What Is a Collection?
By the end of this lesson, you'll be able to:
- Explain what a collection is and why a single variable per value doesn't scale
- Name Apex's three collection types: List, Set, and Map
- Recognize when a problem calls for a collection instead of individual variables
Prerequisites: Module 14: "Professional Engineering Practices I"
The problem: one variable per value doesn't scale
String customer1 = 'Amara';
String customer2 = 'Ben';
String customer3 = 'Carla';
// ...what about customer 500?This breaks down immediately past a handful of values, and it's completely unworkable when the count isn't known until runtime — like every Account a SOQL query happens to return. A collection solves this: one variable that holds many values at once.
Apex's three collection types, previewed
- List — an ordered collection that allows duplicates. Covered in depth for the rest of this module.
- Set — an unordered collection with no duplicates.
- Map — key-value pairs, for looking values up by a key.
The next module goes deep on Set and Map; this one focuses entirely on List, the collection type you'll reach for by default.
Exercise
As a comment, explain why writing customer1, customer2, customer3... doesn't work for data returned from a SOQL query.
Show hint
Think about whether you know the count in advance.
What Is a Collection? Quiz
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Summary
A collection holds many values in a single variable — Apex provides three kinds (List, Set, Map), each suited to a different job, starting with List in this module.