Function :

 void sendEmail(){
 ...
 }

Class:

 public class Main {
 public void main() {
 ...
 }
 }

Compilation Process:

 avinashbest/ $ javac Main.java

 avinashbest/ $ cd ..

 src/ $ java com.avinashbest.Main

Interesting Facts about Java:

Types:

 public class Main {
 public static void main(String[] args) {
 int x = 1;
 int y = 1;
 }
 }

 public class Main {
 public static void main(String[] args) {
 Point point1 = new Point(1, 1);
 Point point2 = point1;
 }
 }

String:

 public class Main {
 public static void main(String[] args) {
 String message = "Hello World" + "!!";
 System.out.println(message.endsWith("!!"));
 System.out.println(message.startsWith("!!"));
 System.out.println(message.length());
 System.out.println(message.indexOf("l"));
 System.out.println(message.indexOf("sky"));
 System.out.println(message.replace("!", "*"));
 System.out.println(message.toLowerCase(Locale.ROOT));
 System.out.println(message.toUpperCase(Locale.ROOT));
 System.out.println(message.trim()) ; /*To remove unnecessary space.*/
 /*NOTE: IN JAVA, ALL THE METHOD WHICH PERFORM OPERATION ON STRING RETURNS A NEW STRING.*/
 /*ALL THE STRING IN JAVA ARE IMMUTABLE*/
 System.out.println(message);
 }
 }

Escape Sequence:

 public class Main {
 public static void main(String[] args) {
 String message1 = "Hello \"Avinash\"";
 String message2 = "C:\\Windows\\...";
 String message3 = "C:\nWindows\n...";
 String message4 = "C:\tWindows\t...";
 System.out.println(message1);
 System.out.println(message2);
 System.out.println(message3);
 System.out.println(message4);
 }
 }

Arrays:

 public class Main {
 public static void main(String[] args) {
 int[] number = {2, 4, 6, 8, 10};
 System.out.println(number.length);
 Arrays.sort(number);
 System.out.println(Arrays.toString(number));
 }
 }

Multi-Dimensional Arrays:

 public class Main {
 public static void main(String[] args) {
 int[][] numbers = {{1, 2, 3}, {4, 5, 6}};
 System.out.println(Arrays.toString(numbers));
 System.out.println(Arrays.deepToString(numbers));
 }
 }

Arithmetic Expressions:

 public class Main {
 public static void main(String[] args) {
 int result = 10 + 3;
 System.out.println(result);
 result = 10 - 3;
 System.out.println(result);
 result = 10 / 3;
 System.out.println(result);
 result = 10 * 3;
 System.out.println(result);
 result = 10 % 3;
 System.out.println(result);
 }
 }

Increment / Decrement Operator:

 public class Main {
 public static void main(String[] args) {
 int x = 1;
 x++;
 System.out.println(x);
 ++x;
 System.out.println(x);
 x = 1;
 int y = x++; // post-increment
 System.out.println(x + "\t" + y);
 y = ++x; //pre-increment
 System.out.println(x + "\t" + y);
 x += 2; //x = x + 2
 System.out.println(x);
 x -= 2; //x = x - 2
 System.out.println(x);
 x *= 2; //x = x * 2
 System.out.println(x);
 x /= 2; //x = x / 2
 System.out.println(x);
 x %= 2; //x = x % 2
 System.out.println(x);
 }
 }

Order of Operations:

 public class Main {
 public static void main(String[] args) {
 int x = (10 + 3) * 2;
 }
 }

Type Casting:

 public class Main {
 public static void main(String[] args) {
 /*Implicit Casting*/
 /*byte > short > int > long > float > double*/
 short x = 1;
 int y = x + 2;
 System.out.println(y);
 /*Explicit Casting*/
 double a = 1.1;
 int b = (int) (a + 2);
 System.out.println(b);
 /*Parsing String in Integer*/
 String str = "123";
 System.out.println(Integer.parseInt(str));
 }
 }

The Math Class:

 public class Main {
 public static void main(String[] args) {
 System.out.println(Math.round(1.2F));
 System.out.println(Math.round(1.2));
 System.out.println(Math.ceil(1.2));
 System.out.println(Math.floor(1.2));
 System.out.println(Math.max(200, 300));
 System.out.println(Math.min(200, 300));
 System.out.println(Math.random());//by default return a doubleType random number b/w 0 to 1
 System.out.println(Math.round(Math.random() * 100));
 System.out.println((int) (Math.random() * 100));
 }
 }

