Java Basic

Biswas Parajuli Jun 13, 2026 77 views 0 downloads
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Introduction to Java

java is a high- level, class-based, object-oriented programming language. Which developed by Sun Microsystems in 1995. It follows the principle: "Write Once, Run Anywhere".

History of Java

Java was originally designed by James Gosling at Sun Microsystems. Originally called Oak, it was renamed Java in 1995. Sun Microsystem was acquired by Oracle  in 2010.


Year Event
1991 Project Oak started by James Gosling at Sun Microsystems
1995 Java 1.0 released, renamed from Oak to Java
1996 JDK 1.0 officially released
2004 Java 5 (Tiger) - Generics, Annotations, Enums 
2010 Oracle acquires Sun Microsystems 
2014 Java 8 - Lambda expressions, Stream API
2017 Java 9 - Module System
2021 Java 17 LTS, Java 21 LTS and beyond 



Feature of Java

Feature Description
Simple Easy syntax, remove complex features like pointers.
Object- oriented Everything is an object, supports encapsulation, inheritance, polymorphism, abstraction
Platform Independent Complied to bytecode, runs on any OS with JVM
Secure  No explicit pointer, bytecode verification, security manager
Robust  Strong memory management, exception handeling, type checking
Multithreaded  Built-in support for concurrent programming
High Performance
 JIT compiler optimizes bytecode at runtime
Distributed
 Built-in networking support (RMI, CORBA)
Dynamic
 Classes loaded at runtime; supports reflection
Architecture Neutral
 Bytecode not tied to any hardware architecture


JDK, JRE, JVM

Understanding the Java ecosystem is fundamental:

JVM (Java Virtual Machine)

Java Virtual Machine (JVM) is  a virtual machine that executes Java bytecode. It enables Java programs to run on different operating systems without modification.

Work of JVM

  1. Loads Java Bytecode (.class files)
  2. Verifies bytecode for security.
  3. Converts bytecode into machine code using the JIT (Just-In-Time) Compiler.
  4. Executes the program.
  5. Manages memory through Garbage Collection.

Features :

  1. Platform independent
  2. Automatic memory management
  3. Security and performance optimization 

JRE (Java Runtime Environment)

The JRE provides the environment required to run Java applications. It contains the JVM and supporting libraries

Work of JRE:

  1. Provides Java class libraries.
  2. Supplies JVM for execution.
  3. Handles runtime resources required by Java programs.

Components of JRE:

  • JVM
  • Core Java Libraries
  • Supporting files

Features:

  • Runs Java applications
  • Does not include development tools such as compiler

JDK (Java Development Kit)

The JDK is a complete package for Java development. It contains JRE along with development tools.

Work of JDK:

  • Compiles Java source code into bytecode.
  • Provides tools for debugging and documentation.
  • Includes JRE to run programs.

Important JDK Tools:

  • jar – Archive Tool
  • java – Program Launcher
  • jdb – Debugger
  • javac – Java Compiler
  • javadoc – Documentation Generato

Features:

  • Used by developers
  • Contains JRE and JVM
  • Enables coding, compiling, debugging, and execution



Java program structure

Java Program Structure

Every Java program must follow a specific structure:

// File: HelloWorld.java


package com.example;     // 1. Package declaration (optional)
import java.util.Scanner;      // 2. Import statements (optional)
public class HelloWorld {         
          // 3. Class declaration (must match filename
         // Instance variable String message;
        // 4. Main method - entry point of every Java program

      public static void main(String[] args) {
          // 5. Statements System.out.println("Hello, World!");
      }
}

Explain:

public  : Access modifier - visible to all

class  HelloWorld : Class name must match the file name exactly

public static void main : Entry point - JVM calls this method to start the program

String[ ] args : Command - line arguments passed to the program

System.out.println() : Prints text to console with a newkine at the end


Note: 

- The class name Must match the filename.

- Java is case-sensitive ('main' and 'Main' are different)




Comments On Java

Comments are non-executable statements used to explain the code. The Java compiler ignores comments entirely. They improve code readability and documentation.

