Does String.join return an empty string for an empty list?

Yes. String.join on an empty List returns an empty string, not null, and it does not throw. Our test asserts that joinNames(new List<Account>(), ', ') equals ''.

Is a Set ordered in Apex?

No. A Set is an unordered collection with no index access; you can only iterate over it. Salesforce documents that the iteration order is deterministic, but if you need a specific order, copy the values into a List and sort it.

Are Map keys case-sensitive in Apex?

Yes, for String keys. Two keys that differ only by case are separate entries, and put, get, containsKey and remove treat them as different keys.

How many elements can an Apex collection hold?

There is no fixed item limit for a List, Set or Map. The heap size limit applies instead, which the Winter '27 governor limits page lists as 10 MB synchronous and 25 MB asynchronous.

Where are String.join and replaceAll documented?

Both are in the String Class page of the Apex Reference: Apex Reference: String Class . Map behavior such as null keys and overwrites is on the Apex Developer Guide: Maps page.

Collections in Salesforce Apex: List, Set, Map, String.join

Read partners[0] from a query that returned no rows and Apex stops with System.ListException: List index out of bounds: 0. Ask a map for a key it does not have, add 1 to the answer, and you get a NullPointerException one line later.

Both bugs come from treating a List, a Set and a Map as if they behaved the same. We built one small class on five partner accounts at Ridgeline Supply in a Winter ’27 Developer Edition org, ran it, tested it, and broke it on purpose, so every output below is real.

By the end you will know which of the collections in salesforce apex to reach for, how to group records in a Map, and how to turn any list into a clean string with String.join.

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