Variables

A variable is a named container that stores a piece of data your program can work with and change while it runs. In Java, every variable must have a declared type, so the compiler always knows what kind of data it is allowed to hold.

int age = 25;
System.out.println(age);

The general pattern for declaring and initializing a variable is: type name = value;. Here, int is the type, age is the name, and 25 is the value assigned with the = operator. You don't have to assign a value right away — you can declare a variable first and give it a value later, as long as you assign something before you try to use it.

Variable names in Java must start with a letter, an underscore (_), or a dollar sign ($) — never a digit — and cannot be one of Java's reserved keywords (like class or int). Names are case-sensitive, so score and Score are two different variables. By convention, Java developers write variable names in camelCase: the first word is lowercase, and each following word starts with a capital letter, e.g. totalScore or firstName.

In Java, variable names conventionally use ____, where the first word is lowercase and every following word starts with a capital letter, such as totalScore.

A variable can also be reassigned: given a new value after it was already initialized. The new value simply replaces the old one — the old value is gone. The new value must still be compatible with the variable's declared type, since Java is statically typed and never lets a variable suddenly hold a different kind of data.

int lives = 3;
lives = lives - 1;
System.out.println(lives); // 2

What happens when you reassign a variable that already has a value?

Once you declare a variable's type, you must write that type again every time you assign it a new value.