What Are Variables?
Variables in Java act like storage boxes that hold data values.
Each variable has:
- a data type (what kind of data it stores)
- a name (how we refer to it)
- a value (the data it holds)
Common Types of Variables in Java
Java supports many data types. Some of the most frequently used ones
are:
- String – stores text values such as "Welcome"
- int – stores whole numbers like 10 or -45
- float – stores decimal numbers like 12.75
- char – stores a single character such as 'X'
- boolean – stores logical values: true or false
Creating (Declaring) Variables
To create a variable in Java, you need to:
- Specify the data type
- Provide a variable name
- Assign a value (optional at first)
General Syntax
dataType variableName = value;
Example: String Variable
The example below creates a text variable and displays it.
String city = "Chennai";
System.out.println(city);
Example: Integer Variable
To store a whole number, use the int type.
int score = 90;
System.out.println(score);
Declaring First, Assigning Later
You can create a variable first and give it a value afterward.
int temperature;
temperature = 32;
System.out.println(temperature);
Updating Variable Values
A variable’s value can be changed after declaration. The new value replaces
the old one.
int level = 1;
level = 2; // value updated
System.out.println(level);
Final (Constant) Variables
If a value should never change, use the final keyword. Final variables are
also called constants.
final int MAX_LIMIT = 100;
// MAX_LIMIT = 120; // This will cause an error
Once assigned, a final variable cannot be modified.
Using Multiple Data Types Together
The example below demonstrates declaring variables of different types in
one program.
int items = 3;
float price = 49.99f;
char grade = 'A';
boolean isAvailable = true;
String product = "Notebook";
System.out.println(items);
System.out.println(price);
System.out.println(grade);
System.out.println(isAvailable);
System.out.println(product);
Java Printing Variables
Displaying Variable Values
The println() method is commonly used to output variable values to the
console.
To display text together with a variable, Java uses the + operator.
Example: Text with a Variable
String user = "Anita";
System.out.println("Welcome, " + user);
This joins the text and the variable value into a single output.
Combining Variables Together
The + operator can also be used to join the values of two variables.
Example: Joining Two Strings
String city = "New ";
String country = "Delhi";
String location = city + country;
System.out.println(location);
Meaning of the + Operator in Java
The + symbol behaves differently depending on the data type:
- With strings → it joins values together (concatenation)
- With numbers → it performs arithmetic addition
Example: Adding Numbers
int a = 8;
int b = 12;
System.out.println(a + b);
Output
20
Step-by-Step Explanation
- a stores the value 8
- b stores the value 12
- Java adds both values and prints the result 20
Mixing Text and Numbers
When text and numbers are used in the same statement, Java processes the
expression from left to right.
Example Without Parentheses
int marks1 = 4;
int marks2 = 7;
System.out.println("Total marks: " + marks1 + marks2);
Output
Total marks: 47
Java converts numbers into text after encountering a string.
Using Parentheses for Correct Calculation
To force Java to calculate numbers first, use parentheses.
System.out.println("Total marks: " + (marks1 + marks2));
Output
Total marks: 11
Declaring Several Variables at Once
Java allows you to create multiple variables of the same data type in a
single statement by separating them with commas.
Traditional Way (One Variable per Line)
int a = 10;
int b = 20;
int c = 30;
System.out.println(a + b + c);
Shorter Way (Single Line Declaration)
int a = 10, b = 20, c = 30;
System.out.println(a + b + c);
Both approaches work the same way. The second option reduces the number of
lines, while the first may be easier to read for beginners.
Assigning One Value to Multiple Variables
Java also allows assigning the same value to several variables in a single
statement.
Example
int p, q, r;
p = q = r = 15;
System.out.println(p + q + r);
Output
45
In this example:
- All three variables receive the value 15
- The sum of the variables is then printed
What Are Identifiers?
In Java, every variable, method, or class must have a unique name.
These names are called identifiers.
Identifiers help the compiler and programmers recognize and work with
data stored in memory.
They can be:
- Short (like i, j)
- Descriptive (like studentCount, totalMarks)
Using meaningful names makes your code easier to read and
maintain.
Choosing Good Identifier Names
Example
// Clear and descriptive
int secondsPerMinute = 60;
// Valid, but unclear
int s = 60;
The first example clearly explains what the value represents.
Rules for Naming Identifiers in Java
When creating identifiers, keep the following rules in mind:
- Names may include letters, numbers, underscores (_), and dollar signs ($)
- The first character must be a letter, underscore, or dollar sign
- Identifiers cannot contain spaces
- By convention, variable names start with a lowercase letter
- Java is case-sensitive (count and Count are different)
- Java keywords cannot be used as identifiers
Examples of Invalid Identifiers
The following examples will cause compilation errors:
// Incorrect identifier names
int 5items = 10; // Cannot begin with a
number
int total cost = 50; // Spaces are not allowed
int boolean = 1; // Java keyword cannot be
used
What Is a Constant in Java?
In Java, a constant is a variable whose value cannot be modified
once it is assigned. To create a constant, you use the final
keyword.
Once a variable is marked as final, it becomes read-only throughout
the program.
Example: Using final Variables
final int MAX_USERS = 100;
// Trying to change the value will cause an error
MAX_USERS = 120; // Compilation error
The compiler prevents reassignment because MAX_USERS is declared as
final.
When Should You Use final?
Use final when a value:
- Should remain constant throughout the program
- Represents fixed data like limits, configuration values, or universal facts
Common Examples
final int DAYS_IN_WEEK = 7;
final int CURRENT_YEAR = 2026;
These values are not expected to change during program
execution.
Naming Convention for Constants
By convention:
- Constant names are written in UPPERCASE
- Words are separated using underscores (_)
final double PI_VALUE = 3.14159;
This naming style is recommended, not mandatory, but it improves
readability and helps other developers quickly identify
constants.
