Java Packages

// Jab hum package declare NAHI karte // class automatically "default package" mein jaati hai // Production mein DEFAULT PACKAGE use karna AVOID karo! public class HelloJava { public static void main(String[] args) { System.out.println("Namaste! Main default package mein hoon."); System.out.println("Mere paas koi package nahi hai."); System.out.println("Yeh sirf beginners ke liye theek hai."); System.out.println("Real projects mein hamesha package use karo!"); } }

// PROBLEM DEMONSTRATE KARTE HAIN // java.util.Date aur java.sql.Date dono ka naam "Date" hai // Package ki wajah se dono alag hain — koi conflict nahi! public class NameConflict { public static void main(String[] args) { // Full Qualified Name use karke dono alag classes use kar sakte hain java.util.Date utilDate = new java.util.Date(); java.sql.Date sqlDate = new java.sql.Date(0); System.out.println("=== Package Ne Naam Conflict Solve Kiya ==="); System.out.println("java.util.Date → " + utilDate.getClass().getName()); System.out.println("java.sql.Date → " + sqlDate.getClass().getName()); System.out.println("\nDono 'Date' hain lekin ALAG packages mein!"); System.out.println("Agar package nahi hota toh → COMPILE ERROR!"); } }

// Package organization ka faayda dikhate hain // Imagine karo ek project mein ye sab classes hain public class PackageDemo { public static void main(String[] args) { System.out.println("=== E-Commerce Project Package Structure ==="); System.out.println(); // Without packages — CHAOS System.out.println("❌ Bina Package ke (All in one folder):"); String[] chaos = { "User.java", "Product.java", "Order.java", "Payment.java", "UserService.java", "ProductService.java", "UserRepo.java", "UserController.java", "DBHelper.java", "EmailUtil.java" }; for (String f : chaos) System.out.println(" 📄 " + f); System.out.println(); System.out.println("✅ Package ke saath (Organized):"); String[][] organized = { {"com.shop.model", "User, Product, Order, Payment"}, {"com.shop.service", "UserService, ProductService"}, {"com.shop.repository", "UserRepo, ProductRepo"}, {"com.shop.controller", "UserController, OrderController"}, {"com.shop.util", "DBHelper, EmailUtil"} }; for (String[] pkg : organized) System.out.printf(" 📁 %-25s → %s%n", pkg[0], pkg[1]); } }

import java.util.ArrayList; import java.math.BigDecimal; public class PackageTypes { public static void main(String[] args) { // ── Built-in package classes use kar rahe hain ── ArrayList<String> builtIn = new ArrayList<>(); builtIn.add("java.lang → String, Math, System, Thread"); builtIn.add("java.util → ArrayList, HashMap, Scanner"); builtIn.add("java.io → File, BufferedReader, FileWriter"); builtIn.add("java.math → BigInteger, BigDecimal"); builtIn.add("java.net → URL, Socket, HttpURLConnection"); System.out.println("=== Built-in Java Packages ==="); builtIn.forEach(p -> System.out.println(" 📦 " + p)); // Math class (java.lang.Math) — auto-imported, explicit import nahi chahiye System.out.println("\n--- java.lang.Math se calculations ---"); System.out.println("sqrt(625) = " + Math.sqrt(625)); System.out.println("pow(2, 10) = " + (int)Math.pow(2, 10)); // BigDecimal (java.math) — banking ke liye BigDecimal balance = new BigDecimal("99999.75"); System.out.println("\n--- java.math.BigDecimal ---"); System.out.println("Bank Balance: ₹" + balance); } }

import java.util.ArrayList; import java.util.Date; public class PackageInfo { public static void main(String[] args) { // Kisi bhi class ka package info nikaal sakte hain Class<?>[] classes = { String.class, ArrayList.class, Date.class, Math.class, Integer.class, java.io.File.class, java.math.BigDecimal.class }; System.out.println("=== Class ka Package Information ==="); System.out.printf("%-16s %-30s%n", "CLASS", "PACKAGE"); System.out.println("─".repeat(48)); for (Class<?> c : classes) { Package pkg = c.getPackage(); System.out.printf("%-16s %s%n", c.getSimpleName(), pkg != null ? pkg.getName() : "(default)"); } // Number of classes info System.out.println("\nTotal classes checked: " + classes.length); } }