Formatting Numbers:

 public class Main {
 public static void main(String[] args) {
 NumberFormat currency = NumberFormat.getCurrencyInstance();
 String result = currency.format(123456.789);
 System.out.println(result);
 NumberFormat percent = NumberFormat.getPercentInstance();
 String percentage = percent.format(0.1);
 System.out.println(percentage);
 System.out.println(NumberFormat.getCurrencyInstance().format(134343134313L));
 }
 }

Reading Input:

 public class Main {
 public static void main(String[] args) {
 Scanner scanner = new Scanner(System.in);
 System.out.print("Age: ");
 byte age = scanner.nextByte();
 System.out.print("Name: ");
 // String name = scanner.next(); //returns the next token
 String name = scanner.nextLine().trim(); //returns next line
 System.out.println("You are " + age);
 System.out.println("Your name " + name);
 }
 }

Mortgage Calculator - v1.0:

 public class Main {
 public static void main(String[] args) {
 final byte MONTHS_IN_YEAR = 12;
 final byte PERCENT = 100;
 Scanner scanner = new Scanner(System.in);
 System.out.print("Principal: ");
 int principal = scanner.nextInt();
 System.out.print("Annual Interest Rate: ");
 float annualInterest = scanner.nextFloat();
 float monthlyInterest = annualInterest / PERCENT / MONTHS_IN_YEAR;
 System.out.print("Period (Years): ");
 byte years = scanner.nextByte();
 int numberOfPayments = years * MONTHS_IN_YEAR;
 /*calculating mortgage*/
 double mortgage = principal * (monthlyInterest * Math.pow(1 + monthlyInterest, numberOfPayments) 
 / (Math.pow(1 + monthlyInterest, numberOfPayments) - 1));
 System.out.println("Mortgage: " + NumberFormat.getCurrencyInstance().format(mortgage));
 }
 }

Comparison Operators:

 public class Main {
 public static void main(String[] args) {
 int x = 1;
 int y = 1;
 System.out.println(x == y);
 System.out.println(x != y);
 System.out.println(x > y);
 System.out.println(x >= y);
 System.out.println(x < y);
 System.out.println(x <= y);
 }
 }

Logical Operators:

 public class Main {
 public static void main(String[] args) {
 int temperature = 22;
 boolean isWarm = temperature > 20 && temperature < 30;
 System.out.println(isWarm);
 boolean hasHighIncome = true;
 boolean hasGoodCredit = true;
 boolean hasCriminalRecord = false;
 boolean isEligible = (hasGoodCredit || hasHighIncome) && !hasCriminalRecord;
 System.out.println(isEligible);
 }
 }

if-statement:

 public class Main {
 public static void main(String[] args) {
 int temperature = 32;
 if (temperature > 32) {
 System.out.println("It's a hot day");
 System.out.println("Drink water");
 } else if (temperature > 20) {
 System.out.println("Beautiful day");
 } else {
 System.out.println("Cold day");
 }
 /*Simplifying if-statement*/
 int income = 120_000;
 boolean hasHighIncome;
 if (income > 100_000) {
 hasHighIncome = true;
 } else {
 hasHighIncome = false;
 }
 /*Above Expression can be simplified by professional be like*/
 int income = 120_000;
 boolean hasHighIncome = income > 100_000;
 }
 }

Ternary Operators:

 public class Main {
 public static void main(String[] args) {
 int income = 120_000;
 String className;
 if (income > 100_000) {
 className = "First";
 } else {
 className = "Economy";
 }
 /*Simplification can be done as*/
 income = 120_000;
 className = "Economy";
 if (income > 100_000) {
 className = "First";
 }
 /*More Simplification can be done using Ternary Operator*/
 income = 120_000;
 className = (income > 100_000) ? "First" : "Economy";
 }
 }

Switch Statements:

 public class Main {
 public static void main(String[] args) {
 String role = "admin";
 switch (role) {
 case "admin":
 System.out.println("You're an admin");
 break;
 case "moderator":
 System.out.println("You're a moderator");
 break;
 default:
 System.out.println("You're a guest");
 }
 if (role == "admin") {
 System.out.println("You're an admin");
 } else if (role == "moderator") {
 System.out.println("You're a moderator");
 } else {
 System.out.println("You're a guest");
 }
 }
 }

