Sets

A set is an unordered collection of elements that do not contain any duplicates. Set elements can be of any data type—primitive types, collections, sObjects, user-defined types, and built-in Apex types.

This table represents a set of strings that uses city names:

'San Francisco'

'New York'

'Paris'

'Tokyo'

Sets can contain collections that can be nested within one another. For example, you can have a set of lists of sets of Integers. A set can contain up to four levels of nested collections inside it, that is, up to five levels overall.

To declare a set, use the Set keyword followed by the primitive data type name within <> characters. For example:

The following example shows how to create a set with two hardcoded string values.

To access elements in a set, use the system methods provided by Apex. For example:

The following example shows how to create a set from elements of another set.

For more information, including a complete list of all supported set system methods, see Set Class.

Note the following limitations on sets:

Unlike Java, Apex developers do not need to reference the algorithm that is used to implement a set in their declarations (for example, HashSet or TreeSet). Apex uses a hash structure for all sets.
A set is an unordered collection—you can’t access a set element at a specific index. You can only iterate over set elements.
The iteration order of set elements is deterministic, so you can rely on the order being the same in each subsequent execution of the same code.

Apex Developer Guide

Sets