// File location: com/school/Student.java // Compile: javac -d . Student.java // Run: java com.school.Student package com.school; // ← MANDATORY: File ki pehli line public class Student { private String name; private int rollNo; private double percentage; private String grade; public Student(String name, int rollNo, double pct) { this.name = name; this.rollNo = rollNo; this.percentage = pct; this.grade = calcGrade(pct); } private String calcGrade(double pct) { if (pct >= 90) return "A+"; if (pct >= 80) return "A"; if (pct >= 70) return "B"; if (pct >= 60) return "C"; return "F"; } public void display() { System.out.printf("%-5d %-15s %6.1f%% Grade: %s%n", rollNo, name, percentage, grade); } public static void main(String[] args) { Student[] students = { new Student("Rahul Kumar", 101, 92.5), new Student("Priya Sharma", 102, 85.0), new Student("Amit Verma", 103, 73.8), new Student("Neha Singh", 104, 58.2) }; System.out.println("Package: com.school"); System.out.println("Roll Name %age Grade"); System.out.println("─".repeat(42)); for (Student s : students) s.display(); } }

package com.bank; public class Account { private String accNo; private String owner; private double balance; private int txCount = 0; public Account(String accNo, String owner, double initBalance) { this.accNo = accNo; this.owner = owner; this.balance = initBalance; } public void deposit(double amount) { if (amount <= 0) { System.out.println("❌ Invalid amount!"); return; } balance += amount; txCount++; System.out.printf("[TX-%03d] ✅ Credited ₹%8.2f | Balance: ₹%.2f%n", txCount, amount, balance); } public void withdraw(double amount) { if (amount <= 0) { System.out.println("❌ Invalid amount!"); return; } if (amount > balance) { System.out.println("❌ Insufficient Balance!"); return; } balance -= amount; txCount++; System.out.printf("[TX-%03d] ✅ Debited ₹%8.2f | Balance: ₹%.2f%n", txCount, amount, balance); } public void statement() { System.out.println("\n────── Account Statement ──────"); System.out.println("Account: " + accNo + " | Owner: " + owner); System.out.printf("Balance: ₹%.2f | Transactions: %d%n", balance, txCount); System.out.println("──────────────────────────────"); } public static void main(String[] args) { System.out.println("=== Package: com.bank ==="); Account acc = new Account("SBI-001", "Rahul Kumar", 10000.0); acc.deposit(5000); acc.withdraw(2500); acc.deposit(1500); acc.withdraw(20000); // should fail acc.statement(); } }

// File: com/shapes/Shape.java package com.shapes; public interface Shape { double area(); double perimeter(); default String info() { return String.format("%s → Area: %.2f | Perimeter: %.2f", getClass().getSimpleName(), area(), perimeter()); } } // File: com/shapes/Circle.java package com.shapes; public class Circle implements Shape { private double r; public Circle(double r) { this.r = r; } public double area() { return Math.PI * r * r; } public double perimeter() { return 2 * Math.PI * r; } } // File: com/shapes/Rectangle.java package com.shapes; public class Rectangle implements Shape { private double l, w; public Rectangle(double l, double w) { this.l = l; this.w = w; } public double area() { return l * w; } public double perimeter() { return 2 * (l + w); } } // File: com/shapes/Triangle.java package com.shapes; public class Triangle implements Shape { private double a, b, c; public Triangle(double a, double b, double c) { this.a=a; this.b=b; this.c=c; } public double area() { double s = (a+b+c)/2; return Math.sqrt(s*(s-a)*(s-b)*(s-c)); } public double perimeter() { return a + b + c; } } // Test class (same package mein) package com.shapes; public class ShapeTest { public static void main(String[] args) { Shape[] shapes = { new Circle(7), new Rectangle(6, 4), new Triangle(3, 4, 5) }; System.out.println("=== Package: com.shapes ==="); for (Shape s : shapes) System.out.println(" → " + s.info()); } }

package com.ecom; public enum OrderStatus { PENDING("⏳", "Order received, processing..."), CONFIRMED("✅", "Payment confirmed"), PACKED("📦", "Items packed"), SHIPPED("🚚", "Out for delivery"), DELIVERED("🎉", "Successfully delivered"), CANCELLED("❌", "Order cancelled"); private final String icon, desc; OrderStatus(String icon, String desc) { this.icon=icon; this.desc=desc; } public void print() { System.out.println(" Status: " + icon + " " + name() + " — " + desc); } } // Order class — same package package com.ecom; public class Order { private int id; private String item; private OrderStatus status; public Order(int id, String item) { this.id = id; this.item = item; this.status = OrderStatus.PENDING; } public void update(OrderStatus s) { this.status = s; System.out.println("\n[Order #" + id + "] Item: " + item); s.print(); } public static void main(String[] args) { Order o = new Order(501, "Laptop — Dell Inspiron"); o.update(OrderStatus.CONFIRMED); o.update(OrderStatus.PACKED); o.update(OrderStatus.SHIPPED); o.update(OrderStatus.DELIVERED); } }

