带有 try 和 catch 的 Do-While 循环退出循环而不要求新输入

Question

我必须创建一个 do-while 循环来验证任何非双精度且非零的用户输入。我一直在这里环顾四周并谷歌搜索，我正在创建一个无限循环，但我通过清除输入来解决这个问题。但是，现在程序不再要求用户输入，并且循环正在中断。不太确定我打破了什么。这只是代码的一部分，我还必须对 wallWidth 执行此 do-while 循环。

public class Paint1 {

    public static void main(String[] args) {  
        Scanner scnr = new Scanner(System.in);  
        double wallHeight = 0.0;  
        double wallWidth = 0.0;  
        double wallArea = 0.0;  
        double gallonsPaintNeeded = 0.0;  
        boolean decimal = false;  

        
        final double squareFeetPerGallons = 350.0;  
        
        // Implement a do-while loop to ensure input is valid
        // Prompt user to input wall's height
        do {  
            try {  
                System.out.println("Enter wall height (feet): ");  
                wallHeight = scnr.nextDouble();  
                
                if (wallHeight <= 0) { //non zero numbers  
                    throw new Exception("Invalid Input");  
                }  
                decimal = true;  
            }  
            catch (Exception e) {  
                System.out.println("Invalid Input");  
                scnr.nextDouble(); //wipe out user input  
            }  

        } while (!decimal);  

循环停止并在清除无效输入后仍然创建异常而不继续。

Enter wall height (feet): 
thirty
Invalid Input
Exception in thread "main" java.util.InputMismatchException
 at java.base/java.util.Scanner.throwFor(Scanner.java:939)
 at java.base/java.util.Scanner.next(Scanner.java:1594)
 at java.base/java.util.Scanner.nextDouble(Scanner.java:2564)
 at Paint1.main(Paint1.java:31)

Answer 1

发生的情况是，当您尝试使用

scnr.nextDouble()

擦除用户输入时，它会抛出 InputMistMatchException，但不会处理该异常。这是因为提供的输入与预期类型不匹配。要在您的环境中克服这个问题，您可以使用

scnr.nextLine()

。因为无论你输入什么，它都可以被解释为字符串。

Answer 2

在尝试使用 double 读取

Scanner.hasNextDouble()

之前，您应该

 检查是否有可用的 

double。同样，在读取（并丢弃）任何非

Scanner

标记之前，您应该检查是否存在带有

Scanner.hasNextLine()

的输入行。就像，double

Answer 3

do {
    try {
        System.out.println("Enter wall height (feet): ");
        // Is there a double available?
        if (scnr.hasNextDouble()) {
            wallHeight = scnr.nextDouble();
            if (wallHeight <= 0) {
                throw new Exception("Invalid Input");
            }
            decimal = true;
        } else if (scnr.hasNextLine()) {
            // Consume the next non-double token
            scnr.nextLine();
        } else {
            // There's no more input, end the loop
            break;
        }
    } catch (Exception e) {
        System.out.println("Invalid Input");
    }
} while (!decimal);

控制台输出：

Scanner userInput = new Scanner(System.in);
String ls = System.lineSeparator();

double feet;
String inFeet = "";
while (inFeet.isEmpty()) {
    System.out.print("Enter wall height (in feet): ");
    inFeet = userInput.nextLine().trim();
    /* Is the User entry a string representation of a unsigned
       integer or floating point value and is that value greater
       than 0?              */
    if (!inFeet.matches("\\d+(\\.\\d+)?") || Double.parseDouble(inFeet) <= 0) {
         System.out.println("Entry (" + inFeet + "): -> Error - Invalid Wall "
                          + "Height Entry! Try again..." + ls);
         inFeet = "";  // Empty `inFeet` to ensure re-loop:
    }
}
// If code makes it to here then the entry is valid!

feet = Double.parseDouble(inFeet);
System.out.println("User Entry: -> " + feet + " feet.");

如果需要，可将 

Enter wall height (in feet): 0
Entry (0): -> Error - Invalid Wall Height Entry! Try again...

Enter wall height (in feet): 14.25
User Entry: -> 14.25 feet.

循环转换为

while

。