Sets

Set<String> uniqueEmails = new Set<String>();
uniqueEmails.add('amara@example.com');
uniqueEmails.add('ben@example.com');
uniqueEmails.add('amara@example.com'); // silently ignored — already present

System.debug(uniqueEmails.size()); // 2, not 3

1. Set<String> uniqueEmails = new Set<String>(); — an empty Set of String values.
2. uniqueEmails.add('amara@example.com'); — adds the first email.
3. uniqueEmails.add('ben@example.com'); — adds a second.
4. uniqueEmails.add('amara@example.com'); — unlike List (Module 15), adding a value already in a Set does nothing — no error, no duplicate, just silently ignored.
5. (blank line)
6. System.debug(uniqueEmails.size()); — logs 2, not 3.

This single trait is the whole reason to reach for Set instead of List.

List<String> industries = new List<String>{'Retail', 'Manufacturing', 'Retail', 'Healthcare', 'Manufacturing'};
Set<String> uniqueIndustries = new Set<String>(industries);

System.debug(uniqueIndustries.size()); // 3

1. List<String> industries = new List<String>{'Retail', 'Manufacturing', 'Retail', 'Healthcare', 'Manufacturing'}; — a list with two repeated values.
2. Set<String> uniqueIndustries = new Set<String>(industries); — passing a List directly into new Set<String>(...) is the fastest way to deduplicate it — every duplicate is automatically dropped, no loop required.
3. (blank line)
4. System.debug(uniqueIndustries.size()); — logs 3 — the three unique values.

Set<String> vipCustomerIds = new Set<String>{'C001', 'C002', 'C003'};

if (vipCustomerIds.contains('C002')) {
    System.debug('This is a VIP customer.');
}

1. Set<String> vipCustomerIds = new Set<String>{'C001', 'C002', 'C003'}; — a set of ids.
2. (blank line)
3. if (vipCustomerIds.contains('C002')) { — contains() checks whether a value exists in a Set — typically much faster than looping through a List checking each item one by one, especially as the collection grows large.
4. System.debug('This is a VIP customer.'); — runs since 'C002' is present.
5. } — closes the if.