Java Fundamentals - Data Types

1. CLASSPATH

CLASSPATH is an environment variable which tells the Java compiler and the JVM where to look for Java class files.

Key idea: CLASSPATH helps Java locate compiled class files and other required classes during compilation and execution.

2. Identifier

Identifiers are programmer-designed tokens. They are used for naming:

Classes

Methods

Variables

Objects

Labels

Packages

Interfaces

3. Identifier Rules

Identifiers can contain alphabets, digits, underscore (_), and dollar sign ($) characters.
An identifier must not begin with a digit.
Uppercase and lowercase letters are distinct.

Examples

studentName
student_name
student1
$amount
_count

4. Data Type

A data type specifies the size and type of values that can be stored. It is an attribute that tells the computer something about the kind of data being represented.

Remember: A data type determines what kind of value a variable can store and the amount of memory associated with that value.

5. Primitive Data Types

The source organizes Java primitive data types into integer, real, character and other categories.

Category	Representation	Bytes	Default Value
Integer	int	4	0
Integer	short	2	0
Integer	long	8	0
Real	float	4	0.0f
Real	double	8	0.0d
Character	char	2	\u0000
Other	byte	1	0
Other	boolean	1	false
Object reference	Reference	—	null

Note: The table preserves the representation, byte sizes and default values as presented in the source PDF.

6. Variable

An identifier whose value can change during the execution of a program is called a variable.

Example

int age = 20;
age = 21;

Here, age is a variable because its value can change during program execution.

7. Constant

An identifier whose value cannot change during the execution of a program is called a constant.

Example

final int MAX = 100;

The value of MAX cannot be reassigned after initialization.

8. How to Declare a Variable

The source gives two basic forms of variable declaration.

Data type var_name;

Data type var_name = value;

Examples

int number;

int number = 10;

String name = "Java";

9. Statement

Statements are roughly equivalent to sentences in natural languages. A statement forms a complete unit of execution.

Important: Every statement must end with a semicolon (;).

Examples

int a = 10;

a = a + 5;

System.out.println(a);

10. Block

A block is a group of zero or more statements between balanced braces and can be used anywhere.

{
    int a = 10;
    int b = 20;
    System.out.println(a + b);
}

Key idea: A block is enclosed between { and } and groups statements into a single unit.

11. Expression

An expression is a collection of operators and operands.

An operand can be a variable, constant or method invocation.

Example

a + b * c

Part	Role
a, b, c	Operands
+, *	Operators
a + b * c	Expression

12. Quick Revision

Topic	Key Point
CLASSPATH	Environment variable that tells the Java compiler and JVM where to find Java class files.
Identifier	Programmer-designed name for classes, methods, variables, objects, labels, packages and interfaces.
Identifier Rule	Can contain letters, digits, underscore and dollar sign, but cannot start with a digit; case matters.
Data Type	Specifies the size and type of values that can be stored.
Primitive Data Types	int, short, long, float, double, char, byte and boolean.
Variable	An identifier whose value can change during program execution.
Constant	An identifier whose value cannot change during program execution.
Variable Declaration	`Data type var_name;` or `Data type var_name = value;`
Statement	A complete unit of execution; statements end with a semicolon.
Block	Group of zero or more statements between balanced braces.
Expression	Collection of operators and operands.