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();
}
}
```
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(); } }
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