Variables

COMP122 Lectures 2021-02-17

A variable is a named memory location that can be changed during the program’s execution. It has three properties:

1. A type.
2. A value.
3. A name or identifier.

Identifiers

Not all identifiers are valid. They can be an sequence of alphanumeric characters, $ and _ unless:

• They start with a number.
• It is a keyword.

Identifiers are case sensitive.

Conventions

• Class names start in capital letters.
• Method and variable names start in lower case letters.
• CamelCase is used if several words make up and identifier.

Example

In the following code all of the following keywords are identifiers:

HelloWorld, main, String, args, System, out and println.

public class HelloWorld {

	// Main Method
	public static void main(String[] args) {
		System.out.println(*"Hello World");
	}
}

Variable Declarations

A declaration statement declares a variable to be of a particular type and is of the form:

<type> <variableName>;

where <type> is a type and <variableName> a legal identifier.

double currentWeight;
int studentsLearningJava;
int maximumValue;

Variables must be declared before you can use them.

Variable Assignments

An assignment statement stores a value in a variable as follows:

<variableName> = <value>;

• <variableName> may be any already declared identifier.
• <value> is a value of the same type as <variableName>.

Declaration and Initialisation

We can also combine declaration a variable with the assignment of an initial value (initialisation) as follows:

<type> <variableName> = <value>;

double currentWeight = 122.5;
int studentsLearningJava = 78;