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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

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

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

4. String Literals

Sequence of characters enclosed in double quotes.

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

5. Boolean Literals

Represents true or false values.

Example Notes
true Boolean true value
false Boolean false value

6. Null Literal

Represents no object reference.

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

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

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

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

🔹 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.

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

🔹 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.

Print Hello World

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }   
}

Common Regex Symbols (Java)

Note: Java me regex likhte waqt backslash (\) ko escape karna padta hai.
Isliye \d ko Java string me "\\d" likhte hain.

Character Classes

Regex Java String Description
\d \\d Digit (0–9)
\D \\D Non-digit
\w \\w Word character (a–z, A–Z, 0–9, _)
\W \\W Non-word character
\s \\s Whitespace (space, tab, newline)
\S \\S Non-whitespace

Quantifiers

Regex Description
* 0 or more times
+ 1 or more times
? 0 or 1 time
{n} Exactly n times
{n,} At least n times
{n,m} Between n and m times

Anchors

Regex Description
^ Start of string
$ End of string

Character Sets

Regex Description
[abc] Matches a, b, or c
[a-z] Lowercase letters
[A-Z] Uppercase letters
[0-9] Digits
[^0-9] Not a digit

Groups & Alternation

Regex Description
(abc) Group
` `

Common Patterns

Pattern Description
\\d+ One or more digits
\\d{10} 10-digit mobile number
[a-zA-Z]+ Alphabets only
\\w+@\\w+\\.\\w+ Basic email format

All Types of Operators in Java

Java me operators ka use operations perform karne ke liye hota hai.

1. Arithmetic Operators

Operator Name Example
+ Addition a + b
- Subtraction a - b
* Multiplication a * b
/ Division a / b
% Modulus a % b

2. Unary Operators

Operator Name Example
+ Unary Plus +a
- Unary Minus -a
++ Increment a++, ++a
-- Decrement a--, --a
! Logical NOT !true

3. Assignment Operators

Operator Example Meaning
= a = 5 Assign
+= a += 2 a = a + 2
-= a -= 2 a = a - 2
*= a *= 2 a = a * 2
/= a /= 2 a = a / 2
%= a %= 2 a = a % 2

4. Relational (Comparison) Operators

Operator Description Example
== Equal to a == b
!= Not equal to a != b
> Greater than a > b
< Less than a < b
>= Greater than or equal a >= b
<= Less than or equal a <= b

5. Logical Operators

Operator Name Example
&& Logical AND (&&) a > 5 && b < 10
` `
! Logical NOT (!) !(a > b)

6. Bitwise Operators

Operator Name Example
& AND a & b
` ` OR
^ XOR a ^ b
~ Complement ~a

7. Shift Operators

Operator Name Example
<< Left Shift a << 1
>> Right Shift a >> 1
>>> Unsigned Right Shift a >>> 1

8. Ternary Operator

Operator Description Example
?: Conditional max = (a > b) ? a : b;

9. instanceof Operator

Operator Description Example
instanceof Object type check obj instanceof String

10. Special Operators

Operator Description Example
. Access member obj.method()
[] Array access arr[0]
() Method call method()
new Object creation new Scanner()

Operator Precedence (High → Low)

Priority Operators
High (), [], .
++, --, !
*, /, %
+, -
<, >, <=, >=
==, !=
&&
`
Low =, +=, -=

Operator Precedence Table (High → Low)

Operator Type Category Operators Associativity
Postfix Increment / Decrement a++, a-- Left to Right
Unary Prefix, Sign, Logical NOT ++a, --a, +a, -a, !a, ~a Right to Left
Arithmetic Multiplicative *, /, % Left to Right
Arithmetic Additive +, - Left to Right
Shift Bitwise Shift <<, >>, >>> Left to Right
Relational Comparison <, >, <=, >=, instanceof Left to Right
Equality Equality Check ==, != Left to Right
Bitwise AND & Left to Right
Bitwise XOR ^ Left to Right
Bitwise OR ` Left to Right
Logical Logical AND && Left to Right
Logical Logical OR ` Left to Right
Ternary Conditional ?: Right to Left
Assignment Assignment / Shorthand =, +=, -=, *=, /=, %= Right to Left

Bitwise Operator

Operator Name Description (What it does) Example Code Binary Operation (Example) Output
& Bitwise AND Dono bits 1 ho tab result 1 int a=5,b=3; a & b; 0101 & 0011 = 0001 1
| Bitwise OR Koi ek bit bhi 1 ho to result 1 `int a=5,b=3; a b;` `0101 0011 = 0111` 7
^ Bitwise XOR Bits different ho tab result 1 int a=5,b=3; a ^ b; 0101 ^ 0011 = 0110 6
~ Bitwise NOT (Complement) Bits ko invert karta hai (1 → 0, 0 → 1) int a=5; ~a; ~0101 = 1010 -6
<< Left Shift Bits ko left shift karta hai (2ⁿ se multiply) int a=5; a << 1; 0101 << 1 = 1010 10
>> Right Shift Bits ko right shift karta hai (2ⁿ se divide) int a=10; a >> 1; 1010 >> 1 = 0101 5
>>> Unsigned Right Shift Right shift + sign bit fill with 0 int a=-10; a >>> 1; Sign bit ignored Varies