oop - I wrote a java program but it seems to be an infinite loop

Question

import java.util.Scanner;

public class FirstJava {
    public static void main(String args[]) {
        final int NUMBER_OF_QUESTIONS = 5;
        int correctCount = 0;
        int count = 0;

        Scanner input = new Scanner(System.in);

        while (count < NUMBER_OF_QUESTIONS) {
            int number1 = (int) (Math.random() * 10);
            int number2 = (int) (Math.random() * 10);

            if (number1 < number2) {
                int temp = number1;
                number1 = number2;
                number2 = temp;
            }

            System.out.print("What is " + number1 + " - " + number2 + "? ");
            int answer = input.nextInt();

            while (number1 - number2 != answer) {
                System.out.println("Wrong, try again.");
                System.out.println("What is " + number1 + " - " + number2 + "? ");
            }

            System.out.println("Correct!");
            count++;
            correctCount++;

        }
        System.out.println("You got " + correctCount + " correct!");
    }
}

I think there is something wrong with the second while loop but I can't figure out what. The goal is to ask subtraction question until the user gets it right. Can someone help me with this?

Answer 1

The problem is that the inner loop never returns once entered since the condition never changes. You can ask for the user input directly inside the condition:

System.out.print("What is " + number1 + " - " + number2 + "? ");

while (input.nextInt() != number1 - number2) {
    System.out.println("Wrong, try again.");
    System.out.print("What is " + number1 + " - " + number2 + "? ");
}

---

A few side-notes for the rest of the code:

• Instead of (int) (Math.random() * 10), I suggest to use Random#nextInt().
• The outer while-loop can be replaced by an easier to read for-loop.
• Use camelCase for the variable NUMBER_OF_QUESTIONS or move it to the class as a static member.
• Use Java notation (String[] args) for the argument array of the main method instead of C-style (String args[]).
• By the way... correctCount will always be the same and might be removed if not used.

Here is the adapted class with above suggestions:

import java.util.Random;
import java.util.Scanner;

public class SubtractionQuiz {

    private static final int NUMBER_OF_QUESTIONS = 5;

    public static void main(String[] args) {
        final Scanner input = new Scanner(System.in);
        final Random random = new Random();

        int correctCount = 0;
        for (int count = 0; count < NUMBER_OF_QUESTIONS; count++) {
            int number1 = random.nextInt(10);
            int number2 = random.nextInt(10);

            // order numbers
            if (number1 < number2) {
                int temp = number1;
                number1 = number2;
                number2 = temp;
            }

            System.out.print("What is " + number1 + " - " + number2 + "? ");
            while (input.nextInt() != number1 - number2) {
                System.out.println("Wrong, try again.");
                System.out.print("What is " + number1 + " - " + number2 + "? ");
            }
            System.out.println("Correct!");
            correctCount++;
        }

        System.out.println("You got " + correctCount + " correct!");
    }
}