Lists

List<String> customerNames = new List<String>();
customerNames.add('Amara');
customerNames.add('Ben');
customerNames.add('Amara'); // duplicates are allowed

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

1. List<String> customerNames = new List<String>(); — List<String> declares a list that only holds String values — Apex enforces this type at compile time.
2. customerNames.add('Amara'); — add() appends to the end.
3. customerNames.add('Ben'); — appends a second value.
4. customerNames.add('Amara'); — unlike a Set (next module), a List happily allows the same value more than once.
5. (blank line)
6. System.debug(customerNames.size()); — logs 3 — all three additions counted, including the duplicate.

List<String> customerNames = new List<String>{'Amara', 'Ben', 'Carla'};

System.debug(customerNames[0]); // 'Amara' — first item, index 0
System.debug(customerNames[2]); // 'Carla'

customerNames.remove(1); // removes 'Ben'
System.debug(customerNames); // ['Amara', 'Carla']

1. List<String> customerNames = new List<String>{'Amara', 'Ben', 'Carla'}; — a list built with three starting values.
2. (blank line)
3. System.debug(customerNames[0]); — indexes start at 0, exactly like the List<Integer> quantities examples back in Module 7's loops — logs 'Amara'.
4. System.debug(customerNames[2]); — logs 'Carla', the third item.
5. (blank line)
6. customerNames.remove(1); — remove(index) deletes the item at that position and shifts everything after it down by one — removes 'Ben'.
7. System.debug(customerNames); — logs ['Amara', 'Carla'].

List<Integer> scores = new List<Integer>{50, 20, 90};
System.debug(scores); // [50, 20, 90] — exact insertion order, unchanged

1. List<Integer> scores = new List<Integer>{50, 20, 90}; — a list built in this specific order.
2. System.debug(scores); — logs [50, 20, 90] — exact insertion order, unchanged. A List always keeps items in the order they were added (or explicitly sorted, covered later in this module) — the key trait that distinguishes it from Set, which makes no order guarantee at all.