Lists: A Quick Foundation Before Looping

Every variable you've written so far holds exactly one value:

String customerName = 'Acme Corp';

1. String customerName = 'Acme Corp'; — one variable, one value.

But real Salesforce work is rarely about one record — it's dozens, hundreds, or thousands of Accounts, Cases, or Contacts at once. You need a single variable that can hold many values together. That's what a List is for.

List<String> customerNames = new List<String>();
customerNames.add('Acme Corp');
customerNames.add('Globex');
customerNames.add('Initech');

1. List<String> customerNames = new List<String>(); — List<String> means "a List that holds String values specifically" (the type inside <...> says what the List is allowed to hold). new List<String>() creates a brand-new, empty List — the same new keyword you've already used for other types.
2. customerNames.add('Acme Corp'); — .add(...) appends one item to the end of the List.
3. customerNames.add('Globex'); — appends a second item.
4. customerNames.add('Initech'); — appends a third.

A List can hold any type — List<Integer>, List<Boolean>, even List<Account> once you've learned about Salesforce records in Apex — but the idea is identical every time: one variable, many values of the same type.

List<String> customerNames = new List<String>{'Acme Corp', 'Globex', 'Initech'};

1. List<String> customerNames = new List<String>{'Acme Corp', 'Globex', 'Initech'}; — produces the exact same List as three separate .add() calls — just written in one line, with the starting values listed inside { }.

You'll see this shorthand constantly from here on, since it's more compact once you already know what values a List should start with.

System.debug(customerNames[0]); // 'Acme Corp' — the FIRST item
System.debug(customerNames[1]); // 'Globex'    — the SECOND item
System.debug(customerNames.size()); // 3 — how many items are in the List

1. System.debug(customerNames[0]); — [0] reads the FIRST item — indexes start counting at 0, not 1, which trips up almost everyone at first.
2. System.debug(customerNames[1]); — [1] is the SECOND item.
3. System.debug(customerNames.size()); — .size() returns how many items are in the List right now — 3 — which is exactly what the rest of this module's loops need in order to know when to stop.

Every lesson from here on in this module shows you how to run the same block of code once for every item in a List, automatically, instead of writing customerNames[0], customerNames[1], customerNames[2]... by hand. That only makes sense once you can already read and write a List yourself — which is exactly what this lesson just taught. The full "Collections: Lists" module, later in the course, goes much deeper (sorting, more iteration patterns, and List's siblings Set and Map) — this lesson is only the minimum foundation this module needs right now.