Single Line Comment

Single-Line Comment

Syntax: // comment text

// This is a single-line comment
int age = 20; // age of the student

// You can also use it to temporarily disable code.
System.out.println("This line is disabled");




Muti-Line Comment


Syntax:  /* comment */

Used to generate API documentation using the javadoc tool. Placed before classes, methods, and fields

/*
    * This class represents a Student.                                                                                                    
    *
    * @author YourName 
    * @version 1.0 
*/
public class Student {
 /*
    * Calculates the grade of a student.
    *
    * @param marks marks obtained (0-100)
    * @return grade as a character (A/B/C/F)
 */
  public char getGrade(int marks) {
    if (marks >= 90) return 'A';  
    else if (marks >= 75) return 'B';
    else if (marks >= 60) return 'C';
    else return 'F';
  }
}

Documentation Comments

Syntax:  /** comment */

Used to generate API documentation using the javadoc tool. Placed before classes, methods, and fields

/**
    * This class represents a Student.                                                                                                    
    *
    * @author YourName 
    * @version 1.0 
*/
public class Student {
 /**
    * Calculates the grade of a student.
    *
    * @param marks marks obtained (0-100)
    * @return grade as a character (A/B/C/F)
 */
  public char getGrade(int marks) {
    if (marks >= 90) return 'A';  
    else if (marks >= 75) return 'B';
    else if (marks >= 60) return 'C';
    else return 'F';
  }
}

Variables

A variable is a named memory location used to store data.

Type Of Variable



1. Local Variable

Declared inside a method, constructor, or block.


public void display(){
    int x = 10;
    System.out.println(x);
}

  • Accessible only within that method/block.
  • Must be initialized before use.


2. Instance Variable

Declared inside a class but outside methods, without static.


class Student {
     String name;   // instance variable
}


  • Each object gets its own copy.
  • Also called a member variable.


3. Static (Class) Variable

Declared with the static keyword.


class Student {
     static String school = "ABC School";
}


Note:  Java does not have true global variables like some languages.
A public static variable is often used when global-like access is needed.
Variable Naming Rules
  • Variable Naming Rules
  • Cannot begin with a digit
  • Cannot use Java reserved keywords (int, class, for, etc.)
  • Case-sensitive: age, Age, AGE are three different variables
  • No spaces allowed in variable names
  • Convention: use camelCase for variable names (firstName, totalMarks)

Data Types


Data types specify the type and size of data that can be stored in a variable. Java is a statically-typed language — every variable must have a declared type

Java has two categories of data types:

  • Primitive Data Types : built-in types (byte, short, int, long, float, double, char, boolean)
  • Non-Primitive Types : String, Arrays, Classes, Interfaces


Primitive Data Types

Type Size Default Example
byte 1 byte 0 byte b = 100;
short 2 bytes 0 short x = 500;
int 4 bytes 0 int x = 100000; 
long 8 bytes 0L long l = 100L;
float 4 bytes 0.0f float f = 3.14f;
double 9 bytes 0.0d double d = 3.14;
char 2 bytes \u000 char c = 'A';
boolean 1 bit false boolean b = true;

Example of each Primitive Type:


public class PrimitiveDemo {
  public static void main(String[] args) {

    //byte smallest integer type
    byte b = 127; // range: -128 to 127
    System.out.println("byte: " + b);

    //short larger than byte
    short s = 32767; // range: -32768 to 32767
    System.out.println("short: " + s);

    //int most commonly used integer type
    int i = 2147483647; // range: -2147483648 to
    System.out.println("int: " + i);

    //long largest integer type
    long l = 9223372036854775807L; // range: -9223372036854775808 to 9223372036854775807
    System.out.println("long: " + l);

    //float single-precision floating point
    float f = 3.14f; // range: approximately ±1.4E-45 to ±3.4E38
    System.out.println("float: " + f);

    //double double-precision floating point
    double d = 3.141592653589793; // range: approximately ±4.9E-324 to ±1.7E308
    System.out.println("double: " + d);

    //char single 16-bit Unicode character
    char c = 'A'; // range: '\u0000' (0) to '\uffff' (65535)
    System.out.println("char: " + c);

    //boolean true or false
    boolean bool = true;
    System.out.println("boolean: " + bool);

  }
}

Note
Common Mistake: float needs 'f' suffix (3.14f), long needs 'L' suffix (100L). Without suffix, 3.14 is treated as double.
Memory Tip: byte(1) < short(2) < int(4) < long(8) — doubles in size each time!


