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Lists, Sets and Maps

Lists, Sets and Maps

These three collections are the backbone of every bulk-safe Apex program.

List: ordered, allows duplicates

List<String> heroes = new List<String>{'Nomsa', 'Sipho'};
heroes.add('Thandi');
String first = heroes[0];     // or heroes.get(0)
Integer n = heroes.size();

A SOQL query returns a List<SObject>. You'll use Lists constantly.

Set: unordered, unique values

Set<Id> accountIds = new Set<Id>();
accountIds.add(someId);
accountIds.add(someId);       // ignored, already there
Boolean has = accountIds.contains(someId);
Set<String> fromList = new Set<String>(heroes); // dedupe a list

Sets are perfect for collecting Ids to use in WHERE Id IN :accountIds.

Map: key → value

Map<String, Integer> xpByHero = new Map<String, Integer>();
xpByHero.put('Nomsa', 120);
Integer xp = xpByHero.get('Nomsa');    // 120
Integer missing = xpByHero.get('Bob'); // null, not an error
Boolean known = xpByHero.containsKey('Nomsa');

And the most useful constructor in all of Apex:

Map<Id, Account> accountsById = new Map<Id, Account>([SELECT Id, Name FROM Account]);

Your mission

loot already exists. Build uniqueLoot (a Set) and lootCounts (a Map counting each item).

🦝 Maps are Quill's favourite data structure. You'll understand why in the bulkification chapter.