// ✅ SPECIFIC IMPORT — har class individually import karo import java.util.ArrayList; import java.util.HashMap; import java.util.LinkedList; import java.util.TreeMap; public class SpecificImport { public static void main(String[] args) { // ArrayList — dynamic array ArrayList<String> fruits = new ArrayList<>(); fruits.add("🍎 Apple"); fruits.add("🥭 Mango"); fruits.add("🍌 Banana"); fruits.add("🍊 Orange"); System.out.println("ArrayList: " + fruits); // HashMap — key-value pairs HashMap<String, Integer> prices = new HashMap<>(); prices.put("Apple", 80); prices.put("Mango", 60); prices.put("Banana", 40); prices.put("Orange", 50); System.out.println("Prices: " + prices); // LinkedList — doubly linked list LinkedList<Integer> queue = new LinkedList<>(); queue.add(10); queue.add(20); queue.add(30); System.out.println("Queue peek: " + queue.peek() + " | size: " + queue.size()); // TreeMap — sorted key-value TreeMap<String, Integer> sorted = new TreeMap<>(prices); System.out.println("Sorted Prices: " + sorted); } }

// Wildcard import — saari classes ek saath import java.util.*; public class WildcardDemo { public static void main(String[] args) { // java.util.* se milne waali classes List<String> list = new ArrayList<>( Arrays.asList("Zara", "Amit", "Priya", "Kiran")); // Collections utility class System.out.println("Before sort: " + list); Collections.sort(list); System.out.println("After sort: " + list); Collections.reverse(list); System.out.println("Reversed: " + list); // Set — unique values only Set<Integer> set = new HashSet<>( Arrays.asList(5, 3, 8, 3, 1, 5)); // duplicates removed System.out.println("Set (unique): " + new TreeSet<>(set)); // Stack Stack<String> stack = new Stack<>(); stack.push("First"); stack.push("Second"); stack.push("Third"); System.out.println("Stack top: " + stack.peek()); System.out.println("Popped: " + stack.pop() + " | Remaining: " + stack.size()); // NOTE: wildcard * sirf java.util tak — sub-packages nahi! // java.util.concurrent.* ko alag import karna hoga! System.out.println("\n⚠️ import java.util.* se java.util.concurrent milta NAHI!"); } }

// Koi bhi import nahi! Seedha full name use kar rahe hain // Yeh tab useful hai jab name conflicts ho public class FullyQualified { public static void main(String[] args) { // java.util.Date aur java.sql.Date dono ek saath — ZERO conflict java.util.Date utilDate = new java.util.Date(); java.sql.Date sqlDate = new java.sql.Date(0); System.out.println("util.Date class: " + utilDate.getClass().getName()); System.out.println("sql.Date class: " + sqlDate.getClass().getName()); // ArrayList — fully qualified java.util.ArrayList<String> names = new java.util.ArrayList<>(); names.add("Using fully"); names.add("qualified"); names.add("names!"); System.out.println("\nNames: " + names); // Scanner with fully qualified name System.out.println("\njava.util.Scanner se input le sakte hain"); System.out.println("java.io.PrintWriter se file write kar sakte hain"); System.out.println("\n📝 Note: Yeh readable nahi — import prefer karo!"); } }

// ── File 1: com/utils/Calculator.java ── package com.utils; public class Calculator { public static double add(double a, double b) { return a + b; } public static double subtract(double a, double b) { return a - b; } public static double multiply(double a, double b) { return a * b; } public static double divide(double a, double b) { if (b == 0) throw new ArithmeticException("Division by zero!"); return a / b; } public static double power(double base, double exp) { return Math.pow(base, exp); } public static double percentage(double val, double total) { return (val / total) * 100; } } // ── File 2: Main.java (root mein) ── import com.utils.Calculator; // ← custom package import public class Main { public static void main(String[] args) { System.out.println("=== Custom Package Import Demo ==="); System.out.printf("15 + 7 = %.1f%n", Calculator.add(15, 7)); System.out.printf("15 - 7 = %.1f%n", Calculator.subtract(15, 7)); System.out.printf("15 * 7 = %.1f%n", Calculator.multiply(15, 7)); System.out.printf("22 / 7 = %.4f%n", Calculator.divide(22, 7)); System.out.printf("2^10 = %.0f%n", Calculator.power(2, 10)); System.out.printf("75/100%% = %.1f%%%n", Calculator.percentage(75, 100)); } }

public class BankVault { private String vaultCode = "SECRET-7829"; // ← PRIVATE — sirf is class mein private double totalCash = 9_50_00_000.00; private boolean authenticate(String code) { return vaultCode.equals(code); } // PUBLIC method — bahar wale isi se interact kar sakte hain public boolean access(String enteredCode) { if (authenticate(enteredCode)) { // private method andar se call kar sakte hain System.out.println("✅ Vault open! Cash: ₹" + totalCash); return true; } System.out.println("❌ Wrong code! Access denied."); return false; } public static void main(String[] args) { BankVault vault = new BankVault(); // ❌ Direct access attempt — compile error hoga // System.out.println(vault.vaultCode); // ERROR: private! // vault.authenticate("any"); // ERROR: private! // ✅ Public method se access vault.access("WRONG-CODE"); vault.access("SECRET-7829"); System.out.println("\n📌 private members: class ke bahar koi access nahi!"); } }

