Maps
By the end of this lesson, you'll be able to:
- Declare and initialize a Map with a key type and a value type
- Add, retrieve, and check for the existence of a key
- Explain why a Map lookup by key is faster than searching a List
Prerequisites: "Sets"
Declaring and populating a Map
Map<String, Decimal> productPrices = new Map<String, Decimal>();
productPrices.put('Widget', 49.99);
productPrices.put('Gadget', 89.99);
System.debug(productPrices.get('Widget')); // 49.99Map<String, Decimal> declares a map from String keys to Decimal values. put(key, value) adds or updates an entry; get(key) retrieves the value for that key directly — no looping required.
Checking for a key before using it
Map<String, Decimal> productPrices = new Map<String, Decimal>{'Widget' => 49.99};
if (productPrices.containsKey('Gadget')) {
System.debug(productPrices.get('Gadget'));
} else {
System.debug('No price found for Gadget.');
}Calling get() on a key that doesn't exist returns null rather than throwing an error — but relying on that silently can lead to confusing NullPointerExceptions later. containsKey() checks safely first, exactly the guard-before-use habit from Module 12's "Handle Errors Gracefully" lesson.
Why a Map beats searching a List
// The List approach: search every item, one at a time
for (Product__c p : allProducts) {
if (p.Name == 'Widget') {
System.debug(p.Price__c);
break;
}
}
// The Map approach: direct lookup, no searching
System.debug(productPrices.get('Widget'));Searching a List for a matching item means checking items one by one until you find it (or don't) — slower as the list grows. A Map looks a key up directly, which is why "look this up by Id" is almost always modeled as a Map in real Apex code, especially inside loops where repeated searching would be costly.
Exercise
Build a Map<String, Integer> called stockLevels with three products and their stock counts. Debug the stock level for one of them using get().
Show hint
stockLevels.put('Widget', 40);
Maps Quiz
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Summary
A Map stores key-value pairs, letting you look up a value directly by its key instead of searching through a list one item at a time.