Exercise: FizzBuzz:

 public class Main {
 public static void main(String[] args) {
 Scanner scanner = new Scanner(System.in);
 System.out.print("Number: ");
 int number = scanner.nextInt();
 boolean moduloFive = (number % 5) == 0;
 boolean moduloThree = (number % 3) == 0;
 if (moduloFive && moduloThree) {
 System.out.println("FizzBuzz");
 } else if (moduloFive) {
 System.out.println("Fizz");
 } else if (moduloThree) {
 System.out.println("Buzz");
 } else {
 System.out.println(number);
 }
 }
 }

Loops in Java:

 public class Main {
 public static void main(String[] args) {
 Scanner scan = new Scanner(System.in);
 /*FOR LOOPS*/
 for (int i = 0; i < 5; i++) {
 System.out.println("Hello, Java!");
 }
 /*WHILE LOOPS: USED WHEN DON'T KNOW HOW MANY TIMES WE NEED TO EXECUTE CODE*/
 int i = 0;
 while (i < 5) {
 System.out.println("Hello, Java!");
 i++;
 }

 String input = "";
 while (!input.equals("quit")) {
 System.out.print("Input: ");
 input = scan.next().toLowerCase(Locale.ROOT);
 System.out.println(input);
 }
 /*DO-WHILE LOOPS: SIMILAR TO WHILE-LOOP BUT IT EXECUTES ATLEAST 1 TIME*/
 do {
 System.out.print("Input: ");
 input = scan.next().toLowerCase(Locale.ROOT);
 System.out.println(input);
 } while (!input.equals("quit"));
 }
 }

Break & Continue Keywords:

 public class Main {
 public static void main(String[] args) {
 Scanner scan = new Scanner(System.in);
 String input = "";
 while (true) {
 System.out.print("Input: ");
 input = scan.next().toLowerCase(Locale.ROOT);
 if (input.equals("pass")) {
 continue;/*moves control to the beginning of the loop*/
 }
 if (input.equals("quit")) {
 break;/*break out of the loop*/
 }
 System.out.println(input);
 }
 }
 }

Mortgage Calculator - v2.0:

 public class Main {
 public static void main(String[] args) {
 final byte MONTHS_IN_YEAR = 12;
 final byte PERCENT = 100;
 int principal;
 float annualInterest;
 float monthlyInterest;
 byte years;
 int numberOfPayments;
 Scanner scanner = new Scanner(System.in);
 while (true) {
 System.out.print("Principal (1ドルK - 1ドルM): ");
 principal = scanner.nextInt();
 if (principal >= 1000 && principal <= 1_000_000) {
 break;
 }
 System.out.println("Enter a value between 1ドル000 and 1ドル,000,000");
 }
 while (true) {
 System.out.print("Annual Interest Rate (1% - 30%): ");
 annualInterest = scanner.nextFloat();
 if (annualInterest >= 1 && annualInterest <= 30) {
 monthlyInterest = annualInterest / PERCENT / MONTHS_IN_YEAR;
 break;
 }
 System.out.println("Enter a value between 1 and 30");
 }
 while (true) {
 System.out.print("Period (1 Years- 30 Years): ");
 years = scanner.nextByte();
 if (years >= 1 && years <= 30) {
 numberOfPayments = years * MONTHS_IN_YEAR;
 break;
 }
 System.out.println("Enter a value between 1 and 30");
 }
 /*calculating mortgage*/
 double mortgage = principal * (monthlyInterest * Math.pow(1 + monthlyInterest, numberOfPayments) 
 / (Math.pow(1 + monthlyInterest, numberOfPayments) - 1));
 System.out.println("Mortgage: " + NumberFormat.getCurrencyInstance().format(mortgage));
 }
 }

Creating Methods:

 public class Main {
 public static void main(String[] args) {
 String message = greetUser("Avinash", "Kumar");
 System.out.println(message);
 }
 
 public static String greetUser(String firstName, String lastName){
 return "Hello " + firstName + " " + lastName;
 }
 }

