When I enter A, B, C, or D, it still says That is not a valid answer. Why?

This is my project for school. We were asked to make a little quiz and I want to make sure when the user enters something outside of the options, they will get an error message saying that is not a valid answer, but whatever I enter it still says that is not valid. Why?

import java.util.Scanner;
class Main {
  public static void main(String[] args) {
  String answer;
  int score = 0;
  Scanner input = new Scanner(System.in);
  System.out.println("ALL SUBJECT QUIZ Starting now, please enter the correct option (Letters or T or F) for the next 10 questions");
      System.out.println("Question 1 (Maths): What is the vertex of the equation y=2(x-2)^2+5");
      System.out.println("A. (-2,5)" + "\n" + "B. (2, 5)" + "\n" + "C. (-2,-5)" + "\n" + "D. (2, -5)");
      answer = input.nextLine();
      answer = answer.toUpperCase();
      
      while (!answer.equals("A") || !answer.equals("B") || !answer.equals("C") || !answer.equals("D")) {
        System.out.println("That is not a valid response");
        answer = input.nextLine();
        answer = answer.toUpperCase();
      }
        if (answer.equals("A")) {
          System.out.println("Incorrect, the answer is B");
        } else if (answer.equals("B")){
          System.out.println("Correct");
          score += 1;
        } else if (answer.equals("C")){
          System.out.println("Incorrect, the answer is B");
        } else if (answer.equals("D")){
          System.out.println("Incorrect, the answer is B");
        }

  }
}

Any help would be appreciated!

Let's evaluate your following condition:

!answer.equals("A") || !answer.equals("B") || !answer.equals("C") || !answer.equals("D")

Assume, answer = E

Now, your condition will be evaluated as

true || !answer.equals("B") || !answer.equals("C") || !answer.equals("D") => true

Note that when multiple conditions are joined with ||, the evaluation stops as soon as a true condition is found.

Assume, answer = B

Now, your condition will be evaluated as

true || !answer.equals("B") || !answer.equals("C") || !answer.equals("D") => true

Assume, answer = A

Now, your condition will be evaluated as

false || true || !answer.equals("C") || !answer.equals("D") => true

Thus, you see that irrespective of the answer, the condition always evaluates to true.

Solution:

Change it to

while (!(answer.equals("A") || answer.equals("B") || answer.equals("C") || answer.equals("D")))

which means the answer is not (A, or B, or C, or D)

Alternatively, you can change it to

while (!answer.equals("A") && !answer.equals("B") && !answer.equals("C") && !answer.equals("D"))

which means the answer is not A and the answer is not B and the answer is not C and the answer is not D.

However, for the sake of simplicity, my taste is the first one.

If you look at your while loop condition, you will notice that it will always evaluate to true because the user input can't equal A, B, C, and D all at once.

So the solution here is to change the or's (||) in the condition to and's (&&) since you want the user input to be only one of the following: A, B, C, or D.

while (!answer.equals("A") && !answer.equals("B") && !answer.equals("C") && !answer.equals("D")) {
    System.out.println("That is not a valid response");
    answer = input.nextLine();
    answer = answer.toUpperCase();
}