Java Variables

Variables are containers for storing data values.

In Java, there are different types of variables, for example:

  • String - stores text, such as "Hello". String values are surrounded by double quotes
  • int - stores integers (whole numbers), without decimals, such as 123 or -123
  • float - stores floating point numbers, with decimals, such as 19.99 or -19.99
  • char - stores single characters, such as 'a' or 'B'. Char values are surrounded by single quotes
  • boolean - stores values with two states: true or false

Declaring (Creating) Variables

To create a variable, you must specify the type and assign it a value:

Syntax

type variableName = value;

Where type is one of Java's types (such as int or String), and variableName is the name of the variable (such as x or name). The equal sign is used to assign values to the variable.

To create a variable that should store text, look at the following example:

Example

Create a variable called name of type String and assign it the value "John".
Then we use println() to print the name variable:

String name = "John";
System.out.println(name);

To create a variable that should store a number, look at the following example:

Example

Create a variable called myNum of type int and assign it the value 15:

int myNum = 15;
System.out.println(myNum);

You can also declare a variable without assigning the value, and assign the value later:

Example

int myNum;
myNum = 15;
System.out.println(myNum);

Note that if you assign a new value to an existing variable, it will overwrite the previous value:

Example

Change the value of myNum from 15 to 20:

int myNum = 15;
myNum = 20;  // myNum is now 20
System.out.println(myNum);

Final Variables

If you don't want others (or yourself) to overwrite existing values, use the final keyword (this will declare the variable as "final" or "constant", which means unchangeable and read-only):

Example

final int myNum = 15;
myNum = 20;  // will generate an error: cannot assign a value to a final variable

Other Types

A demonstration of how to declare variables of other types:

Example

int myNum = 5;
float myFloatNum = 5.99f;
char myLetter = 'D';
boolean myBool = true;
String myText = "Hello";

You will learn more about data types in the next section.

Printing Variables

Display Variables

The println() method is often used to display variables.

To combine both text and a variable, use the + character:

Example

String name = "John";
System.out.println("Hello " + name);

You can also use the + character to add a variable to another variable:

Example

String firstName = "John ";
String lastName = "Doe";
String fullName = firstName + lastName;
System.out.println(fullName);

For numeric values, the + character works as a mathematical operator (notice that we use int (integer) variables here):

Example

int x = 5;
int y = 6;
System.out.println(x + y); // Print the value of x + y

From the example above, you can expect:

  • x stores the value 5
  • y stores the value 6
  • Then we use the println() method to display the value of x + y, which is 11

Declare Multiple Variables

To declare more than one variable of the same type, you can use a comma-separated list:

Example

Instead of writing:

int x = 5;
int y = 6;
int z = 50;
System.out.println(x + y + z);

You can simply write:

int x = 5, y = 6, z = 50;
System.out.println(x + y + z);

One Value to Multiple Variables

You can also assign the same value to multiple variables in one line:

Example

int x, y, z;
x = y = z = 50;
System.out.println(x + y + z);

Java Identifiers

Identifiers

All Java variables must be identified with unique names.

These unique names are called identifiers.

Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).

Note: It is recommended to use descriptive names in order to create understandable and maintainable code:

Example

// Good
int minutesPerHour = 60;

// OK, but not so easy to understand what m actually is
int m = 60;

The general rules for naming variables are:

  • Names can contain letters, digits, underscores, and dollar signs
  • Names must begin with a letter
  • Names should start with a lowercase letter, and cannot contain whitespace
  • Names can also begin with $ and _ (but we will not use it in this tutorial)
  • Names are case-sensitive ("myVar" and "myvar" are different variables)
  • Reserved words (like Java keywords, such as int or boolean) cannot be used as names

Java Variables - Examples

Real-Life Examples

Often in our examples, we simplify variable names to match their data type (myInt or myNum for int types, myChar for char types, and so on). This is done to avoid confusion.

However, for a practical example of using variables, we have created a program that stores different data about a college student:

Example

// Student data
String studentName = "John Doe";
int studentID = 15;
int studentAge = 23;
float studentFee = 75.25f;
char studentGrade = 'B';

// Print variables
System.out.println("Student name: " + studentName);
System.out.println("Student id: " + studentID);
System.out.println("Student age: " + studentAge);
System.out.println("Student fee: " + studentFee);
System.out.println("Student grade: " + studentGrade);

Calculate the Area of a Rectangle

In this real-life example, we create a program to calculate the area of a rectangle (by multiplying the length and width):

Example

// Create integer variables
int length = 4;
int width = 6;
int area;

// Calculate the area of a rectangle
area = length * width;

// Print variables
System.out.println("Length is: " + length);
System.out.println("Width is: " + width);
System.out.println("Area of the rectangle is: " + area);