Open in app

Sign in

Write

Sign in

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:

Sign up to discover human stories that deepen your understanding of the world.

Free

Distraction-free reading. No ads.

Organize your knowledge with lists and highlights.

Tell your story. Find your audience.

Membership

Read member-only stories

Support writers you read most

Earn money for your writing

Listen to audio narrations

Read offline with the Medium app

Core Java Tutorial
Core Java
Java Reference
Avinashbest
Codewithmosh
Avinash Kumar
Avinash Kumar

Written by Avinash Kumar

22 Followers
3 Following

Software Engineer at Bobble AI

No responses yet

Write a response

What are your thoughts?

Help

Status

About

Careers

Press

Blog

Privacy

Terms

Text to speech

Teams