java do while循环在满足条件后不断循环

Question

我是一名新的 Java 程序员，我正在编写一个程序，为 3 台打印机设置 3 个型号。如果用户输入错误的值，我希望它继续向用户询问型号。我让它工作，但前提是用户的第一个值是 3 台打印机之一的编号。如果第一个值不是可能的值之一，而第二个输入是它仍然不断重复循环。

package printing;

import java.util.Scanner;

public class newClass {

    public static void main(String[] args) {

        int count = 0;

        String machine1 = "546";
        String machine2 = "892";
        String machine3 = "127";


        Scanner s = new Scanner(System.in);

        System.out.print("Model Number:");
        String modelNumber = s.nextLine();
        // increment count if first input value is wrong
        if (!s.equals(machine1) || !s.equals(machine2) || !s.equals(machine3))
            count++;

        // if user inputs right value
        while (true) {
            if (modelNumber.equals(machine1)) {
                System.out.println("Machine 1 is online");
                break;
            }
            if (modelNumber.equals(machine2)) {
                System.out.println("Machine 2 is online");  
                break;
            }
            if (modelNumber.equals(machine3)) {
                System.out.println("Machine 3 is online");
                break;
            }

            // keep looping if user does not input values for machine1, machine2 or machine3
            do {
                System.out.println("Try again");
                System.out.print("Model Number:");
                String modelNumberFalse = s.nextLine();
                /* each time user gets value wrong the count variable goes up by 1 and
                   the loop breaks when count reaches 3 */
                count++;
                if (count == 3)
                    break;
            } while (!s.equals(machine1) || (!s.equals(machine2)) || (!s.equals(machine3)) && (count < 2));

        }
    }
}

此外，每次用户输入错误的值时，我都希望计数变量递增，直到达到 3 并且 do while 循环中断，但在我输入错误值超过 3 次后，它一直在询问型号。

Answer 1

有几个问题。这一行是错误的：

while(!s.equals(machine1) || (!s.equals(machine2)) || (!s.equals(machine3)) && (count < 2));

s 是 Scanner，而不是 String，这不是有效的比较。用modelNumber 代替s 得到：

while(!modelNumber.equals(machine1) || (!modelNumber.equals(machine2)) || (!modelNumber.equals(machine3)) && (count < 2));

这不能为假，除非 modelNumber、machine1、machine2 和 machine3 都是相同的值。

另外，测试计数会搞砸这一切，而且是多余的，因为您正在测试它并在循环中中断。

应该是

while(!modelNumber.equals(machine1) 
    && (!modelNumber.equals(machine2)) 
    && (!modelNumber.equals(machine3)));

见DeMorgan's Laws。应用这条规则给出了

while(!(modelNumber.equals(machine1)
    || modelNumber.equals(machine2)
    || modelNumber.equals(machine3)))

这可能更容易阅读。

另外，如果你用“return”代替“break;”随着对 do-while 条件的更改，它可以工作。所以还有其他事情发生。在内部 do-while 中调用 break 会导致控制返回到外部 while 循环的顶部。添加一个在中断之前设置并在外部 while 循环中测试的布尔标志将是解决此问题的一种方法。或者只使用 return。

Answer 2

import java.util.Scanner;

public class newClass
{

    public static void main(String[] args)
    {
        int count = 0; 
        String machine1 = "546";
        String machine2 = "892";
        String machine3 = "127";

        Scanner s = new Scanner(System.in);

        while (true)
        {
            System.out.print("Model Number:");
            String modelNumber = s.nextLine();
            // increment count if first input value is wrong
            if ((!modelNumber.equals(machine1)) || (!modelNumber.equals(machine2)) || (!modelNumber.equals(machine3)))
                count++;

            if (count == 3)
            {
                System.out.println("You have utilized your maximum number of try's");
                break;
            }

            if (modelNumber.equals(machine1))
            {
                System.out.println("Machine 1 is online");
                break;
            }
            if (modelNumber.equals(machine2))
            {
                System.out.println("Machine 2 is online");
                break;
            }
            if (modelNumber.equals(machine3))
            {
                System.out.println("Machine 3 is online");
                break;
            }

            System.out.println("Try again");

        }
    }
}

希望这能解决你的问题

Answer 3

• 首先：在需要 NOT AND 时使用 NOT OR
• 第二：你在重复 没有正当理由的测试

在您的代码中，您最终会重复相同的测试。这意味着，当您添加机器时，您将不得不在多个地方更新您的代码。

软件的第一条规则是不要重复自己。当下一个人被要求对条件进行判断时，他/她会找到第一个代码块并对其进行编辑，并且可能永远不会注意到重复的代码块。复制粘贴的代码是根源或许多未来的错误。

您可以像这样简化代码，使每次检查只进行一次：

import java.util.Scanner;

public class newClass {

    public static void main(String[] args) {

        int count = 0;

        // for extra credit, try to make this an ArrayList
        // so you can keep adding models as needed
        // then you would adjust your tests to leverage the ArrayList
        // search functions
        String machine1 = "546";
        String machine2 = "892";
        String machine3 = "127";


        Scanner s = new Scanner(System.in);

        // when using a while loop, it is good practice to use a boolean
        // as your logic expands, multiple tests in the loop may set the
        // boolean to true or false
        // it is cumbersom to have large blocks of code in your while check
        boolean keepOnTrucking = true;

        System.out.print("Enter Model Number:");
        while (keepOnTrucking) {

            String modelNumber = s.nextLine();

            // when using multiple tests, it is good
            // to give each test its own line and end the line
            // with the logical operator that joins it to the next
            // it makes it easier to read
            // and easier to edit (add or remove tests)

            // Logical operator note:
            // Your test was: not A OR not B OR not C
            // This would NEVER work, as A != B != C
            // If a user entered machine2, the check would
            // fail for !(machine1), OR !(machine2) OR !(machine3)
            // because while (!s.equals(machine2)) would say false
            // (!s.equals(machine1)) would say true, and the OR
            // chain would stop and count it as an error.
            // Instead you want:
            // !(machine1) && !(machine2) && !(machine3)
            // Thus to to error, it has to not any of the machines.
            // If it is true for all three nots, then you have an error
            if (!machine1.equals(modelNumber) && 
                !machine2.equals(modelNumber) &&
                !machine3.equals(modelNumber)) {

                // you only increment on failure
                count++;

                // nice developers give meaningful feed back to users
                if (count>=3) {
                    System.out.print("Out of guesses! Go Away!"); // even when it is mean

                    // since you are nested in one while loop,
                    // this will break you out
                    break;
                } else {
                    System.out.print("Not a valid model number, please re-enter:");
                }
            } else {

                // the found a machine, so exit the while loop
                keepOnTrucking = false;

                if (machine1.equals(modelNumber)) {
                    System.out.println("Machine 1 is online");
                } else if (machine1.equals(modelNumber)) {
                    System.out.println("Machine 2 is online");
                } else { // since this ins the only one left, you don't need an if clause
                    System.out.println("Machine 3 is online");
                }
            }

        }
    }
}