✅ FULL DETAILED NOTES ON JAVA VARIABLES

1. 🧠 What Are Variables?

A variable is a named container for storing data. You can think of it like a labeled box in your memory where you can keep values and use them throughout your program.

🔁 “Variable” comes from “vary” — meaning the value it holds can change over time.

2. ✏️ Variables in Java vs. Math

In Math:

A = 10
B = 20
Area = A * B

In Java:

int a = 10;
int b = 20;
int area = a * b;

Java variables are just like in math — they hold values, like numbers or words — but Java requires you to specify a type.

3. 🛠 Declaring a Variable in Java

Syntax:

<type> <variableName> = <value>;

Example:

int a = 5;
String name = "Tony Stark";

Part	Description
int	Data type (integer)
a	Variable name (identifier)
=	Assignment operator
5	Value stored in variable

💡 The semicolon (;) ends the statement — mandatory in Java.

4. 🧱 Java's Memory Model: How Variables Work Under the Hood

Java uses RAM (memory) to store data in small blocks.

Each variable reserves a block of memory.

That block has:

• A name (e.g., a)
• A type (e.g., int)
• A value (e.g., 10)
• A memory address (behind the scenes)

Example:

int c = 25;

➡️ Java reserves memory block labeled c, stores 25, and marks it as type int.

🧠 Analogy: Think of memory like a city with houses. Each house has:

• An address
• A name on the mailbox (variable name)
• A resident (value stored)

5. 🔖 Identifiers: Naming Variables

An identifier is the name you give to variables, functions, classes, etc.

✅ Rules for identifiers in Java:

• Must start with a letter (a-z, A-Z), underscore _, or dollar sign $
• Cannot start with a digit
• Cannot be a Java keyword (like class, int, static)
• Case-sensitive (Name ≠ name)
• Use camelCase (userName, totalAmount)

6. 🔢 Types of Variables: Data Types in Java

Java is statically typed, so every variable must have a data type.

✴️ Common Data Types:

Data Type	Description	Example
int	Whole numbers	int age = 25;
double	Decimal numbers	double pi = 3.14;
char	Single characters	char grade = 'A';
String	Sequence of characters	String name = "Tony";
boolean	true or false values	boolean isActive = true;

7. ♻️ Changing Variable Values

int a = 10;
a = 50; // Now a holds 50

➡️ The old value is overwritten.

8. 🖨 Printing Variable Values

System.out.println(a); // prints the value of a

➡️ The console will show the current value stored in a.

9. 🔄 Assigning One Variable to Another

int a = 10;
int b = 5;
a = b; // Now a = 5

➡️ The value of b is copied into a.

🧠 Important: Only the value is copied — not a reference (unless you're using objects).

10. 🧠 Expert Tips & Real-World Insight

• Memory efficiency: Always choose the smallest data type that fits the value.
• Naming: Use descriptive variable names (age, salary, isLoggedIn) — improves code readability.
• Variable scope: A variable only exists within its scope (e.g., inside a method or block).
• Immutable values: final variables cannot be reassigned.

final int maxLimit = 100;

11. 📋 Example Code (With Output)

public class VariableDemo {
    public static void main(String[] args) {
        int a = 10;
        int b = 5;
        int c = 25;
        String name = "Tony Stark";

        System.out.println(a);        // Output: 10
        System.out.println(b);        // Output: 5
        System.out.println(c);        // Output: 25
        System.out.println(name);     // Output: Tony Stark

        a = 50;
        System.out.println(a);        // Output: 50

        a = b;
        System.out.println(a);        // Output: 5
    }
}

12. 📌 Revision Notes

• Java variables store data in memory using named containers.
• Each variable has a type, a name, and a value.
• Variables use memory blocks, each with a unique address.
• You can change a variable’s value after declaring it.
• Assigning one variable to another copies the value.
• System.out.println() prints the current value.
• Identifiers must follow naming rules.
• Java is strictly typed — all variables need a type.

13. 🧠 Memory Tricks (to help you retain this)

• "Variable = Box": Always think of a variable as a labeled box.
• "Type First": In Java, always tell the type first (int, String, etc.).
• "Copy, Don't Link": In simple variables like int, assigning one to another copies the value, not the reference.
• Use Pseudocode: Practice writing logic in plain English, then in Java.
• Quiz Yourself: Try predicting output of small code snippets.
• Repeat With Purpose: Recode the examples 3x on your own — one with changes.

14. 💼 Interview Questions + Model Answers

Q1: What is a variable in Java?

Answer:
A variable in Java is a named memory location used to store a value. The value can be changed during program execution. Variables must be declared with a type that tells the compiler what kind of data it will store (e.g., int, String, etc.). For example:

int score = 100;

Here, score is the variable name, storing an integer value 100.

Q2: What is the difference between int a = b; and int a == b;?

Answer:

• int a = b; is assignment — it assigns the value of b to a.
• a == b is a comparison — it checks whether a and b are equal and returns a boolean (true or false).

Q3: How does Java manage memory when variables are created?

Answer:
When a variable is created, Java reserves a specific block of memory to store its value. The size of the memory depends on the variable's type (e.g., int uses 4 bytes). The variable name is a reference to that memory location, and each memory block has a unique address in RAM.

Q4: Can you reassign a value to a variable in Java?

Answer:
Yes. Variables in Java are mutable (unless declared final). You can change the value at any time after it's declared:

int age = 25;
age = 30; // age now holds 30

Q5: What are valid variable names in Java?

Answer:

• Start with a letter, underscore _, or dollar sign $
• Cannot be a keyword (e.g., class, int)
• Cannot start with a digit
• Are case-sensitive

Examples:

int userAge;  // valid
int _id;      // valid
int $salary;  // valid
int 1num;     // invalid