🧠 Understanding Variables and Memory

let a = 27;

• The value 27 is stored in RAM (Random Access Memory).
• A variable a acts as a reference or pointer to that memory address.

🔁 Reference and Assignment

• a = 97 ➝ changes the reference to a new value.
• Each assignment changes the reference address of a.

a = null;

• null means the reference is empty.
• In Java, when a variable becomes null, the Garbage Collector clears the memory previously held by that variable.

🧩 Data Types in Java

Java uses data types to define the kind of value a variable can hold.

📌 Variable Creation Syntax

datatype variableName = value;

Multiple variables of the same data type

data_type variableName1 = value1, variableName2 = value2;

Example : String firstName="Arunkumar", lastName = "Selvam";

Syntax of Variable Declaration & Initialization

data_type variableName; (Variable Declaration)
data_type variaableName = value; (Variable Initialization)

String userName; // Variable Declaration
String userName = "Arunkumar"; //Variable Initialization

✍️ Examples of Data Types

📦 Java Data Type Categories

• Primitive Data Types (Built-in language types):

  • char, int, float, long, double, boolean

• User-Defined Data Types:

  • String (Actually a class, not a primitive)

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🧮 Binary & Decimal Conversion Study Notes

📌 1 Byte = 8 Bits

A byte contains 8 bits, and each bit is either 0 or 1:

00000000

🔁 Decimal to Binary Conversion

Example: Convert 27 to binary

27 ÷ 2 = 13, remainder 1  
13 ÷ 2 = 6,  remainder 1  
6 ÷ 2  = 3,  remainder 0  
3 ÷ 2  = 1,  remainder 1  
1 ÷ 2  = 0,  remainder 1

Binary Representation of 27 = 11011

🔁 Binary to Decimal Conversion

Example: Convert 11011 to Decimal

(1 × 2⁴) + (1 × 2³) + (0 × 2²) + (1 × 2¹) + (1 × 2⁰)
= 16 + 8 + 0 + 2 + 1
= 27

🧠 More Practice: Convert 97 to Binary

97 ÷ 2 = 48, remainder 1  
48 ÷ 2 = 24, remainder 0  
24 ÷ 2 = 12, remainder 0  
12 ÷ 2 = 6,  remainder 0  
6 ÷ 2  = 3,  remainder 0  
3 ÷ 2  = 1,  remainder 1  
1 ÷ 2  = 0,  remainder 1

Binary of 97 = 1100001

🧮 Convert 1100001 to Decimal

(1 × 2⁶) + (1 × 2⁵) + (0 × 2⁴) + (0 × 2³) + (0 × 2²) + (0 × 2¹) + (1 × 2⁰)
= 64 + 32 + 0 + 0 + 0 + 0 + 1
= 97

Variable Naming Convention

• Variable names are case-sensitive.
• In Java, variable names should follow camelCase (e.g., firstName, lastName).
• They should not start with special characters like & (ampersand), $ (dollar), or _ (underscore).
• Avoid using single-character variables like x, y, or z.

Bit - Binary Digit - 0, 1s

Base 2 - 0, 1

Base 8 - 01234567

Base 10 - 0123456789

Base 16 - 0123456789ABCDEF

ASCII - American Standard for Code Information Interchange