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Core Java Basics: Quick Reference

Avinash Kumar
9 min readAug 6, 2021
Java

Function:

  • A block of code that performs a task.
void sendEmail(){
        ...
    }
  • camelNamingConvention

Class:

  • A container for related functions.
public class Main {
        public void main() {
            ...
        }
    }
  • PascalNamingConvention

Compilation Process:

Compilation Process
  • JRE: Java Runtime Environment
JRE
avinashbest/ $ javac Main.javaavinashbest/ $ cd ..src/ $ java com.avinashbest.Main

Interesting Facts about Java:

  • Java was developed by James Gosling in 1995.

Types:

  • Primitive: for storing simple values, like numbers, characters, booleans.
  • How primitive types are stored in memory.
public class Main {
        public static void main(String[] args) {
            int x = 1;
            int y = 1;
        }
    }
  • Reference: for storing complex objects, like date, mail, message
  • How primitive types are stored in memory.
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 ($1K - $1M): ");
                principal = scanner.nextInt();
                if (principal >= 1000 && principal <= 1_000_000) {
                    break;
                }
                System.out.println("Enter a value between $1000 and $1,000,000");
            }            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 ($1K - $1M): ", 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 ($1K - $1M): ", 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

  • Changing the structure of the code without changing its behaviour.
  • Keep your methods short
  • Extract repetitive patterns
  • Extract highly related statements

Types of Errors

  • Compile — Time Errors
  • Run — Time Errors

Running a Java Archive (.jar) in local:

$ java -jar file-name.jar

Thank You!

Credits:

Core Java Tutorial
Core Java
Java Reference
Avinashbest
Codewithmosh

--

--

Avinash Kumar

Written by Avinash Kumar

21 Followers

Software Engineer at Bobble AI

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