Variables and Data Types

Variables and Data Types in Java

In Java, variables are used to store data values. Java is a statically-typed language, meaning you must declare the type of a variable before using it. Variables are essential for storing and manipulating data in programs.

Example:


// Variables in Java
int age = 25;  // Integer
double height = 5.9;  // Double
String name = "Alice";  // String
boolean isStudent = true;  // Boolean

Common Data Types in Java:

int: Whole numbers (e.g., 10, -5).
double: Decimal numbers (e.g., 3.14, -0.001).
String: Text data (e.g., "Hello", "Java").
boolean: True or False values.
char: Single characters (e.g., 'A', 'b').

Variable Declaration and Initialization

Variables in Java must be declared with a specific data type before they can be used. They can be initialized at the time of declaration or later.


int x; // Declaration
x = 10; // Initialization

double pi = 3.14; // Declaration and initialization

Type Casting

Type casting is the process of converting one data type into another. Java supports both implicit (automatic) and explicit (manual) type casting.


// Implicit Casting (Widening)
int num = 10;
double decimal = num; // Automatically converts int to double

// Explicit Casting (Narrowing)
double decimal = 10.5;
int num = (int) decimal; // Manually converts double to int

Constants

Constants are variables whose values cannot be changed once assigned. In Java, constants are declared using the final keyword.


final double PI = 3.14159;
// PI = 3.14; // This will cause a compilation error

Variable Scope

The scope of a variable determines where it can be accessed. Variables can have local scope (within a method) or class-level scope (within a class).


public class Main {
    static int classVariable = 10; // Class-level variable

    public static void main(String[] args) {
        int localVariable = 20; // Local variable
        System.out.println("Class Variable: " + classVariable);
        System.out.println("Local Variable: " + localVariable);
    }
}

Primitive vs Reference Types

Java has two categories of data types: primitive types (e.g., int, double) and reference types (e.g., String, arrays, objects).


// Primitive Types
int num = 10;
double decimal = 3.14;

// Reference Types
String name = "Alice";
int[] numbers = {1, 2, 3};

Next: Control Structures