// File 1: com/demo/Helper.java package com.demo; class Helper { // ← DEFAULT class — no public keyword String message = "Hello"; // ← DEFAULT field void greet(String name) { // ← DEFAULT method System.out.println(message + ", " + name + "! (from com.demo.Helper)"); } } // File 2: com/demo/Tester.java — SAME PACKAGE — can access! package com.demo; public class Tester { public static void main(String[] args) { Helper h = new Helper(); // ✅ Same package — accessible h.message = "Namaste"; // ✅ Default field — accessible h.greet("Rahul"); // ✅ Default method — accessible h.message = "Aadab"; h.greet("Priya"); System.out.println("\n📌 Default access = Same Package Only!"); System.out.println(" Doosre package se Helper ka use NAHI hoga!"); } } // File 3: com/other/Outside.java — DIFFERENT PACKAGE — Error! // package com.other; // import com.demo.Helper; ← ERROR: Helper is not public // public class Outside { // Helper h = new Helper(); ← COMPILE ERROR! // }

// com/vehicle/Vehicle.java package com.vehicle; public class Vehicle { protected String brand; // ← protected — subclass accessible protected int speed; // ← protected private String engineCode; // ← private — nobody else public Vehicle(String brand, int speed) { this.brand = brand; this.speed = speed; this.engineCode = "ENG-" + (int)(Math.random() * 9999); } protected void showBase() { // ← protected method System.out.println("Brand: " + brand + " | Speed: " + speed + " km/h"); } } // com/car/Car.java — DIFFERENT PACKAGE — extends Vehicle package com.car; import com.vehicle.Vehicle; public class Car extends Vehicle { private int doors; private String type; public Car(String brand, int speed, int doors, String type) { super(brand, speed); this.doors = doors; this.type = type; } public void display() { showBase(); // ✅ protected — accessible in subclass System.out.println("Type: " + type + " | Doors: " + doors); // System.out.println(engineCode); // ❌ COMPILE ERROR — private! } public static void main(String[] args) { Car[] cars = { new Car("Maruti Swift", 150, 4, "Hatchback"), new Car("Honda City", 180, 4, "Sedan"), new Car("BMW M3", 290, 2, "Sports") }; for (Car c : cars) { c.display(); System.out.println("──"); } } }

public class AccessDemo { public String publicVar = "PUBLIC — sabko dikhe"; protected String protectedVar = "PROTECTED — pkg + subclass"; String defaultVar = "DEFAULT — sirf same package"; private String privateVar = "PRIVATE — sirf yahan"; public void showAll() { // Is class ke andar SABHHI accessible hain System.out.println("=== Andar se (Same Class) ==="); System.out.println("✅ public: " + publicVar); System.out.println("✅ protected: " + protectedVar); System.out.println("✅ default: " + defaultVar); System.out.println("✅ private: " + privateVar); } public static void main(String[] args) { AccessDemo obj = new AccessDemo(); obj.showAll(); System.out.println("\n=== Main se (Same Class) ==="); System.out.println("✅ public: " + obj.publicVar); System.out.println("✅ protected: " + obj.protectedVar); System.out.println("✅ default: " + obj.defaultVar); System.out.println("✅ private: " + obj.privateVar); System.out.println("\n=== Access Modifier Summary ==="); System.out.println("private → Only THIS class"); System.out.println("default → Same package only"); System.out.println("protected → Package + subclasses"); System.out.println("public → Everywhere"); } }

