Java Do While Statement有两个条件

Question

I'm trying to learn java but I'm stuck trying to do a single program which concerns Do While Statement with two conditions. Specifically, I want a method to run until the user write "yes" or "no". Well, down there is my thing, what is wrong with it?

    String answerString;
    Scanner user_input = new Scanner(System.in);
    System.out.println("Do you want a cookie? ");

    do{
    answerString = user_input.next();
    if(answerString.equalsIgnoreCase("yes")){
        System.out.println("You want a cookie.");
    }else if(answerString.equalsIgnoreCase("no")){
        System.out.println("You don't want a cookie.");
    }else{
        System.out.println("Answer by saying 'yes' or 'no'");
    }while(user_input == 'yes' || user_input == 'no');
    }
}}

Answer 1

I'd do something similar to Tim's answer. But to do things the way you were trying to do them, you have a lot of problems that need to be fixed:

(1) String literals in Java are surrounded by double quote marks, not single quote marks.

(2) user_input is a Scanner . You can't compare a scanner to a string. You can only compare a String to another String . So you should be using answerString in your comparison, not user_input .

(3) Never use == to compare strings. StackOverflow has 953,235 Java questions, and approximately 826,102 of those involve someone trying to use == to compare strings. (OK, that's a slight exaggeration.) Use the equals method: string1.equals(string2) .

(4) When you write a do-while loop, the syntax is do , followed by { , followed by the code in the loop, followed by } , followed by while(condition); . It looks like you put the last } in the wrong place. The } just before the while belongs to the else , so that doesn't count; you need another } before while , not after it.

(5) I think you were trying to write a loop that keeps going if the input isn't yes or no . Instead, you did the opposite: you wrote a loop that keeps going as long as the input is yes or no . Your while condition should look something like

while (!(answerString.equals("yes") || answerString.equals("no")));

[Actually, it should be equalsIgnoreCase to be consistent with the rest of the code.] ! means "not" here, and note that I had to put the whole expression in parentheses after the ! , otherwise the ! would have applied only to the first part of the expression. If you're trying to write a loop that does "Loop until blah-blah-blah", you have to write it as "Loop while ! (blah-blah-blah)".

Answer 2

I might opt for a do loop which will continue to take in command line user input until he enters a "yes" or "no" answer, at which point the loop breaks.

do {
    answerString = user_input.next();

    if ("yes".equalsIgnoreCase(answerString)) {
        System.out.println("You want a cookie.");
        break;
    } else if ("no".equalsIgnoreCase(answerString)) {
        System.out.println("You don't want a cookie.");
        break;
    } else {
        System.out.println("Answer by saying 'yes' or 'no'");
    }
} while(true);