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

<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.

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.

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.

double d = 12.34;
float f = 12.34f;

Scientific notation:

double d = 1.2e3; // 1200

3. Boolean Literals

boolean b = true;

Valid values:

• true

• false

Invalid:

boolean b = 1; // not allowed

4. Character Literals

Single character enclosed in single quotes.

char ch = 'A';

Valid:

• 'A', '1', '\n', '\t'

Invalid:

• ''

• 'AB'

5. String Literals

Enclosed in double quotes.

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.

String s = null;

• Default value for reference types

• Not equal to 0