// java.lang AUTOMATIC import hota hai — explicitly import nahi karna! public class JavaLangDemo { public static void main(String[] args) { // ── String class ── String s = " Hello Backend World! "; System.out.println("Original: '" + s + "'"); System.out.println("Trimmed: '" + s.trim() + "'"); System.out.println("Upper: '" + s.trim().toUpperCase() + "'"); System.out.println("Contains 'Backend': " + s.contains("Backend")); System.out.println("Replace: '" + s.trim().replace("Backend", "Java") + "'"); // ── Math class ── System.out.println("\n--- Math Operations ---"); System.out.printf("sqrt(144) = %.1f%n", Math.sqrt(144)); System.out.println("min(5,9) = " + Math.min(5, 9)); System.out.println("max(5,9) = " + Math.max(5, 9)); System.out.println("abs(-42) = " + Math.abs(-42)); System.out.printf("PI = %.5f%n", Math.PI); // ── Integer Wrapper ── System.out.println("\n--- Integer Wrapper Class ---"); System.out.println("parseInt('42'): " + Integer.parseInt("42")); System.out.println("toBinary(10): " + Integer.toBinaryString(10)); System.out.println("toHex(255): " + Integer.toHexString(255)); System.out.println("MAX_VALUE: " + Integer.MAX_VALUE); // ── System class ── System.out.println("\n--- System Class ---"); System.out.println("JVM Version: " + System.getProperty("java.version")); System.out.println("OS Name: " + System.getProperty("os.name")); System.out.println("Time (ms): " + System.currentTimeMillis()); } }

import java.util.*; public class JavaUtilDemo { public static void main(String[] args) { // ── ArrayList ── List<String> cities = new ArrayList<>( Arrays.asList("Mumbai", "Delhi", "Indore", "Pune", "Bengaluru")); Collections.sort(cities); System.out.println("Sorted cities: " + cities); System.out.println("Min city: " + Collections.min(cities)); // ── HashMap ── Map<String, Integer> pop = new HashMap<>(); pop.put("Mumbai", 2080); pop.put("Delhi", 1900); pop.put("Indore", 360); pop.put("Pune", 700); System.out.println("\nCity populations (lakhs):"); new TreeMap<>(pop).forEach((k,v) -> System.out.printf(" %-12s → %d L%n", k, v)); // ── Optional (Java 8) ── Optional<String> found = cities.stream() .filter(c -> c.startsWith("I")) .findFirst(); System.out.println("\nCity starting with I: " + found.orElse("Not found")); // ── Random — OTP generate karo ── Random rnd = new Random(); int otp = 100000 + rnd.nextInt(900000); System.out.println("Generated OTP: " + otp); // ── Queue (LinkedList as Queue) ── Queue<String> ticketQ = new LinkedList<>(); ticketQ.add("Person1"); ticketQ.add("Person2"); ticketQ.add("Person3"); System.out.println("\nTicket Queue:"); while (!ticketQ.isEmpty()) System.out.println(" Serving: " + ticketQ.poll()); } }

import java.io.*; public class JavaIODemo { public static void main(String[] args) { String file = "students.csv"; // ── Write to file ── try (BufferedWriter bw = new BufferedWriter(new FileWriter(file))) { bw.write("Name,Marks,Grade\n"); bw.write("Rahul,88,A\n"); bw.write("Priya,95,A+\n"); bw.write("Amit,72,B\n"); bw.write("Neha,65,C\n"); System.out.println("✅ File written: " + file); } catch (IOException e) { System.out.println("❌ Write error: " + e.getMessage()); } // ── Read from file ── try (BufferedReader br = new BufferedReader(new FileReader(file))) { System.out.println("\n📄 Reading CSV file:"); System.out.printf("%-12s %-8s %-6s%n", "Name", "Marks", "Grade"); System.out.println("─".repeat(28)); String line = br.readLine(); // skip header while ((line = br.readLine()) != null) { String[] parts = line.split(","); System.out.printf("%-12s %-8s %-6s%n", parts[0], parts[1], parts[2]); } } catch (IOException e) { System.out.println("❌ Read error: " + e.getMessage()); } // File info File f = new File(file); System.out.printf("%nFile: %s | Size: %d bytes | Exists: %b%n", f.getName(), f.length(), f.exists()); f.delete(); // cleanup System.out.println("Cleaned up. Exists now: " + f.exists()); } }

import java.math.BigDecimal; import java.math.BigInteger; import java.math.RoundingMode; public class BigDecimalDemo { public static void main(String[] args) { // PROBLEM: double precision issue System.out.println("=== Floating Point Problem ==="); System.out.println("double 0.1+0.2 = " + (0.1 + 0.2)); // 0.30000000000000004 !!! // SOLUTION: BigDecimal System.out.println("BigDecimal: " + new BigDecimal("0.1").add(new BigDecimal("0.2"))); // Banking use case System.out.println("\n=== Salary Calculation ==="); BigDecimal salary = new BigDecimal("85000.00"); BigDecimal taxRate = new BigDecimal("0.30"); BigDecimal pfRate = new BigDecimal("0.12"); BigDecimal tax = salary.multiply(taxRate); BigDecimal pf = salary.multiply(pfRate); BigDecimal inHand = salary.subtract(tax).subtract(pf); System.out.println("Gross Salary : ₹" + salary); System.out.println("Tax (30%) : ₹" + tax); System.out.println("PF (12%) : ₹" + pf); System.out.println("In-hand : ₹" + inHand); // EMI calculation System.out.println("\n=== Home Loan EMI ==="); BigDecimal loan = new BigDecimal("5000000"); BigDecimal months = new BigDecimal("240"); BigDecimal emi = loan.divide(months, 2, RoundingMode.HALF_UP); System.out.println("₹50L / 20yr EMI (simple) = ₹" + emi); // BigInteger — 25! factorial System.out.println("\n=== BigInteger — Large Factorial ==="); BigInteger fact = BigInteger.ONE; for (int i=1; i<=25; i++) fact = fact.multiply(BigInteger.valueOf(i)); System.out.println("25! = " + fact); System.out.println("Digits: " + fact.toString().length()); } }

