⚡ Variables & Data Types in Java ⭐

After understanding Java basics, the next important concept is Variables and Data Types. Every Java program stores and manipulates data, and this is done using variables. Let’s understand everything step by step.

✅ 1️⃣ What is a Variable?

A variable is a container that stores data. Think of it like a box that holds values.

Example: int age = 25;

Here:
- int → data type
- age → variable name
- 25 → value stored in variable

Simple Structure:
data_type variable_name = value;

Example:

int number = 10;
double salary = 50000.50;
char grade = 'A';

✅ 2️⃣ Rules for Naming Variables

Java has some rules for variable names.

✔ Must start with letter, _ or $
✔ Cannot start with a number
✔ Cannot use Java keywords

Valid examples:
- int age;
- double salary;
- String studentName;

Invalid examples:
- int 1age;
- double student-name;

✅ 3️⃣ Data Types in Java

Java has two main types of data types.

1️⃣ Primitive Data Types
2️⃣ Non-Primitive Data Types

🔹 4️⃣ Primitive Data Types

Primitive types store simple values directly in memory. Java has 8 primitive data types.

- byte: 1 byte (e.g., byte a = 10;)
- short: 2 bytes (e.g., short b = 100;)
- int: 4 bytes (e.g., int age = 25;)
- long: 8 bytes (e.g., long population = 8000000000L;)
- float: 4 bytes (e.g., float price = 12.5f;)
- double: 8 bytes (e.g., double salary = 50000.75;)
- char: 2 bytes (e.g., char grade = 'A';)
- boolean: 1 bit (e.g., boolean isTrue = true;)

🔹 5️⃣ Non-Primitive Data Types

Non-primitive types store references to objects.
Examples: String, Arrays, Classes, Objects, Interfaces

Example:

String name = "Java";
int[] numbers = {1, 2, 3, 4};

Difference:
- Primitive: Stores value, fixed size, faster.
- Non-Primitive: Stores reference, dynamic size, slightly slower.

🔹 6️⃣ Type Casting

Type casting means converting one data type to another. There are two types.

⭐ 1. Implicit Casting (Automatic): Smaller type → Larger type.
Example:

int number = 10;
double value = number; 

⭐ 2. Explicit Casting (Manual): Larger type → Smaller type.
Example:

double price = 99.99;
int value = (int) price; // Output: 99

🔹 7️⃣ Constants in Java (final keyword)

A constant is a variable whose value cannot change. Java uses the final keyword.

Example:

final double PI = 3.14159;

Constants are usually written in UPPERCASE.

🔥 Example Program (Variables in Java)

class VariablesDemo {
    public static void main(String[] args) {
        int age = 25;
        double salary = 50000.75;
        char grade = 'A';
        boolean isWorking = true;

        System.out.println("Age: " + age);
        System.out.println("Salary: " + salary);
        System.out.println("Grade: " + grade);
        System.out.println("Working: " + isWorking);
    }
}

Output:

Age: 25
Salary: 50000.75
Grade: A
Working: true

⭐ Common Interview Questions

1️⃣ What are the 8 primitive data types in Java?
2️⃣ What is the difference between primitive and non-primitive data types?
3️⃣ What is type casting in Java?
4️⃣ What is the difference between implicit and explicit casting?
5️⃣ What is the purpose of the final keyword?

🔥 Quick Revision

- Variables → containers for storing data.
- Primitive types: byte, short, int, long, float, double, char, boolean.
- Non-primitive types: String, Arrays, Objects, Classes.
- Type casting: Implicit → automatic; Explicit → manual.
- Constant: Created using the final keyword.

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⚡ Methods in Java (Functions) ⭐

Now you’ve reached a very important concept — Methods.

This is where your code becomes clean, reusable, and interview-ready.

✅ 1️⃣ What is a Method?

👉 A method is a block of code that performs a task.

Instead of writing the same code again and again → you reuse it.

🔹 Example Without Method:

System.out.println("Hello");
System.out.println("Hello");
System.out.println("Hello");

🔹 With Method:

void sayHello() {
System.out.println("Hello");
}
👉 Now you can call it multiple times.

✅ 2️⃣ Method Syntax

returnType methodName(parameters) {
// code
}

Example:
void greet() {
System.out.println("Hello Java");
}

✅ 3️⃣ Calling a Method

class Test {
static void greet() {
System.out.println("Hello");
}

public static void main(String[] args) {
greet(); // method call
}
}

🔹 4️⃣ Types of Methods

1️⃣ Without parameters, no return
2️⃣ With parameters
3️⃣ With return value
4️⃣ With parameters + return

⭐ 1. No Parameters, No Return

static void show() {
System.out.println("Java");
}

⭐ 2. With Parameters

static void add(int a, int b) {
System.out.println(a + b);
}
Call:
add(5, 3);

⭐ 3. With Return Value

static int square(int x) {
return x x;
}
Call:
int result = square(4);

⭐ 4. Parameters + Return

static int add(int a, int b) {
return a + b;
}

🔹 5️⃣ Method Overloading (Important ⭐)

👉 Same method name, different parameters

Example:
static int add(int a, int b) {
return a + b;
}

static double add(double a, double b) {
return a + b;
}

👉 Java decides method based on arguments

🔹 6️⃣ Recursion (Interview Favorite ⭐)

👉 Method calling itself

Example:

static void printNumbers(int n) {
if (n == 0) return;

System.out.println(n);
printNumbers(n - 1);
}
Call:
printNumbers(5);

Output:
5
4
3
2
1

🔥 7️⃣ Important Keywords

- return: sends value back
- void: no return value
- static: no object needed
- parameters: input values

🔥 Example Program
class MethodDemo {

static int add(int a, int b) {
return a + b;
}

public static void main(String[] args) {

int result = add(10, 5);
System.out.println(result);

}
}

⭐ Common Interview Questions
- What is a method?
- Difference between function and method?
- What is method overloading?
- What is recursion?
- Difference between void and return?

🔥 Quick Revision

- Method → reusable code
- Parameters → input
- Return → output
- Overloading → same name, different args
- Recursion → method calls itself

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