. But under the hood, they work quite differently. This post will walk you through the key contrasts between Arrays and ArrayLists, backed with examples to make things crystal clear.

By the end, you’ll not only be able to answer this question confidently but also know when to use which.

1. Fixed vs. Dynamic Size

Arrays: When an array is created, its size must be explicitly defined during initialization. If the size is omitted, the code will result in a compilation error, and once defined, the size cannot be changed.

int[] numbers = new int[5];  // Size fixed at 5
// Adding a 6th element will throw an ArrayIndexOutOfBoundsException
numbers[5] = 6;  // Exception: Index 5 out of bounds for length 5

ArrayLists: With ArrayLists, size is dynamic—they grow (and shrink) automatically as you add or remove elements. Even though you can provide an initial capacity, it’s optional.

  ArrayList<Integer> nums = new ArrayList<>(1);  // Initial capacity set to 1
  nums.add(1); 
  nums.add(2);  // No issues even if it exceeds initial capacity!

Takeaway: Use arrays when size is fixed. Use ArrayLists when you need flexibility.

2. Handling Primitives and Objects

Arrays: Arrays can store both primitives (like int, char) and objects.

// Array of primitives
char[] letters = new char[3];

// Array of objects
ArrayListAndArrayDifferences[] objArray = new ArrayListAndArrayDifferences[2];

ArrayLists: ArrayLists only store objects, so if you want to store primitives, you’ll need to use wrapper classes (like Integer for int).

  // Using Integer instead of int
  ArrayList<Integer> nums = new ArrayList<>();
  nums.add(10);

Common Pitfall in Interviews:

If you try to use a primitive type directly with ArrayList (e.g., ArrayList<int>), it will result in a compilation error.

3. Accessing and Modifying Elements

Both Arrays and ArrayLists allow access to elements by index, but the way they do it differs.

Array: You use the index directly to access or modify elements.

  numbers[1] = 42;  // Assigning a value
  System.out.println(numbers[1]);  // Accessing the value

ArrayList: ArrayLists use methods like add() and get() to handle operations.

  nums.add(42);  // Adding element
  System.out.println(nums.get(0));  // Accessing element

Interview Tip:

Mention that using methods for element access in ArrayLists makes the code cleaner but slightly less performant than direct array access.

4. Performance and Operations

Arrays: Arrays are more performance-oriented because they don’t have overhead like resizing. However, managing elements can be cumbersome since there are no built-in methods for things like removing or searching.

ArrayLists: ArrayLists offer handy methods such as add(), remove(), and indexOf(), which simplify working with dynamic data. But the flexibility comes with a slight performance cost due to resizing and wrapping/unwrapping of primitive values.

5. Checking Size

Array: To find the size of an array, you use the length field.

  System.out.println(numbers.length);  // Output: 5

ArrayList: With ArrayLists, you call the size() method.

  System.out.println(nums.size());  // Output: 2

Summary Table: Arrays vs. ArrayLists

Feature	Array	ArrayList
Size	Fixed	Dynamic
Type of Elements	Primitives and Objects	Objects only
Performance	Faster	Slight overhead
Operations	Manual with operators	Built-in methods like `add()`
Size Retrieval	`length` field	`size()` method

Which One to Use?

Use Arrays when:
- The size is known and fixed.
- You need fast access and minimal overhead.
Use ArrayLists when:
- The size of the collection can vary.
- You need built-in methods to make element management easier.

Interview Pro Tip

When faced with this question in an interview, don’t just list the differences—demonstrate your understanding by giving practical examples. Bonus points if you can explain the trade-offs between performance and usability.

For example:

Arrays are faster, but managing them is more manual.
ArrayLists are easier to work with but come with some overhead.

Conclusion

Understanding the difference between Arrays and ArrayLists is essential not just for interviews but also for writing efficient Java code. Knowing when to choose one over the other demonstrates that you understand both performance considerations and practical coding scenarios.

Stay tuned for the next posts in this series, where we’ll dive deeper into the Collections Framework and other essential data structures, like on HashMap and HashSet!