/* * 📁 Hospital Management System * com.hospital.model → Patient, Doctor, Appointment * com.hospital.service → PatientService, DoctorService * com.hospital.billing → Bill, Insurance * com.hospital.util → DateHelper, Validator */ // ── com/hospital/model/Patient.java ── package com.hospital.model; public class Patient { private int id; private String name, bloodGroup; private int age; public Patient(int id, String name, int age, String blood) { this.id=id; this.name=name; this.age=age; this.bloodGroup=blood; } public int getId() { return id; } public String getName() { return name; } public int getAge() { return age; } public String getBlood() { return bloodGroup; } @Override public String toString() { return String.format("[P%03d] %-15s Age:%-3d Blood:%s", id, name, age, bloodGroup); } } // ── com/hospital/service/PatientService.java ── package com.hospital.service; import com.hospital.model.Patient; // ← sub-package alag import import java.util.*; public class PatientService { private List<Patient> patients = new ArrayList<>(); public void register(Patient p) { patients.add(p); System.out.println("✅ Registered: " + p.getName()); } public void listAll() { System.out.println("\n📋 All Patients:"); patients.forEach(System.out::println); } public Optional<Patient> findByBlood(String bg) { return patients.stream().filter(p -> p.getBlood().equals(bg)).findFirst(); } } // ── Main entry point ── import com.hospital.model.Patient; // ← model sub-package import com.hospital.service.PatientService; // ← service sub-package public class HospitalApp { public static void main(String[] args) { PatientService svc = new PatientService(); svc.register(new Patient(1, "Rahul Sharma", 35, "B+")); svc.register(new Patient(2, "Priya Patel", 28, "O+")); svc.register(new Patient(3, "Amit Verma", 45, "A-")); svc.register(new Patient(4, "Neha Singh", 32, "B+")); svc.listAll(); svc.findByBlood("O+").ifPresent(p -> System.out.println("\nO+ donor found: " + p.getName())); } }

import static java.lang.Math.*; // Math ke sab static members import static java.lang.System.out; // System.out ko sirf 'out' se use karo public class StaticImportMath { public static void main(String[] args) { // Math. prefix ki zaroorat nahi! out.println("=== Static Import Demo ==="); out.println("PI = " + PI); out.println("E = " + E); out.printf("sqrt(625) = %.1f%n", sqrt(625)); out.printf("pow(2, 10) = %.0f%n", pow(2, 10)); out.printf("sin(PI/2) = %.1f%n", sin(PI / 2)); out.printf("cos(0) = %.1f%n", cos(0)); out.printf("abs(-99) = %d%n", abs(-99)); out.printf("ceil(4.2) = %.1f%n", ceil(4.2)); out.printf("floor(4.9) = %.1f%n", floor(4.9)); out.printf("log(E) = %.1f%n", log(E)); // Geometry calculations — clean code! out.println("\n--- Circle (radius=10) ---"); double r = 10; out.printf("Area = %.4f%n", PI * pow(r, 2)); out.printf("Perimeter = %.4f%n", 2 * PI * r); out.printf("Diagonal = %.4f%n", sqrt(2) * r); // Hypotenuse of 3-4-5 triangle out.printf("%nHypotenuse(3,4) = %.1f%n", sqrt(pow(3, 2) + pow(4, 2))); } }

// ── AppConstants.java ── package com.config; public class AppConstants { public static final String HOST = "localhost"; public static final int PORT = 8080; public static final String DB_URL = "jdbc:mysql://localhost/mydb"; public static final int MAX_THREADS = 50; public static final int SESSION_TIMEOUT = 3600; public static final String VERSION = "v2.4.1"; public static String getBaseUrl() { return "http://" + HOST + ":" + PORT; } public static String getApiUrl() { return getBaseUrl() + "/api"; } } // ── Server.java — static import use karo ── import static com.config.AppConstants.*; // AppConstants. prefix nahi chahiye public class Server { public static void main(String[] args) { // AppConstants prefix nahi lagate! System.out.println("╔══════════════════════════════╗"); System.out.println("║ Server Configuration ║"); System.out.println("╚══════════════════════════════╝"); System.out.println("Version: " + VERSION); System.out.println("Host: " + HOST); System.out.println("Port: " + PORT); System.out.println("DB URL: " + DB_URL); System.out.println("Threads: " + MAX_THREADS); System.out.println("Timeout: " + SESSION_TIMEOUT + " sec"); System.out.println("Base URL: " + getBaseUrl()); System.out.println("API URL: " + getApiUrl()); System.out.println("\n✅ Server started on " + getBaseUrl()); } }

