CalculatorApp.java

public class CalculatorApp {
  public static void main (String[] args) {
    Calculator calculator = new Calculator();

    System.out.println("======== Welcome to the calculator app =========");
    String choice = "y";

    while (choice.equalsIgnoreCase("y")) {
      CalculatorApp.displayMenu();

      String option = Console.getString("Please select from the above options: ");
      
      switch (option) {
        case "1":
          calculator.getInput();
          break;
        case "2":
          calculator.setStrategy(new AdditionStrategy());
          CalculatorApp.printResult(calculator.getResult());
          break;
        case "3":
          calculator.setStrategy(new SubtractionStrategy());
          CalculatorApp.printResult(calculator.getResult());
          break;
        case "4":
          calculator.setStrategy(new MultiplicationStrategy());
          CalculatorApp.printResult(calculator.getResult());
          break;
        case "5":
          calculator.setStrategy(new DivisionStrategy());
          CalculatorApp.printResult(calculator.getResult());
          break;
        default:
          return;
      }

      choice = Console.getString("\nDo you wish to continue (y): ");
    }
  }

  public static void printResult (double output) {
    System.out.println("The output is: " + output);
    System.out.println();
  }
  
  public static void displayMenu () {
    System.out.println();
    System.out.println("1 - Input numbers");
    System.out.println("2 - Addition operation (a + b)");
    System.out.println("3 - Subtraction operation (a - b)");
    System.out.println("4 - Multiply operation (a * b)");
    System.out.println("5 - Division operation (a/b)");
    System.out.println("6 - Exit \n");
  }
}

class Calculator {
  private CalculationStrategy strategy;
  private double a;
  private double b;
  
  public void getInput () {
    System.out.println();

    a = Console.getDouble("Enter the value of a: ");
    b = Console.getDouble("Enter the value of b: ");
  }

  public void setStrategy (CalculationStrategy s) {
    strategy = s;
  }
  public double getResult () {
    return strategy.calculate(a, b);
  }
}

interface CalculationStrategy {
  public double calculate(double a, double b);
}

class AdditionStrategy implements CalculationStrategy {
  public double calculate (double a, double b) {
    return a + b;
  }
}

class Console {
  private static Scanner sc = new Scanner(System.in);

  public static String getString(String prompt) {
    System.out.print(prompt);
    String s = sc.next();
    sc.nextLine();
    return s;
  }

  public static double getDouble(String prompt) {
    double d = 0;
    boolean isValid = false;

    while (!isValid) {
      System.out.print(prompt);
      if (sc.hasNextDouble()) {
        d = sc.nextDouble();
        isValid = true;
      } else {
        System.out.println("Error! Invalid decimal value. Try again.");
      }
      sc.nextLine();
    }

    return d;
  }
}

class DivisionStrategy implements CalculationStrategy {
  public double calculate (double a, double b) {
    return a / b;
  }
}

class MultiplicationStrategy implements CalculationStrategy {
  public double calculate (double a, double b) {
    return a * b;
  }
}

class SubtractionStrategy implements CalculationStrategy {
  public double calculate (double a, double b) {
    return a - b;
  }
}