Introduction to Sets

A list is great when order matters and repeats are fine. Sometimes you want the opposite: a bag of values where each value appears once and the order does not matter. That collection is a Set.

Creating a Set

You declare a set by naming the type it holds inside angle brackets:

Set<String> tags = new Set<String>();
tags.add('urgent');
tags.add('review');
// tags now holds urgent and review

You can also fill a set at the moment you create it, or build one directly from a list:

Set<Integer> codes = new Set<Integer>{ 100, 200, 300 };

List<String> names = new List<String>{ 'Ana', 'Miguel' };
Set<String> nameSet = new Set<String>(names);

The Methods You Will Use Most

• add(value) puts a value into the set
• contains(value) hands back true if the value is already there
• size() tells you how many values the set holds
• remove(value) takes a value back out
• isEmpty() tells you whether the set holds nothing at all

Set<String> tags = new Set<String>{ 'urgent', 'review' };
Boolean isUrgent = tags.contains('urgent'); // true
Integer howMany = tags.size();              // 2

No Index, No Order

A set has no positions, so there is no tags.get(0). Asking a set for its first element is not a thing you can do. If you need to look at every value, use a for-each loop, and do not count on the order staying the same.

Why This Matters

contains() is the reason sets exist. Checking whether a value is already in a set is fast and reads clearly, which makes sets the right tool for membership questions like "have I already seen this record id".