/* * ═══════════════════════════════════════════════════════ * REAL INDUSTRY PACKAGE EXAMPLES * ═══════════════════════════════════════════════════════ * * ── Famous Libraries ── * org.springframework.boot ← Spring Boot * com.google.gson ← Google Gson * org.apache.commons.lang3 ← Apache Commons * io.netty.channel ← Netty * org.hibernate.orm ← Hibernate ORM * com.fasterxml.jackson ← Jackson JSON * * ── Famous Companies ── * com.flipkart.zeus.catalog ← Flipkart * com.paytm.payments.gateway ← Paytm * com.zomato.order.management ← Zomato * in.ola.ride.booking ← Ola * com.byju.learning.content ← BYJU's * * ── Standard Spring Boot Project ── * com.company.myapp ← root package * com.company.myapp.model ← data classes * com.company.myapp.dto ← data transfer objects * com.company.myapp.repository ← database layer * com.company.myapp.service ← business logic * com.company.myapp.controller ← REST APIs * com.company.myapp.config ← configurations * com.company.myapp.exception ← custom exceptions * com.company.myapp.security ← auth/security * com.company.myapp.util ← helper utilities */ package com.mycompany.hrms.employee; public class Employee { private String empId, name, dept; private double salary; public Employee(String id, String name, String dept, double sal) { this.empId=id; this.name=name; this.dept=dept; this.salary=sal; } public static void main(String[] args) { Employee[] team = { new Employee("E01", "Rahul", "Backend", 75000), new Employee("E02", "Priya", "Frontend", 65000), new Employee("E03", "Amit", "DevOps", 85000), new Employee("E04", "Kiran", "Database", 72000) }; System.out.println("Package: com.mycompany.hrms.employee"); System.out.println("ID Name Department Salary"); System.out.println("─".repeat(42)); double total = 0; for (Employee e : team) { System.out.printf("%-5s %-10s %-14s ₹%.0f%n", e.empId, e.name, e.dept, e.salary); total += e.salary; } System.out.println("─".repeat(42)); System.out.printf("Total Payroll: ₹%.0f/month%n", total); } }

// ════════════════════════════════════════ // ❌ MISTAKE 1: Wrong Order // ════════════════════════════════════════ // import java.util.List; ← GALAT: pehle import // package com.myapp; ← GALAT: baad mein package // Error: "class, interface, or enum expected" // ✅ SAHI ORDER: // 1) package com.myapp; // 2) import java.util.List; // 3) public class MyClass { ... } // ════════════════════════════════════════ // ❌ MISTAKE 2: File name mismatch // ════════════════════════════════════════ // File: student.java (lowercase s) // Class: public class Student {} ← ERROR! // Fix: File must be Student.java (capital S) // ════════════════════════════════════════ // ❌ MISTAKE 3: Wildcard sub-package myth // ════════════════════════════════════════ // import java.util.*; ← java.util.concurrent NAHI milega! // AtomicInteger x = new AtomicInteger(); ← COMPILE ERROR! // Fix: import java.util.concurrent.atomic.AtomicInteger; // ════════════════════════════════════════ // ✅ CORRECT DEMONSTRATION // ════════════════════════════════════════ package com.correct; // Step 1: package import java.util.ArrayList; // Step 2: import import java.util.concurrent.atomic.AtomicInteger; // sub-pkg alag! public class CorrectDemo { // Step 3: class (file: CorrectDemo.java) public static void main(String[] args) { ArrayList<String> errors = new ArrayList<>(); errors.add("Wrong import order"); errors.add("File name case mismatch"); errors.add("Wildcard = sub-package myth"); errors.add("Uppercase package names"); errors.add("Multiple package declarations"); System.out.println("Top Java Package Mistakes:"); for (int i=0; i<errors.size(); i++) System.out.printf("%d. ❌ %s%n", i+1, errors.get(i)); // AtomicInteger — java.util.concurrent.atomic (NOT in java.util.*) AtomicInteger counter = new AtomicInteger(0); counter.incrementAndGet(); counter.incrementAndGet(); System.out.println("\nAtomicInteger (sub-package): " + counter.get()); System.out.println("✅ Correctly imported via full sub-package path!"); } }

