Apex Collections Cheat Sheet: List vs Set vs Map (with the Patterns You Actually Use)

Hey friends,

Use a List when order matters or you're about to insert or update records. Use a Set when you need unique values, usually Ids for a WHERE Id IN :ids filter. Use a Map when you need to find something by key quickly, usually a record by its Id. Apex has only these three collections, and no arrays apart from lists.

These definitions come from Trailhead's Apex & .NET Basics and Apex Database units.

The whole cheat sheet in one table.

List

An ordered, indexed collection, like an array. The first element is index 0.

1List<String> colors = new List<String>{ 'red', 'green', 'blue' };
2colors.add('yellow');
3String first = colors[0];

Query results come back as lists, and DML takes lists. That's why insert accts; on a list beats inserting inside a loop.

Set

An unordered collection with no duplicates. Trailhead's example: a set is commonly used to store Ids because they're always unique, and then used in a SOQL WHERE clause.

1Set<Id> acctIds = new Set<Id>();
2for (Contact c : Trigger.new) { acctIds.add(c.AccountId); }
3List<Account> accts = [SELECT Id, Name FROM Account WHERE Id IN :acctIds];

Map

Key-value pairs. Keys are unique and each maps to one value. Trailhead's example maps Accounts to their Ids, which you can build straight from a query:

1Map<Id, Account> accountMap = new Map<Id, Account>([SELECT Id, Name FROM Account]);
2Account a = accountMap.get(someId);

The pattern: Set to query, Map to look up, List to save

This is the shape of almost every bulk-safe trigger, and it's the heart of my trigger best practices:

1. Loop the records once and collect the Ids you need in a Set<Id>.
2. Run one query with WHERE Id IN :ids and put the results in a Map<Id, SObject>.
3. Loop again, look each related record up with map.get(), and add changes to a List.
4. One DML statement on the list, outside the loop.

One query and one DML, no matter how many records. That keeps you far from the governor limits.

Exam traps

• Index out of bounds: reading list[0] on an empty query result throws an exception. Check isEmpty() first.
• Sets don't keep order. Don't rely on iteration order.
• Map.get on a missing key returns null, not an error. Check containsKey() or null.
• Adding a duplicate to a Set silently does nothing.

Keep studying

• SOQL relationship queries.
• DML vs Database methods.
• My 8-week PD1 study plan.

Hope this helps!

Best,

Nick