java - 谓词方法在是/否/否则回答不回答的情况下返回重复提示

Question

我目前正在关注 Erik Roberts 所著的《Java 的艺术与科学》一书。当在代码中测试是/否/替代答案时，我会因为没有答案而被问两次。

/*
 * File: YesNoQuestion.java
 * ------------------------
 * This program asks the user a question and expects a yes / no
 * answer. It is also exercise 7 in Chapter 5.
 * "Write a predicate method askYesNoQuestion(prompt) that prints 
 * the string prompt as a question for the user and then waits 
 * for a response. If the user enters the string "yes", the 
 * askYesNoQuestion method should return true; if the user enters 
 * "no", the method should return false. If the user enters 
 * anything else, askYesNoQuestion should remind the user that it 
 * is seeking a yes-or-no answer and then repeat the question."
 */

import acm.program.*;

public class YesNoQuestion extends ConsoleProgram {

    public void run () {
        String prompt = "Are you over 18 year old?";
        if (askYesNoQuestion(prompt) == true) {
            println("Evaluated true.");
        }
        else if (askYesNoQuestion(prompt) == false) {
            println ("Evaluated false");
        }
    }
/* Predicate method returns true if user enters the string "yes"
 * and false if the user enters "no", else informs user that it is 
 * expecting a yes or no answer.    
 */
    private boolean askYesNoQuestion(String prompt) {
        String userInput = readLine(prompt);
        boolean trueOrFalse = true;
        while(true) {
            if (userInput.equalsIgnoreCase("yes")){
                trueOrFalse = true;
                break;
            }
            else if (userInput.equalsIgnoreCase("no")) {
                trueOrFalse = false;
                break;
            }
            else {
                println("Please answer yes or no");
                userInput = readLine(prompt);
            }
        }
        return trueOrFalse;
    }
}

这是输出的屏幕截图。谁能帮我理解为什么？ Screenshoted output

Answer 1

在此代码中:

if (askYesNoQuestion(prompt) == true) {
    println("Evaluated true.");
}
else if (askYesNoQuestion(prompt) == false) {
    println ("Evaluated false");
}

我们可以看到您调用了 askYesNoQuestion(prompt) 两次。每次您调用它时，它都会提出问题并从用户那里得到另一个响应。

只需调用一次:

if (askYesNoQuestion(prompt) == true) {
    println("Evaluated true.");
} else {
    println("Evaluated false.");
}

如果 if 条件不匹配，则该方法一定返回 false，因此无需再次调用该方法并获取另一个结果。

此外，在语句中测试 boolean 值的正常方法如下:

if (askYesNoQuestion(prompt)) {
    ...

==true 部分是多余的。