Chapter 11 Exercise 16, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

11.16 (Addition quiz)
Rewrite Listing 5.1 RepeatAdditionQuiz.java to alert the user if an answer is
entered again. Hint: use an array list to store answers.

import java.util.ArrayList;
import java.util.Scanner;

public class Exercise_16 {

    public static void main(String[] args) {
        int number1 = (int)(Math.random() * 10);
        int number2 = (int)(Math.random() * 10);

        // Create a Scanner
        Scanner input = new Scanner(System.in);
        ArrayList<Integer> repeatedAnswers = new ArrayList<>();

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

        while (number1 + number2 != answer) {
            if (repeatedAnswers.contains(answer)) {
                System.out.println("You already answered " + answer);
            }
            System.out.print("Wrong answer. Try again. What is " + number1 + " + " + number2 + "? ");
            repeatedAnswers.add(answer);
            answer = input.nextInt();
        }

        System.out.println("You got it!");
    }
}