java - 如何修复我的 Java 代码以验证变量？

Question

我目前正在计算机科学课上做一个项目，我们应该验证变量的每个字符，看看它是否合法。如果以数字开头，则是非法的。如果它以特殊字符开头，那么它是合法的，但风格很糟糕。如果它有空格，它又是非法的。我现在发布我当前的代码:

import java.util.Scanner;

public class classOfValidation {

    public static void main(String[] args) {    
        Scanner scan = new Scanner(System.in);    
        String theVariable = null;

        System.out.println("This program checks the validity of variables");    
        System.out.println("Please enter a variable (or press 'q' to quit");

        theVariable = scan.nextLine();

        do {        
            System.out.println("The variable is illegal");    
            theVariable = scan.nextLine();    
        } while (theVariable.startsWith("[0123456789]"));

        do {    
            System.out.println("The variable is illegal");    
            theVariable = scan.nextLine();    
        } while (theVariable.contains("[ ]"));

        do {    
            System.out.println("The variable is legal, but has bad style");    
            theVariable = scan.nextLine();    
        } while (theVariable.startsWith("[!@#$%^&*]"));         
    }    
}

如果你还看不出我是编程新手，而且我可能很困惑。如果您有任何建议或其他需要我解释的内容，请发表评论。谢谢大家

Answer 1

您可以使用单个 regex 通过 String#matches() 方法验证您的输入。但对于您提供的示例，您应该使用 while 循环，而不是 do-while ，因为在 do while 情况下，您总是在检查条件之前运行它的主体一次。所以，你最好这样做:

theVariable = scan.nextLine();

while (theVariable.startsWith("[0123456789]")) {
    System.out.println("The variable is illegal");
    theVariable = scan.nextLine();
}

while (theVariable.contains("[ ]")) {
    System.out.println("The variable is illegal");
    theVariable = scan.nextLine();
}

while (theVariable.startsWith("[!@#$%^&*]")) {
    System.out.println("The variable is legal, but has bad style");
    theVariable = scan.nextLine();
}

第二个，在您的解决方案中，您使用 String.startsWith() 方法并向其传递一些正则表达式。查看 javadoc 来了解此方法。那里说:

Tests if this string starts with the specified prefix.

这意味着，此方法不支持正则表达式，而只是检查字符串是否以传递的字符串开头。所以，你的条件似乎永远不会成为现实。我不认为有人会输入 [0123456789] 或 [!@#$%^&*] 。

另外，任何条件都会检查一次，但之后用户可以修改输入，并且预览通过的条件将不会再次检查。看来，在某些情况下最好使用 continue 和 break 进入无限循环，例如:

 //infinit loop, until user enter the `q` or the input is correct
 while (true) {

    //read the input
    theVariable = scan.nextLine();

    //chtck, whether is `quit` command entered
    if ("q".equals(theVariable)) {
        break;
    }

    //if string starts with digit or contains some whitespaces
    //then print alert and let the user to 
    //modify the input in a new iteration
    if (theVariable.matches("^\d+.*|.*\s+.*")) {
        System.out.println("The variable is illegal");
        continue;
    }

    //if string contains some special characters print alert 
    //and let the user to modify the input in a new iteration
    if (theVariable.matches("^[!@#$%^&*].*")) {
        System.out.println("The variable is legal, but has bad style");
        continue;
    }

    //if all the conditions checked, then break the loop
    break;
}