Java Variables

1. Variable Declaration:

int age;          // Declaration of an integer variable named 'age'
double salary;    // Declaration of a double variable named 'salary'
String name;      // Declaration of a String variable named 'name'

2. Variable Initialization:

int age = 25;               // Initialization of the 'age' variable
double salary = 50000.75;   // Initialization of the 'salary' variable
String name = "John";       // Initialization of the 'name' variable

3. Variable Types:

Primitive Data Types: Hold simple values directly.

int count = 10;
double pi = 3.14;
char grade = 'A';
boolean flag = true;

Reference Data Types: Hold references to objects in memory.

String message = "Hello, World!";
MyClass myObject = new MyClass();

4. Variable Naming Rules:

• Variable names are case-sensitive.
• The first character must be a letter, underscore (_), or dollar sign ($).
• Subsequent characters can be letters, digits, underscores, or dollar signs.
• Avoid using Java reserved words (e.g., int, double, class, etc.) as variable names.
• Use meaningful names that describe the variable's purpose.

5. Variable Scope:

• The scope of a variable is the region of the program where the variable is accessible.
• Java has block scope, meaning a variable is only accessible within the block of code where it is declared.
• Class-level variables (fields) have class scope and are accessible throughout the class.

6. Final Variables:

You can use the final keyword to declare a constant variable, meaning its value cannot be changed after initialization.

final double PI = 3.14159;

7. Variable Types in Methods:

• Method parameters and local variables follow the same rules as other variables.
• Method parameters receive values when the method is called.
• Local variables are declared inside a method and have block scope.

public class Example {
    // Class-level variable
    static int globalVar = 100;

    public static void main(String[] args) {
        // Method-level variable
        int localVar = 42;
        
        // Accessing class-level variable
        System.out.println(globalVar);

        // Accessing method-level variable
        System.out.println(localVar);
    }
}

Understanding and using variables effectively is fundamental to programming in Java. They allow you to store and manipulate data, making your programs dynamic and responsive.