Java

Java is a high-level, object-oriented, platform-independent programming language used to build software applications. It was developed by Sun Microsystems in 1995 and is now maintained by Oracle.

Why Java is Popular

Platform Independent

Java follows the rule: “Write Once, Run Anywhere” A Java program can run on Windows, Linux, macOS, etc., without changes.

Object-Oriented

Java is based on concepts like: Class Object Inheritance Polymorphism Encapsulation This makes programs reusable, modular, and easy to maintain.

Secure

Java has: No direct memory access Bytecode verification Automatic garbage collection

Robust & Reliable

Strong memory management Exception handling Less chances of crashes

JDK vs JVM VS JRE

JDK is a complete kit for Java developers. (JDK = JRE, Javac or Java Compiler, Development & Debugger) JVM is a virtual machine that runs Java programs. Using JVM Executes Java bytecode (.class files), Converts bytecode into machine code, Makes Java platform-independent, Manages memory and garbage collection and JRE provides the environment required to run Java applications. (JRE = JVM + Libraries)

Variables

Java Literals

Literals are fixed values assigned directly to variables in Java. They represent constant data.

1. Integer Literals

Whole numbers without decimal point.

Type	Syntax / Example	Notes
Decimal	int a = 10;	Normal integer
Octal	int b = 012;	Starts with 0, decimal 10
Hexadecimal	int c = 0xA;	Starts with 0x, decimal 10
Binary	int d = 0b1010;	Starts with 0b, decimal 10
Long	long l = 100000L;	Use L or l
Readable	int p = 1_000_000;	_ allowed for readability

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2. Floating-Point Literals

Numbers with decimal point.

Type	Syntax / Example	Notes
Float	float f = 10.5f;	Use f or F
Double	double d = 20.75;	Default type for decimals
Scientific	double e = 1.2e3;	1200 in decimal

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3. Character Literals

Single character enclosed in single quotes.

Example	Notes
'A'	Single English letter
'9'	Digit as character
'#'	Symbol
'\u0041'	Unicode character (A)
'\n'	Escape sequence (new line)
'\t'	Escape sequence (tab)
'\''	Single quote character

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4. String Literals

Sequence of characters enclosed in double quotes.

Example	Notes
"Hello"	Text
"Sujit Kumar Singh"	Multiple words
"Line1\nLine2"	Escape sequences supported

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5. Boolean Literals

Represents true or false values.

Example	Notes
true	Boolean true value
false	Boolean false value

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6. Null Literal

Represents no object reference.

Example	Notes
String str = null;	Only for reference types, not primitive

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Notes

1. Integer types: byte, short, int, long
2. Floating types: float, double
3. Characters: Single quotes ' '
4. Strings: Double quotes " "
5. Boolean: true or false
6. Null: For reference variables only
7. Java supports underscore _ in numeric literals for readability

Data Type

Premitive Data Type

Data Type	Description	Size (Fixed)	Example	Type
byte	Very small integer	1 byte	byte a = 10;	Integer (Integral)
short	Small integer	2 bytes	short b = 100;	Integer (Integral)
int	Integer value (whole numbers)	4 bytes	int c = 1000;	Integer (Integral)
long	Large integer	8 bytes	long d = 100000L;	Integer (Integral)
float	Single-precision decimal number	4 bytes	float e = 3.14f;	Floating-Point
double	Double-precision decimal number	8 bytes	double f = 3.14159;	Floating-Point
char	Single Unicode character	2 bytes	char g = 'A';	Character Type
boolean	True or false	1 byte*	boolean h = true;	Boolean

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Example

int age = 20;
boolean isRealAge = true;
float myMarks = 70.0f;
double myHeight = 5.7;
String name = "Sujit Kumar Singh";
byte level = 3; (Use when Memory very limited)
short year = 2025; (Use when memory need are small but larger than byte)
long population = 140000000L; (Use only when large value needs to be store)
char myChar = 'S'; (It is use in single single quotes with single character)

Non Premitive Data Type

Data Type	Description	Size	Example	Type
String	Sequence of characters (String class)	Depends	String a = "Hello";	Text / Reference
Array	Collection of elements of same data type	Depends	int[] b = {1, 2, 3};	Collection
Class	Blueprint for creating objects	Depends	class Student { }	User-Defined Type
Object	Parent class of all Java classes	Depends	Object obj = new Object();	Universal Reference
Interface	Blueprint with abstract methods	Depends	interface Shape { }	Abstract Type
Enum	Set of named constants	Depends	enum Day { MON, TUE };	Special Class

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Java Literals – Table Reference

Literals are fixed values assigned directly to variables in Java. They represent constant data.

Literal Type	Example Syntax	Notes / Output
Integer (Decimal)	int a = 10;	Normal integer
Integer (Octal)	int b = 012;	Starts with 0, decimal 10
Integer (Hexadecimal)	int c = 0xA;	Starts with 0x, decimal 10
Integer (Binary)	int d = 0b1010;	Starts with 0b, decimal 10
Long	long l = 10000000000L;	Use L or l for long values
Readable Integer	int r = 1_000_000;	_ can be used for readability
Float	float f = 10.5f;	Use f or F
Double	double d1 = 20.75;	Default type for decimals
Scientific (Double)	double d2 = 1.2e3;	1.2 × 10³ = 1200
Char	char ch = 'A';	Single character, uses single quotes
Unicode Char	char ch = '\u0041';	Unicode character
String	String name = "Sujit";	Sequence of characters
Boolean	boolean isJavaFun = true;	Only true or false
Null	String str = null;	Represents no reference
Escape in String	String msg = "Hello\nJava";	Supports escape sequences like \n, \t

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🔹 Quick Tips

1. Integer types: byte, short, int, long
2. Floating types: float, double
3. Char vs String: 'A' → char, "A" → String
4. Boolean literals are only true or false.
5. Null literal can be assigned only to reference types.

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Java Escape Sequences – Table Reference

Escape sequences in Java are special characters preceded by a backslash \, used in strings and char literals.

Escape Sequence	Meaning / Usage	Example Code	Output Example
\n	New line	System.out.println("Hello\nJava");	Hello Java
\t	Horizontal tab	System.out.println("A\tB\tC");	A B C
\\	Backslash	System.out.println("C:\\Users");	C:\Users
\'	Single quote	char ch = '\'';	'
\"	Double quote	System.out.println("He said \"Hi\"");	He said "Hi"
\r	Carriage return (cursor to line start)	System.out.println("12345\rAB");	AB345
\b	Backspace (removes previous char)	System.out.println("Helloo\b!");	Hell!
\f	Form feed (page break, mostly printing)	System.out.println("Hello\fWorld");	(depends on printer)
\uXXXX	Unicode character	char ch = '\u0041';	A

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🔹 Quick Tips

1. Escape sequences work only in char or string literals.
2. Always use double backslash \\ for printing a backslash.
3. \b and \r may behave differently in console vs files.
4. Unicode \uXXXX can represent any Unicode character.

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