Question

Creating a calculator for my CS1 class and my code was able to run but it crashes near the end. As far as I know, the error means that it's retrieving "null" from the variable but I haven't seen that kind of error from this kind of situation. I would appreciate any help. I'm just starting up in Java so I apologize if this is a silly question. Thank you!


```
----jGRASP exec: java CS1Calculator

------------

Welcome to the CS1 Calculator!
This calculator can help you with addition, subtraction, multiplication, or division.

------------

Select the problem type you would like to calculate by entering the first letter of the operation.
(A)ddition
(S)ubtraction
(M)ultiplication
(D)ivision
Exception in thread "main" java.lang.NullPointerException
   at CS1Calculator.calculate(CS1Calculator.java:42)
   at CS1Calculator.main(CS1Calculator.java:60)

----jGRASP wedge2: exit code for process is 1.
----jGRASP: operation complete.
```

```
/*
Program: CS1Calculator
Date: 09/10/2019

Description: Simple interactive calculator that's able to calculate integers.
*/

import java.util.Scanner;

public class CS1Calculator {
private int addition, subtraction, multiplication, division;
private static Scanner keyboard;
private String userOperation;

public CS1Calculator () {
// Constructor
Scanner keyboard = new Scanner(System.in);   
int addition = 0;
int subtraction = 0;
int multiplication = 0;
int division = 0;
}

public void printIntro() {
// Ouputs introduction for program
System.out.println("\n------------\n");
System.out.println("Welcome to the CS1 Calculator!");
System.out.println("This calculator can help you with addition, subtraction, multiplication, or division.");
System.out.println("\n------------\n");
}

public void calculate () {
// Asks for preferred operation, calculates answer, and tallies up problems done
System.out.println("Select the problem type you would like to calculate by entering the first letter of the operation.");
System.out.println("(A)ddition");
System.out.println("(S)ubtraction");
System.out.println("(M)ultiplication");
System.out.println("(D)ivision");
String userOperation = keyboard.nextLine();

if(userOperation == "A") {
System.out.println("You have selected addition");
}
else if(userOperation == "S") {
System.out.println("You have selected subtraction.");
}
  
}

public void printRecord () {
// Outputs total of each problem done and overall total problems
}

public static void main(String[] args) {
CS1Calculator cs1c = new CS1Calculator();
cs1c.printIntro();
cs1c.calculate();
cs1c.printRecord();

}

}
```

Answer 1

import java.util.Scanner;

public class CS1Calculator {
    private int addition, subtraction, multiplication, division;
    private static Scanner keyboard;
    private String userOperation;

    public CS1Calculator () {
// Constructor
        keyboard = new Scanner(System.in);
        int addition = 0;
        int subtraction = 0;
        int multiplication = 0;
        int division = 0;
    }

    public void printIntro() {
// Ouputs introduction for program
        System.out.println("\n------------\n");
        System.out.println("Welcome to the CS1 Calculator!");
        System.out.println("This calculator can help you with addition, subtraction, multiplication, or division.");
        System.out.println("\n------------\n");
    }

    public void calculate () {
// Asks for preferred operation, calculates answer, and tallies up problems done
        System.out.println("Select the problem type you would like to calculate by entering the first letter of the operation.");
        System.out.println("(A)ddition");
        System.out.println("(S)ubtraction");
        System.out.println("(M)ultiplication");
        System.out.println("(D)ivision");
        String userOperation = keyboard.nextLine();

        if(userOperation.equals("A")) {
            System.out.println("You have selected addition");
        }
        else if(userOperation.equals("S")) {
            System.out.println("You have selected subtraction.");
        }

    }

    public void printRecord () {
// Outputs total of each problem done and overall total problems
    }

    public static void main(String[] args) {
        CS1Calculator cs1c = new CS1Calculator();
        cs1c.printIntro();
        cs1c.calculate();
        cs1c.printRecord();

    }

}