Section 2.2 Variables and Comments
In Java, a variable is a named storage location that holds a value of a particular data type. Variables are used to access data stored in memory and manipulate that data within a program.
To create a variable it must be declared with a specific data type, which determines the kind of data it can store and the amount of memory to allocate. Java is a statically typed language, which means that the type of a variable is known at at the time your program is compiled, and must be declared explicitly.
Following is the syntax for declaring a variable in Java:
dataType represents the type of data the variable can hold, such as int for integers, double for double-precision floating-point numbers, boolean for Boolean values ’true’ or ’false’, String for text, etc. variableName is an identifier that you choose for the variable that refers to the underlying memory. But don’t worry, we don’t have to think about the memory itself. Java will take care of choosing a location and setting aside the correct number of bytes. It knows how many to set aside using the dataType that you specified when declaring the variable. Also note that the statement ends with a semicolon (;). All Java statements must end with a semicolon.
For example, let’s declare a few variables in Java. Note the // to the right of each declaration. These are Java single-line comments. Anything to the right of a // on a line in Java is ignored. We use single-line comments extensively to explain what is occuring in example programs.
int age; // declare an integer variable named 'age'
double pi; // declare a double variable named 'pi'
boolean isStudent; // declare a boolean variable named 'isStudent'
String name; // declare a String variable named 'name'
Java variable names follow certain rules.
Variable names must start with a letter (upper or lower case), an underscore ( _ ) or a dollar sign ($)
After the first character, variable names may be made up of additional letters (A-Z, a-z), digits (0-9), an underscore ( _ ) and/or a dollar sign ($)
Spaces are not permitted in a variable name.
After declaring a variable you can assign it to an initial value using the assignment operator (=).
age = 25; // assign the value 25 to the 'age' variable
pi = 3.14; // assign the value 3.14 to the 'pi' variable
isStudent = true; // assign the value true to the 'isStudent' variable
name = "Moonman"; // assign the value "Moonman" to the 'name' variable
Optionally, you can combine the declaration of a variable and initial assignment in a single statement:
int age = 25; // declare and initialize the 'age' variable
double pi = 3.14; // declare and initialize the 'pi' variable
boolean isStudent = true; // declare and initialize the 'isStudent' variable
String name = "Moonman"; // declare and initialize the 'name' variable
Once a variable is declared and assigned a value, you can use it in various operations and expressions within your program.
Open a new terminal on your computer or in your editor and start JShell by entering the command jshell -v. The -v at the end of this command tells JShell to run in verbose mode. In verbose mode JShell prints additional information after each expression is evaluated, which is useful while you are still learning. Additional information includes the type of the result just calculated. Following is a sample JShell session for you to try.
jshell -v
| Welcome to JShell -- Version 17.0.6
| For an introduction type: /help intro
jshell> 1
$1 ==> 1
| created scratch variable $1 : int
jshell> 255
$2 ==> 255
| created scratch variable $2 : int
jshell> 1 + 255
$3 ==> 256
| created scratch variable $3 : int
jshell> int x;
x ==> 0
| created variable x : int
jshell> int y = 100;
y ==> 100
| created variable y : int
jshell> x = 200;
x ==> 200
| assigned to x : int
jshell> x + y;
$7 ==> 300
| created scratch variable $7 : int
jshell> /exit
| Goodbye
Java provides different modifier terms that may be used when declaring a variable. For example, you can precede (modify) a declaration with the additional keyword final. This turns your variable into a constant (the value cannot be changed after initialized). We’ll explore these topics in more detail.
You now know how to declare, assign, initialize and use variables in simple mathematical expressions. Variables in Java are essential for storing and manipulating data, and they play a fundamental role in programming with Java.
Activity 2.2.1.
Perform the following in JShell running in verbose mode.
Declare a variable of type double named amount and initialize it to 10.5
Declare a variable of type int named nCats and initialize it to 3.
Enter the expression nCats * amount
Compare the variable types used in the expressed and the type of the computed result? Why do you think Java produced a result of this type?
Answer.jshell> double amount = 10.5;
amount ==> 10.5
| created variable amount : double
jshell> int nCats = 3;
nCats ==> 3
| created variable nCats : int
jshell> nCats * amount
$3 ==> 31.5
| created scratch variable $3 : double
Activity 2.2.2.
Declare two int variables named numer and denom. Intialize numer to 1 and denom to 2. Use these variables and only these variables to compute the value one-half. Is this possible?
Answer.jshell> int numer = 1;
numer ==> 1
| created variable numer : int
jshell> int denom = 2;
denom ==> 2
| created variable denom : int
jshell> numer/denom;
$3 ==> 0
| created scratch variable $3 : int
jshell> // Not possible
Activity 2.2.3.
Declare and initialize variables with names _underscore and $dollar of types boolean.
Answer.jshell> boolean _underscore = true;
_underscore ==> true
| created variable _underscore : boolean
jshell> boolean $dollar = false;
$dollar ==> false
| created variable $dollar : boolean
