Episode 1, Calculator - Coding

Episode 1, Calculator - Coding

Season 3 - I know Kung Fu

Letz’ code it

  1. Create a empty folder with name calculator.
  2. Go inside of this folder.


  1. Create a .java file with the name App.java
  2. This will be the “entry point” in our app
  3. While you’ll type the code below your IDE will show errors, that’s ok, we’ll add all the pieces then all the errors will disappear.
    • Please, do not copy paste.
public class App {

//    we are creating now a application that behaves like a real one
//    let's see what parts do we have in real app
//    1. The entry point (App.java)
//    2. Interaction with the user
//        1. display of messages to the user
//        2. capture the input from the user
//    4. The calculation unit, that does the calculation
//    we are going to create a console app, which looks different, but behaves
//    exactly like a normal one

    // this function can be run
    // this a feature of Java
    // we can have only one such method in a class
    public static void main(String[] args) throws Exception {
        // we have as well the display, let's use it here
        // since the functions are static, we can call the directly
        // without creating a instance!

        // 1st step is to ask for first operand!

        // next we have to wait for user's input!
        // we'll use the Input class and store the result inside a local variable
        int operand1 = Input.getOperand1();

        // next, we need to ask what operation need to be done

        // we'll do a similar step as we've done earlier
        String operation = Input.getOperation();


        int operand2 = Input.getOperand2();

        int result = CalculationUnit.calculate(operand1, operation, operand2);





  1. Create a .java file with the name Display.java
  2. This class will contain only messages that we are going to show to the user
public class Display {

    public static void showWelcomeMessage() {
        System.out.println("Welcome to Basic Calculator");
        System.out.println("please follow the steps for get the calculation done");

    public static void askFirstOperand() {
        // since println() moves the cursor to the next line, we'll use just print()
        // this will give us a nice way to let message and value in same line
        System.out.print("Please provide first operand ");

    public static void askOperation() {
        System.out.print("Please select operation + - / * ");

    public static void askSecondOperand() {
        System.out.print("Please provide second operand ");

    public static void showResult(int result) {
        System.out.println("The result of the operation is = " + result);

    public static void showBye() {
        System.out.println("Thanks for using Basic calculator, see you soon!");


  1. Create a .java file with the name Input.java
  2. This class will contain only messages that we are going to show to the user
  3. In this class we’ll use a class from the JDK (Java Development Kit) Scanner
// in java we have a Scanner class that gives us possibility to read user's input
// when we need to use class from other package we need to explicitly "import" them
// java.util.Scanner = there's somewhere a folder [java] with with a subfolder [util] which contains a file Scanner.java
// in this case the JDK give this class to us
import java.util.Scanner; 

public class Input {

    // we create a variable scanner and assign to it a new instance of scanner
    static Scanner scanner = new Scanner(System.in);

    // this function will read first operand
    public static int getOperand1() {

        int result = scanner.nextInt();

        return result;

    // as you see, here we use variable with name: result, just like in the function getOperand1()
    // it is not same variables, because it is defined in a the scope of a different function
    public static String getOperation() {
        String result = scanner.next();

        return result;

    // this function will read second operand
    // notice any difference between this one and getOperand2() ?
    // can we improve this part ?
    public static int getOperand2() {
        int result = scanner.nextInt();

        return result;


  1. Create a .java file with the name CalculationUnit.java
  2. This class will contain only the calculation part
public class CalculationUnit {

    public static int calculate(int operand1, String operation, int operand2) {
        // we create a "placeholder" variable to use later for holding the result
        int result = 0;

        // find what should we do depending on the selected operation
        switch (operation) {
            case "+":
                    result = operand1 + operand2;
            case "-":
                    result = operand1 - operand2;
            case "*":
                    result = operand1 * operand2;
            case "/":
                    result = operand1 / operand2;

        return result;



  • Open App.java
  • Right click inside the file and click Run


  • Click on little green triangle, that looks like play located on the same line as to public static void main(..).

If you cannot find it then click here for details.

Navigate to the location of the file App.java with cd command, e.g. cd /home/$USER/calculator.

javac App.java
java App

After launching terminal command you’ll see newly generated .class files. You can try to open them with any text editor. They look funny as they are binary files, compiled by javac compiler. The content is bytecode, something understood by the JVM.