When to Use List or Array in Apex Salesforce

Alright, let’s kick things off by laying the foundation. In Apex, you’ve got two main tools to hold a bunch of values: Lists and Arrays. On the surface, they might look like twins - they both store collections of items and allow you to loop through them. But dig a little deeper, and you’ll find they each have their own quirks.

A List in Apex is a part of the standard collection framework. It's dynamic, flexible, and comes with a rich set of methods like add(), remove(), contains() and more. On the flip side, an Array feels like the simpler, old-school sibling - declared with a fixed size and seemingly more limited at first glance.

But here's the twist: Apex Arrays aren't really "traditional" arrays like in Java. More on that in a bit.

Declaring a List and an Array

Here’s a side-by-side comparison of how you declare and use each one:

List:

List<String> fruits = new List<String>();
fruits.add('Apple');
fruits.add('Banana');

Array:

String[] fruits = new String[2];
fruits[0] = 'Apple';
fruits[1] = 'Banana';

Functionally? They behave the same in simple cases. But under the hood? Things get interesting.

Wait, Isn’t Apex Built on Java? Let’s Talk About That

Yep, you're absolutely right. Apex is built as an abstraction over Java, but it’s a highly customized one, tailored for the Salesforce platform.

In Java, arrays are true fixed-size structures. Once you define the size, that’s it - you can’t add or remove elements without doing some acrobatics.

In Apex, though, arrays are more like "syntactic sugar" over lists. That means that even though you declare something as an array, under the hood, it's actually treated as a List. You can access all the same methods, and the system treats arrays and lists as nearly identical types.

Let’s break that down with a quick demo:

Integer[] arr = new Integer[1];
Object obj = (Object) arr;

System.debug(obj instanceof Integer[]);         // true
System.debug(obj instanceof List<Integer>);     // true

Whoa. Both return true. That’s wild.

So what does that mean? It means that Apex arrays are not traditional, fixed-length arrays like they are in Java. You’re not dealing with the same rigid structure. The array you declared is just wearing a different outfit - but underneath, it’s still a collection.

This proves that arrays in Apex are not truly distinct from Lists in terms of behavior. They’re more of a stylistic choice than a fundamentally different data type.

Use Case: When to Choose a List

Alright, let’s talk real-life dev scenarios. You’re writing a trigger, maybe on Contact insert, and you need to gather all related Account IDs for an update. You don’t know how many Contacts are in the Trigger context, and that number can change anytime. You need something flexible.

Boom - List to the rescue.

List<Id> accountIds = new List<Id>();

for (Contact con : Trigger.new) {
    if (con.AccountId != null) {
        accountIds.add(con.AccountId);
    }
}

No need to guess the size ahead of time. Just collect and move on. Lists grow as needed, and they’re super friendly with SOQL queries, DML operations, and just about every part of Apex logic.

Use Case: When to Choose an Array

Now, are there any moments where an array might be a better pick? Sure-when you have a very small, fixed-size collection, and it just feels cleaner to declare it inline.

Let’s say you’re building out weekdays for a dropdown menu. You know there are seven, always.

String[] days = new String[7]{'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'};

You could use a List here too - but if you’re never modifying this list and want to keep things tidy, the array syntax might read cleaner to your eyes.

This is more about developer convenience and readability than functionality.

The Verdict: Do You Ever Really Need Arrays in Apex?

Let’s be blunt: there’s almost no practical reason to prefer arrays over lists in Apex.

Arrays in Apex aren’t truly arrays in the Java sense. They don’t lock you into a fixed size, and under the hood, they’re treated as collections anyway. You can use all the List methods-add(), size(), remove() - on arrays. They’re just a different way of writing the same thing.

But functionally? You’re better off just using Lists for everything else.

So unless you’ve got a good reason and a clean use case, do yourself a favor: stick with Lists. They’re more readable, more dynamic, and more in line with how Apex is meant to be used.

Wrapping It Up

Apex is quirky, and its take on arrays vs. lists is a perfect example. While it borrows from Java, it twists things just enough to throw you off if you're not paying attention. What looks like an array is actually a list in disguise. That’s why sticking to Lists just makes sense in most real-world coding scenarios.

Use Lists for flexibility, dynamic sizing, and better compatibility with Salesforce features. Keep arrays in your back pocket for rare, specific situations where they just look better for a tiny static set.

And remember: in Apex, Arrays aren’t really arrays - they’re just Lists in a trench coat.