/* * 📁 Library Management System — MVC Architecture * com.library.model → Book, Member * com.library.exception → BookNotFoundException, MemberException * com.library.repository → BookRepository, MemberRepository * com.library.service → LibraryService * com.library.controller → LibraryController */ // ════════════ 1. MODEL LAYER ════════════ package com.library.model; public class Book { private String isbn, title, author; private boolean available; public Book(String isbn, String title, String author) { this.isbn=isbn; this.title=title; this.author=author; this.available = true; } public String getIsbn() { return isbn; } public String getTitle() { return title; } public String getAuthor() { return author; } public boolean isAvailable(){ return available; } public void setAvailable(boolean a) { this.available = a; } @Override public String toString() { return String.format("[%s] %-30s by %-15s %s", isbn, title, author, available ? "✅ Available" : "📤 Borrowed"); } } // ════════════ 2. EXCEPTION LAYER ════════════ package com.library.exception; public class BookNotFoundException extends RuntimeException { public BookNotFoundException(String isbn) { super("Book not found: ISBN = " + isbn); } } public class BookNotAvailableException extends RuntimeException { public BookNotAvailableException(String title) { super("Book not available: " + title); } } // ════════════ 3. REPOSITORY LAYER ════════════ package com.library.repository; import com.library.model.Book; import java.util.*; public class BookRepository { private Map<String, Book> db = new HashMap<>(); public void save(Book b) { db.put(b.getIsbn(), b); } public List<Book> findAll() { return new ArrayList<>(db.values()); } public Optional<Book> findByIsbn(String isbn) { return Optional.ofNullable(db.get(isbn)); } public List<Book> findAvailable() { List<Book> avl = new ArrayList<>(); for (Book b : db.values()) if (b.isAvailable()) avl.add(b); return avl; } } // ════════════ 4. SERVICE LAYER ════════════ package com.library.service; import com.library.model.Book; import com.library.repository.BookRepository; import com.library.exception.*; import java.util.List; public class LibraryService { private BookRepository repo = new BookRepository(); public void addBook(Book b) { repo.save(b); } public List<Book> getAllBooks() { return repo.findAll(); } public List<Book> getAvailable(){ return repo.findAvailable(); } public void issueBook(String isbn, String member) { Book book = repo.findByIsbn(isbn) .orElseThrow(() -> new BookNotFoundException(isbn)); if (!book.isAvailable()) throw new BookNotAvailableException(book.getTitle()); book.setAvailable(false); System.out.println("📤 Issued: '"+book.getTitle().trim()+"' to "+member); } public void returnBook(String isbn) { Book book = repo.findByIsbn(isbn) .orElseThrow(() -> new BookNotFoundException(isbn)); book.setAvailable(true); System.out.println("📥 Returned: '"+book.getTitle().trim()+"'"); } } // ════════════ 5. CONTROLLER LAYER ════════════ package com.library.controller; import com.library.model.Book; import com.library.service.LibraryService; public class LibraryController { private LibraryService service = new LibraryService(); public void addBook(String isbn, String title, String author) { service.addBook(new Book(isbn, title, author)); System.out.println("✅ Added: " + title); } public void showAll() { System.out.println("\n📚 ALL BOOKS:"); service.getAllBooks().forEach(System.out::println); } public void showAvailable() { System.out.println("\n✅ AVAILABLE:"); service.getAvailable().forEach(System.out::println); } public void issue(String isbn, String member) { try { service.issueBook(isbn, member); } catch (RuntimeException e) { System.out.println("❌ Error: " + e.getMessage()); } } public void returnBook(String isbn) { try { service.returnBook(isbn); } catch (RuntimeException e) { System.out.println("❌ Error: " + e.getMessage()); } } } // ════════════ 6. APP ENTRY POINT ════════════ import com.library.controller.LibraryController; public class LibraryApp { public static void main(String[] args) { LibraryController ctrl = new LibraryController(); // Add books ctrl.addBook("ISBN001", "Clean Code", "Robert C. Martin"); ctrl.addBook("ISBN002", "Effective Java", "Joshua Bloch"); ctrl.addBook("ISBN003", "Head First Design Patterns", "Freeman"); ctrl.addBook("ISBN004", "Spring in Action", "Craig Walls"); ctrl.showAll(); // Issue books System.out.println(); ctrl.issue("ISBN001", "Rahul"); ctrl.issue("ISBN003", "Priya"); ctrl.issue("ISBN001", "Amit"); // Already issued — error ctrl.showAvailable(); // Return book System.out.println(); ctrl.returnBook("ISBN001"); ctrl.showAvailable(); } }