




- 1Java OOP Introduction
- 2Java Class
- 3Java Class Constructor
- 4Java Class Objects
- 5Java Access Modifiers
- 6Java Static Variables in Classes
- 7Java Static Methods Explained
- 8Java Static Blocks
- 9Java final Variables
- 10Java final Methods
- 11Java final class
- 12Inheritance in Java
- 13Java Method Overriding
- 14Java Abstraction in OOP
- 15Interfaces in Java
- 16Polymorphism in Java
- 17Encapsulation in Java
- 18Java Nested Classes
- 19Java Nested Static Class
- 20Java Anonymous Class
- 21Java Singleton Class
- 22Java Enums
- 23Reflection in Java


Java Variables - Declare, Initialize, and Use
Variables in Java
In Java, a variable is a named container used to store data that can be changed during the execution of a program. Think of it as a labeled box where you can keep values—whether that's numbers, text, or more complex objects.
How to Declare a Variable in Java
Declaring a variable means telling Java what type of data the variable will hold and giving it a name. The basic syntax is:
datatype variableName;
Example:
int age;
String name;
double price;
How to Initialize a Variable
Initialization means assigning a value to the declared variable. You can do this at the time of declaration or later in your code.
1. Declare and Then Initialize
int age;
age = 25;
2. Declare and Initialize Together
String name = "Alice";
Variable Types in Java
Java is a statically-typed language, which means every variable must have a data type. Here are some commonly used types:
Data Type | Description |
---|---|
int | Stores integers (whole numbers), e.g., 1, 100 |
double | Stores decimal numbers, e.g., 3.14 |
char | Stores a single character, e.g., 'A' |
boolean | Stores true or false |
String | Stores sequences of characters, e.g., "Hello" |
Java Variable Naming Rules
Here’s what you need to know to name your variables properly, along with examples for better understanding:
1. Names can contain letters, digits, underscores, and dollar signs
// Valid variable names
int total_Amount = 100;
double price2 = 49.99;
String user$name = "John";
2. Names must begin with a letter, not a digit
// Correct
int count1 = 5;
// Incorrect (will cause a compile-time error)
int 1count = 5;
3. Names are case-sensitive (value
and Value
are different)
int value = 10;
int Value = 20;
System.out.println(value); // Outputs: 10
System.out.println(Value); // Outputs: 20
4. Names should not be Java reserved keywords
// Invalid variable names (these are reserved by Java)
int class = 1; // Error
boolean public = true; // Error
// Correct alternative names
int classNumber = 1;
boolean isPublic = true;
Example: Declaring and Using Variables
public class VariableDemo {
public static void main(String[] args) {
int age = 30;
String name = "John";
boolean isAdult = true;
System.out.println("Name: " + name);
System.out.println("Age: " + age);
System.out.println("Is Adult? " + isAdult);
}
}
Name: John
Age: 30
Is Adult? true
Why Are Variables Important?
Variables make your program flexible and dynamic. They allow you to write code that can adapt based on inputs, conditions, or changing data. Without variables, programming would be static and manual.
Checkpoints for Beginners
- Always declare a variable with its correct type. A mistake like
int name = "Alice";
will throw a compile-time error. - Uninitialized variables cannot be used. For instance, trying to print a variable before assigning a value will result in a compile error.
- Be mindful of types during assignments. For example, you can't directly assign a
double
to anint
without casting.
Best Practices
- Use meaningful variable names like
totalAmount
,isAvailable
, rather thanx
ordata
. - Stick to camelCase naming (e.g.,
userAge
,itemPrice
) for variables. - Initialize variables when possible to avoid accidental use of undefined values.