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:
type variable = value;
Where type is one of Java's types (such as
variable 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:
Create a variable called name of type
String and assign it the value "John":
String name = "John"; System.out.println(name);
To create a variable that should store a number, look at the following 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:
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:
Change the value of
int myNum = 15; myNum = 20; // myNum is now 20 System.out.println(myNum);
However, you can add the
final keyword if you don't want others (or yourself) to overwrite existing values (this will declare the variable as "final" or "constant", which means unchangeable and read-only):
final int myNum = 15; myNum = 20; // will generate an error: cannot assign a value to a final variable
A demonstration of how to declare variables of other types:
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 chapter.
println() method is often used to display variables.
To combine both text and a variable, use the
String name = "John"; System.out.println("Hello " + name);
You can also use the
+ character to add a variable to another variable:
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):
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 Many Variables
To declare more than one variable of the same type, use a comma-separated list:
int x = 5, y = 6, z = 50; System.out.println(x + y + z);
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.
The general rules for constructing names for variables (unique identifiers) are:
- Names can contain letters, digits, underscores, and dollar signs
- Names must begin with a letter
- Names should start with a lowercase letter and it 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
boolean) cannot be used as names