Non-Primitive Data Types

  1. String
  2. Array
String
String is an immutable sequence of characters. It is an object of the java.lang.String class.
public class PrimitiveDemo {
  public static void main(String[] args) {

    //example of string data type
    String name = "John Doe";
    String greeting = "Hello, " + name + "!";
    System.out.println(greeting);

    //Useful string methods
    String s = "Biswas Parajuli";
    System.out.println("Length of the string: " + s.length()); //output => 16
    System.out.println("Uppercase: " + s.toUpperCase()); //output => BISWAS PARAJULI
    System.out.println("Lowercase: " + s.toLowerCase()); //output => biswas parajuli
    System.out.println("Substring (0, 6): " + s.substring(0, 6)); //output => Biswas
    System.out.println("Index of 'Parajuli': " + s.indexOf("Parajuli")); //output => 7
    System.out.println("Replace 'a' with 'o': " + s.replace('a', 'o')); //output => Biswas Porajuli
    System.out.println("Trimmed string: '" + s.trim() + "'"); //output => 'Biswas Parajuli'
    System.out.println("Character at index 0: " + s.charAt(0)); //output => B
    System.out.println("Does the string contain 'Parajuli'? " + s.contains("Parajuli")); //output => true
  }
}

Key Fact:
Strings are immutable — once created, their value cannot change. Operations create new String objects



Arrays

public class PrimitiveDemo {
  public static void main(String[] args) {

    //example of array
    int[] myArray = new int[5];
    myArray[0] = 10;

    // create with new keyword
    int[] myArray2 = new int[5];
    myArray2[0] = 20;
    myArray2[1] = 30;

    // accessing elements of the array
    System.out.println("First element of myArray: " + myArray[0]);
    System.out.println("First element of myArray2: " + myArray2[0]);
    System.out.println("Second element of myArray2: " + myArray2[1]);

    //2d array
    int[][] my2DArray = new int[3][4];
    my2DArray[0][0] = 1;
    System.out.println("Element at [0][0]: " + my2DArray[0][0]);
  }
}

Important:
Array index starts at 0. Last element index = length - 1. Accessing out-of-range index throws ArrayIndexOutOfBoundsException.



Constants

A constant is a variable whose value cannot be changed once assigned. In Java, constants are declared using the final keyword

Syntax:  

final dataType constant_name = value;

Example:


public class PrimitiveDemo {

  //example of constant
    static final double PI = 3.14159;
    static final int MAX_SIZE = 100;
    static final String GREETING = "Hello, World!";

  public static void main(String[] args) {

    final int MAX_SIZE = 50; // This will cause a compile-time error
    System.out.println("The value of PI is: " + PI);
    System.out.println("The maximum size is: " + MAX_SIZE);
    System.out.println(GREETING);

  }
}


Usage Of Final Description
final variable Value cannot be reassigned after initialization
final method cannot be overridden in a subclass
final class Cannot be extended 
💡 Best Practice  :  Use 'static final' for class-level constants so they're shared and accessible without creating object
💡 Important :  A final variable must be initialized either at declaration or in the constructor — but only once!

Operators

↓ Constant (Section 6)

An operator performs an operation on operands. Java provides a rich set of operators.


