Memory Calculator confusion java

Question

In my class we needed to make a memory calculator in Java. Im really new to Java and had help making the program. Turned it in and the teacher said "Please separate the MemoryCalculator class from the class with the main() method. Currently the way you have created the class, there is no reason to create an instance of the class. But the point of the assignment is to use separate classes and objects." Its been a super long week and midterms and just lost at this time. Any help would be great.

import java.util.Scanner;

public class MemoryCalculator {


        private double currentValue;
        //Methods
        //Scanner
        public static int displayMenu(){
                    Scanner input = new Scanner(System.in);
                    System.out.print("Lets do some math! \nMenu \n1. Add \n2. Subtract \n3. Multiply \n4. Divide \n"
                                    + "5. Clear \n6. Quit \n\nWhat would you like to do? ");
                    int menuChoice = input.nextInt();
                    return menuChoice;
        }

        public static double getOperand(String prompt) {
            Scanner input = new Scanner(System. in );
            double operand;
            System.out.println(prompt);
            operand = input.nextDouble();
            return operand;
        }

        //Current Value
        //Gets
        public double getCurrentValue() {
            return currentValue;
        }
        //Setter
        public void setCurrentValue(double currentValue) {  
            this.currentValue = currentValue;  
        }
        //Add
        public void add(double operand2) {
            currentValue += operand2;
        }
        //Subtract
        public void subtract(double operand2) {
            currentValue -= operand2;
        }
        //Multiply
        public void multiply(double operand2) {
            currentValue *= operand2;
        }
        //Divide
        public void divide(double operand2) {
            if (operand2==0){
                            setCurrentValue(0);    
                    }
                    currentValue /=operand2;
        }
        //Clear
        public void clear() {
            currentValue = 0;
        }



        //Main part of the calculator
        public static void main(String[] args) {

            MemoryCalculator instance = new MemoryCalculator();
            double operand;
            boolean repeat = true;


            while (repeat) {
                System.out.println("The current value is: " + instance.getCurrentValue() + "\n");
                int menuChoice;
                menuChoice = displayMenu();

                if (menuChoice > 6 || menuChoice < 1){
                                    System.out.println("I'm sorry, " + menuChoice + " wasn't one of the options\n");
                }

            switch(menuChoice){
                case 1:
                    operand = getOperand("What is the second number?");
                    instance.add(operand);
                    break;
                case 2:
                    operand = getOperand("What is the second number?");
                    instance.subtract(operand);
                    break;
                case 3:
                    operand = getOperand("What is the second number?");
                    instance.multiply(operand);
                    break;
                case 4:
                    operand = getOperand("What is the second number?");
                    instance.divide(operand);
                    break;
                    case 5:
                    instance.clear();
                    break;
                    case 6:
                            System.out.println("Goodbye have a great day");
                            System.exit(0);
                            break;

            }
        }    
    }
}

Answer 1

What it looks like you did with your program was create one, single, class that holds all of the code for your calculator program, within which you instantiated an object of the same class.

What your teacher wants instead, is for you to have two separate classes, one which contains the code that makes the calculator work, and another class where you instantiate an object of the first class, and call the methods contained within that class.

For your assignment, what I would suggest would be to create a new class, perhaps called Main, where your program's Main() method will be, and keep all of the code for the calculator program in the MemoryCalculator class. From there, you can instantiate an object of MemoryCalculator class (which you already did, called instance) and use method calls to reference methods and attributes from within the MemoryCalculator class.

This may require reworking some of your code so that it runs properly, given that you'll be calling most of it from an object of the MemoryCalculator class, but it should be doable.