Declaring and initializing variables

In short: Declaring means giving a variable a name and type so the program reserves memory for it. Initializing means assigning it its first value. Both can be done together on one line.

In more detail: The computer must know beforehand how much memory a variable needs and what it may contain. That happens in the declaration. Afterwards it must receive a valid initial value, otherwise it contains random leftovers or null.

In Depth

Declaration, initialization, assignment:

int age;            // declaration: type and name
age = 17;           // initialization: first value
int height = 180;   // both on one line
age = 18;           // later assignment (change the value)

Step	Meaning	Effect
Declare	Announce name and type	Memory is reserved
Initialize	Assign first value	Variable is usable
Assign	Change value later	Old value is overwritten

In different languages:

Language	Example
Java, C, C#	int x = 5; (specify type)
Python	x = 5 (type is inferred, no declaration needed)
JavaScript	let x = 5;, const PI = 3.14;, formerly var
TypeScript	let x: number = 5;
Kotlin	val x = 5 (immutable), var y = 3
Go	x := 5 or var x int = 5

Important rules:

• Local variables in Java must have a value before being read (otherwise compiler error). Attributes get default values (0, false, null).
• C/C++: Uninitialized local variables contain garbage, a common bug and security risk. Always initialize.
• const/final: Value may be set only once.
• Visibility (scope): A variable applies only in the block where it is declared.
• Names: Meaningful (customerCount instead of a), camelCase in Java, snake_case in Python.
• Several on one line: int a = 1, b = 2; (only when clear).
• Declaring functions: In C you separate declaration (prototype) and definition (body).

Errors: Using a variable before it was declared (“cannot find symbol”, “is not defined”). Declaring twice in the same scope.

See also: Initialise, Multiple variables, Variables