Arithmetic Operators


Used to perform mathematical calculations

Operator Name Example Result
+ Addition 10 + 3 13
- Subtraction 10  -  3 7
* Multiplication 10  *  3 30
/ Division 10  /  3 3 (integer division)
% Modulus 10 % 3 1 (remaining)

Example:

public class ArithmeticDemo {
  public static void main(String[] args) {

    //arithmetic operators
    int a = 10;
    int b = 5;
    //addition
    System.out.println("a + b = " + (a + b));
    //output: a + b = 15 

    //subtraction
    System.out.println("a - b = " + (a - b));
    //output: a - b = 5

    //multiplication
    System.out.println("a * b = " + (a * b));
    //output: a * b = 50
   
    //division
    System.out.println("a / b = " + (a / b));
    //output: a / b = 2

    //modulus
    System.out.println("a % b = " + (a % b));  
    //output: a % b = 0

  }

}


💡 Tricky: 10 / 3 = 3 (not 3.33) for int. Use 10.0/3 or cast to get decimal result
💡 Use case: Modulus (%) is very useful: to check even/odd (n%2==0), to get last digit (n%10), etc.

Relational (Comparison) Operators


Used to compare two values. Always returns a boolean (true or false)

Operators Name Example Result
== Equal to 5 == 5 true
!=  Not equal to 5 != 3 true
> Greater than 5 > 3 true
< Less than 5 < 3 false
>= Greater than or equal to 5 >= 5 true
<= Less than or equal to 3 <= 5 true

Example:

public class RelationalDemo {
  public static void main(String[] args) {

    //relational operators
    int a = 10;
    int b = 5;
    //greater than
    System.out.println("a > b = " + (a > b));
    //output: a > b = true

    //less than
    System.out.println("a < b = " + (a < b));
    //output: a < b = false

    //equal to
    System.out.println("a == b = " + (a == b));
    //output: a == b = false

    //not equal to
    System.out.println("a != b = " + (a != b));
    //output: a != b = true

    //greater than or equal to
    System.out.println("a >= b = " + (a >= b));
    //output: a >= b = true

    //less than or equal to
    System.out.println("a <= b = " + (a <= b));
    //output: a <= b = false
  }
}


Common Mistake :
Don't use == to compare Strings. Use .equals() method: str1.equals(str2). == compares references, not content.


Logical Operation


Used to combine multiple boolean conditions

Operator Name Description Example
&& Logical AND true only if BOTH condition are true (5>3) && (10>7)
| | Logical OR true if AT LEAST ONE condition is true (5>3)  | |  (10<7)
! Logical NOT REVERSES the boolean value ! (5>3)

Example:

public class LogicalDemo {
  public static void main(String[] args) {

    //logical operators
    int a = 10;
    int b = 5;
   
    System.out.println("a > b: " + (a > b)); // true
    System.out.println("a < b: " + (a < b)); // false
    System.out.println("a == b: " + (a == b)); // false
    System.out.println("a != b: " + (a != b)); // true
  }
}

💡 Short-circuit Evaluation:
 && stops at first false; || stops at first true. This is called short-circuit evaluation and can improve performance.


Assignment Operators


Used to assign values to variables

x = 10

Operator Equivalent to Example Result
= a = value a = 10 a = 10
+= a = a + b a += 5 a = 15
-= a = a - b a -= 5 a = 5
*= a = a *  b a *= 2 a = 20
/= a = a / b a /= 4 a = 2
%= a = a % b a %= 3 a = 1

Example

public class AssignmentDemo {
  public static void main(String[ ] args) {

 //assignment operator
    int x = 10;
    System.out.println("Value of x: " + x);
    //output will be: Value of x: 10

    //addition assignment operator
    x += 5; // equivalent to x = x + 5
    System.out.println("After addition assignment (x += 5): " + x);
    //output will be: After addition assignment (x += 5): 15

    //subtraction assignment operator
    x -= 3; // equivalent to x = x - 3
    System.out.println("After subtraction assignment (x -= 3): " + x);
    //output will be: After subtraction assignment (x -= 3): 12

    //multiplication assignment operator
    x *= 2; // equivalent to x = x * 2
    System.out.println("After multiplication assignment (x *= 2): " + x);
    //output will be: After multiplication assignment (x *= 2): 24

    //division assignment operator
    x /= 4; // equivalent to x = x / 4
    System.out.println("After division assignment (x /= 4): " + x);
    //output will be: After division assignment (x /= 4): 6

    //modulus assignment operator
    x %= 3; // equivalent to x = x % 3
    System.out.println("After modulus assignment (x %= 3): " + x);
    //output will be: After modulus assignment (x %= 3): 0

  }
}

💡 Tip: Compound assignment operators are shorthand and also more readable. Prefer a += 5 over a = a + 5.

