Data Types, Variables and Literals

1. Data Types

Definition

A data type defines the type of data that a variable can store.

Types of Data Types

1. Primitive Data Types

Predefined by Java
Store a single value
Efficient in memory and performance

2. Non-Primitive (Reference Data Types)

Created by programmer or provided by Java
Store references (addresses of objects)
Can hold multiple values

Predefined:

String
Array
Wrapper Classes
Collections

User-defined:

Class
Interface
Enum
Annotation

2. Variables

Definition

A variable is a container used to store data in memory.

Syntax

java

<data_type> variable_name = value;

Types of Variables

1. Class Level Variables

Declared inside class but outside methods
Stored in heap memory
Default values assigned by JVM

Types:

Instance Variable
Static Variable

2. Local Variables

Declared inside methods, blocks, or constructors
Stored in stack memory
Must be initialized before use
Accessible only within their scope

final Keyword

Used to declare constants. Value cannot be changed.

java

final int a = 10;

3. Memory (Heap and Stack)

Heap Memory

Stores objects and class-level variables

Stack Memory

Stores method calls and local variables
Each method has its own stack frame

4. Literals

Definition

A literal is a fixed value assigned to a variable.

Types of Literals

1. Integral Literals

Values without decimal points.

java

int a = 10;

Types:

Decimal → 10 (0-9)
Octal → 0123 (0-7)
Hexadecimal → 0x1A (0-9 a-f)
Binary → 0b1010

Octal:

Digits must be between 0–7
Must start with 0

Hexadecimal:

Digits: 0–9, A–F
Must start with 0x or 0X

Binary:

Only 0 and 1
Must start with 0b or 0B

2. Floating-Point Literals

Decimal values.

java

double d = 12.34;
float f = 12.34f;

Scientific notation:

java

double d = 1.2e3; // 1200

3. Boolean Literals

java

boolean b = true;

Valid values:

true
false

Invalid:

java

boolean b = 1; // not allowed

4. Character Literals

Single character enclosed in single quotes.

java

char ch = 'A';

Valid:

'A', '1', '\n', '\t'

Invalid:

''
'AB'

5. String Literals

Enclosed in double quotes.

java

String s = "Hello";

Note:

String is not a primitive data type
It is a class from java.lang package

6. Null Literal

Represents absence of value.

java

String s = null;

Default value for reference types
Not equal to 0