What Is a Variable?

Picture a labelled storage box. The label tells you what's allowed inside — a box labelled "Invoices" shouldn't have a stapler in it. A variable works the same way: it has a name (the label) and a type (what's allowed inside), and it holds one value at a time.

Integer age = 30;

1. Integer age = 30; — declares a new variable named age of type Integer, and assigns it the value 30 in the same statement. The type comes first, then the name, then =, then the value, ending in a semicolon.

Declaring a variable creates the box and labels it. Assigning puts something inside. You can do both at once (as above), or separately:

Integer age;      // declare — an empty, labelled box
age = 30;         // assign — put a value in it
age = 31;         // reassign — swap out what's inside

1. Integer age; — declares age as an Integer, with no value yet — Apex gives it null until something is assigned.
2. age = 30; — assigns 30 to the already-declared age. No type is repeated here; Apex already knows age's type from line 1.
3. age = 31; — reassigns age to a new value, replacing the 30 it held a moment ago.

An Integer box can hold different Integer values over time — but never a String. That restriction is what "strongly-typed" means.

Some languages let a variable hold anything, of any type, at any time. Apex deliberately doesn't — the type is fixed the moment you declare it. This might feel like extra ceremony coming from nothing, but it catches real mistakes early: if you accidentally try to put text into a box labelled "numbers only," Apex refuses to even compile your code, rather than letting the mistake slip through and cause a confusing failure later.

A variable name must start with a letter, can contain letters/numbers/underscores, and can't be an Apex reserved word (like class or if). Beyond the compiler's rules, good names are descriptive — age tells you far more than a or x, and this matters enormously once your classes grow past a few lines.

• Reusing a variable name for something unrelated. Reassigning age partway through to mean something else entirely (say, an account ID) makes code far harder to read — pick a new variable instead.
• Forgetting the semicolon. Every Apex statement ends in ; — a missing one is one of the most common first-week compiler errors.

There's no "opting out" of typing a variable in Apex — but you do choose how specific to be. Declaring something as the general-purpose Object type instead of a specific type like Integer is technically legal but throws away the compiler's ability to catch mistakes for you. Prefer the most specific type that fits, always.

1. Integer age = 30; — declares age and assigns it 30 in one step.
2. System.debug(age); — logs the current value of age — 30 — to the debug log.
3. (blank line, separating the first pair of statements from the second)
4. age = 31; — reassigns age to 31. No Integer keyword needed again — Apex already knows age's type.
5. System.debug(age); — logs age again; this time it prints 31, since line 4 changed it.