Unary Operators

Work on a single operand 

Operator Name Example Description
+ Unary plus +a Represent positive value 
- Unary minus -a Negates the value
++ Pre-increment ++a Increment first, then use value
++ Post-increment a++ Use value first, then increments
-- Pre-decrement --a Decrements first, then uses value
-- Post-decrement a-- Uses value first, then decrements

Example:

public class UnaryOperators {
  public static void main(String[] args) {

    // Unary Operators
    int a = 10;
    System.out.println("Value of a: " + a);
    //output will be: Value of a: 10

    // Unary minus operator
    int b = -a;
    System.out.println("Value of b: " + b);
    //output will be: Value of b: -10

    // Unary plus operator
    int c = +a;
    System.out.println("Value of c: " + c);
    //output will be: Value of c: 10

    // pre- Increment operator
    int d = ++a;
    System.out.println("Value of d: " + d);
    //output will be: Value of d: 11

    // post- Increment operator
    int f = a++;
    System.out.println("Value of f: " + f);
    //output will be: Value of f: 11

    // pre-Decrement operator
    int e = --a;
    System.out.println("Value of e: " + e);
    //output will be: Value of e: 11

    // post-Decrement operator
    int g = a--;
    System.out.println("Value of g: " + g);
    //output will be: Value of g: 11

  }
}

💡 Memory Trick:
Pre(++a): Change then carry
Post(a++): Carry then Change

💡 Important:
Avoid using ++/-- inside complex expression like a = a++ + ++a;   because  this can confusing.

Bitwise Operators


Operate directly on binary (bit-level) representation of numbers.

Operators Name Description Example (a=5 , b=3)
& Bitwise AND  1 only if both bits are 1
 5 & 3 = 1
| Bitwise OR 1 if at least one bit is 1 5 | 3 = 7
^ Bitwise XOR 1 if bits are different 5 ^ 3 = 6
~ Bitwise NOT Inverts all bits ~5 = - 6
<< Left Shift Sheft bits left 5 << 1 = 10
>> Right Shift
 Shift bits right
 5 >> 1 = 2
>>>
 Unsigned Right Shift
 Shift right, fill with 0
5 >>> 1 = 2

Example:

public class Bitwise {
  public static void main(String[] args) {

   //bitwise operators
    int a = 5; // 0101 in binary
    int b = 3; // 0011 in binary

    System.out.println("a & b: " + (a & b)); // 0001 in binary, which is 1
    System.out.println("a | b: " + (a | b)); // 0111 in binary, which is 7
    System.out.println("a ^ b: " + (a ^ b)); // 0110 in binary, which is 6
    System.out.println("~a: " + (~a)); // 1010 in binary
    System.out.println("a << 1: " + (a << 1)); // 1010 in binary, which is 10
    System.out.println("a >> 1: " + (a >> 1)); // 0010 in binary, which is 2
    System.out.println("a >>> 1: " + (a >>> 1)); // 0010 in binary, which is 2

  }
}

💡 Practical Use: Left shift << is a fast way to multiply by powers of 2: a << 3 = a * 8
💡 Practical Use: Bitwise AND with 1 checks if a number is odd: if (n & 1) → odd

Ternary (Conditional) Operator


The ternary operator is a compact if-else written in one line. It takes three operands.

Syntax: 

result = (condition) ? valueIfTrue : valueIfFalse;

Example:

import java.util.Scanner;

public class TernaryOperators {
  public static void main(String[] args) {

    //ternary operator is a shortcut for if-else statement
    //syntax: condition ? expression1 : expression2
    Scanner scanner = new Scanner(System.in);
    System.out.print("Enter a number: ");
    int number = scanner.nextInt();
    String result = (number % 2 == 0) ? "Even" : "Odd";
    System.out.println("The number is " + result);
   
    scanner.close();
   
  }
}

💡 Best Practice:
Use ternary for simple single-line conditions. For complex logic with multiple statements, use if-else for readability.

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