Java Variables
Storing and managing data in your Java programs
📦 What are Java Variables?
Java variables are containers that store data values. Think of them as labeled boxes where you can put different types of information like numbers, text, or true/false values.
public class VariableExample {
public static void main(String[] args) {
int age = 25; // Integer variable
String name = "John"; // String variable
double price = 19.99; // Double variable
System.out.println("Name: " + name);
System.out.println("Age: " + age);
System.out.println("Price: $" + price);
}
}
Output:
Name: John
Age: 25
Price: $19.99
Java Data Types
Integer (int)
Whole numbers without decimals
int count = 42;
int temperature = -10;Double
Numbers with decimal points
double price = 29.99;
double pi = 3.14159;Boolean
True or false values
boolean isActive = true;
boolean isComplete = false;🔹 Creating Variables
To create a variable in Java, you need to specify the data type and give it a name:
public class CreateVariables {
public static void main(String[] args) {
// Syntax: dataType variableName = value;
int studentCount = 30; // Integer
double average = 85.5; // Double
String schoolName = "ABC High"; // String
boolean isOpen = true; // Boolean
// You can also declare first, then assign
int grade;
grade = 95;
System.out.println("School: " + schoolName);
System.out.println("Students: " + studentCount);
System.out.println("Average: " + average + "%");
System.out.println("Open: " + isOpen);
System.out.println("Grade: " + grade);
}
}Output:
School: ABC High
Students: 30
Average: 85.5%
Open: true
Grade: 95
🔹 Variable Rules
Java has specific rules for naming variables:
✅ Valid Variable Names:
- Must start with a letter, underscore (_), or dollar sign ($)
- Can contain letters, numbers, underscores, and dollar signs
- Cannot use Java keywords (like int, class, public)
- Case-sensitive (age and Age are different)
public class VariableRules {
public static void main(String[] args) {
// ✅ Valid variable names
int age = 25;
String firstName = "John";
double _price = 19.99;
boolean $isValid = true;
int number1 = 10;
// ❌ Invalid variable names (these would cause errors)
// int 2number = 5; // Cannot start with number
// String first-name; // Cannot use hyphen
// boolean class = true; // Cannot use Java keywords
System.out.println("Age: " + age);
System.out.println("Name: " + firstName);
System.out.println("Price: $" + _price);
System.out.println("Valid: " + $isValid);
System.out.println("Number: " + number1);
}
}Output:
Age: 25
Name: John
Price: $19.99
Valid: true
Number: 10
🔹 Changing Variable Values
You can change the value of a variable after creating it:
public class ChangeValues {
public static void main(String[] args) {
int score = 0; // Initial value
System.out.println("Initial score: " + score);
score = 50; // Change the value
System.out.println("Updated score: " + score);
score = score + 25; // Add to existing value
System.out.println("Final score: " + score);
String status = "Beginner";
System.out.println("Status: " + status);
status = "Advanced"; // Change string value
System.out.println("New status: " + status);
}
}Output:
Initial score: 0
Updated score: 50
Final score: 75
Status: Beginner
New status: Advanced