同时循环测试条件

Question

I'm trying to execute while loop until the user enter Yes or No.

Why can't there be an OR in the testing function instead of AND?

While using OR, it can be either yes or no .

System.out.print("Enter yes or no: ");
String answer = input.getNext();
while (!answer.equals("yes") && !answer.equals("no")) {
    System.out.println("Enter ONLY yes or no, please: ");
    answer = input.getNext();
}
System.out.println("Thank you!");

Answer 1

Because !answer.equals("yes") && !answer.equals("no") basically says while answer is not "yes" and answer is not "no".

In order to make it an OR you can do it like:

while(!(answer.equals("yes") || answer.equals("no")))
{
}

Which means, while the following condition is not true; condition: answer is "yes" or answer is "no"

Answer 2

while (!answer.equals("yes") && !answer.equals("no")) means that the loop terminates when the condition becomes false. According to de Morgan laws, !(!answer.equals("yes") && !answer.equals("no")) is answer.equals("yes") || answer.equals("no") answer.equals("yes") || answer.equals("no") . That is, the loop terminates when the answer is either "yes" or "no" - that's exactly what you want.

If you change && to || , the loop would terminate when the answer is both "yes" and "no" at the same time, which is impossible.

Answer 3

If you use the short circuit or/and (|| and &&) the system doesn't evaluate the entire expression if it doesn't have to. So if the user enters "no", the first part that checks if it is not "yes" evaluates to true. Since an or condition only needs one true to evaluate to true it doesn't evaluate anything else and just returns.

You need && because you are saying if the answer is not yes and is not no ask for input again. Here is more on short circuit boolean operators: https://users.drew.edu/bburd/JavaForDummies4/ShortCircuitEval.pdf .