Mortgage Calculator - v3.0:

 public class Main {
 public static void main(String[] args) {
 int principal = (int) readNumber("Principal (1ドルK - 1ドルM): ", 1000, 1_000_000);
 float annualInterest = (float) readNumber("Annual Interest Rate (1% - 30%): ", 1, 30);
 byte years = (byte) readNumber("Period (1 Years- 30 Years): ", 1, 30);
 double mortgage = calculateMortgage(principal, annualInterest, years);
 System.out.println("Mortgage: " + NumberFormat.getCurrencyInstance().format(mortgage));
 }
 public static double readNumber(String prompt, double min, double max) {
 Scanner scanner = new Scanner(System.in);
 double value;
 while (true) {
 System.out.print(prompt);
 value = scanner.nextDouble();
 if (value >= min && value <= max) {
 break;
 }
 System.out.println("Enter a value between " + min + " and " + max);
 }
 return value;
 }
 public static double calculateMortgage(int principal, float annualInterest, byte years) {
 final byte MONTHS_IN_YEAR = 12;
 final byte PERCENT = 100;
 float monthlyInterest = annualInterest / PERCENT / MONTHS_IN_YEAR;
 short numberOfPayments = (short) (years * MONTHS_IN_YEAR);
 return principal * (monthlyInterest * Math.pow(1 + monthlyInterest, numberOfPayments) 
 / (Math.pow(1 + monthlyInterest, numberOfPayments) - 1));
 }
 }

Mortgage Calculator - Clean Code:

 public class Main {
 final static byte MONTHS_IN_YEAR = 12;
 final static byte PERCENT = 100;
 public static void main(String[] args) {
 int principal = (int) readNumber("Principal (1ドルK - 1ドルM): ", 1000, 1_000_000);
 float annualInterest = (float) readNumber("Annual Interest Rate (1% - 30%): ", 1, 30);
 byte years = (byte) readNumber("Period (1 Years- 30 Years): ", 1, 30);
 printMortgage(principal, annualInterest, years);
 printPaymentSchedule(principal, annualInterest, years);
 }
 private static void printPaymentSchedule(int principal, float annualInterest, byte years) {
 System.out.println();
 System.out.println("PAYMENTS SCHEDULE:");
 System.out.println("------------------");
 for (short month = 1; month <= years * MONTHS_IN_YEAR; month++) {
 double balance = calculateBalance(principal, annualInterest, years, month);
 System.out.println(NumberFormat.getCurrencyInstance().format(balance));
 }
 }
 private static void printMortgage(int principal, float annualInterest, byte years) {
 System.out.println();
 System.out.println("MORTGAGE");
 System.out.println("--------");
 double mortgage = calculateMortgage(principal, annualInterest, years);
 System.out.println("Monthly Payments: " + NumberFormat.getCurrencyInstance().format(mortgage));
 }
 public static double readNumber(String prompt, double min, double max) {
 Scanner scanner = new Scanner(System.in);
 double value;
 while (true) {
 System.out.print(prompt);
 value = scanner.nextDouble();
 if (value >= min && value <= max) {
 break;
 }
 System.out.println("Enter a value between " + min + " and " + max);
 }
 return value;
 }
 public static double calculateBalance(int principal, float annualInterest, byte years, short numberOfPaymentsMade) {
 float monthlyInterest = annualInterest / PERCENT / MONTHS_IN_YEAR;
 short numberOfPayments = (short) (years * MONTHS_IN_YEAR);
 double balance = principal * (Math.pow(1 + monthlyInterest, numberOfPayments) 
 - Math.pow(1 + monthlyInterest, numberOfPaymentsMade)) 
 / (Math.pow(1 + monthlyInterest, numberOfPayments) - 1);
 return balance;
 }
 public static double calculateMortgage(int principal, float annualInterest, byte years) {
 float monthlyInterest = annualInterest / PERCENT / MONTHS_IN_YEAR;
 short numberOfPayments = (short) (years * MONTHS_IN_YEAR);
 return principal * (monthlyInterest * Math.pow(1 + monthlyInterest, numberOfPayments) 
 / (Math.pow(1 + monthlyInterest, numberOfPayments) - 1));
 }
 }

Refactoring

Types of Errors

Running a Java Archive (.jar) in local:

 $ java -jar file-name.jar

Thank You !