如何检查用户输入的数据类型有效性（Java Scanner类） [英] how to check the data type validity of user's input (Java Scanner class)

问题描述

我正在尝试制作一个简单的用户界面，要求用户输入双重类型的数字，如果他们的输入不是双重类型，程序应该继续打印提示，直到用户输入有效的双重类型。我的下面的代码还没有完成，因为当一个用户键入一个有效的double类型时，除非用户键入另一个double类型的数字，否则程序不会执行任何操作。我猜在while循环中的条件（sc.hasNextDouble（））消耗第一个有效的输入。如何纠正？感谢很多

I am trying to make a simple UI that asks users to input double-type numbers, if theirs input is not of double type, the program should keep printing the prompt until user inputs a valid double type. My code below is not quite working yet, because when a user types in a valid double type, the program does not do anything unless the user types another double type number. I guess the condition (sc.hasNextDouble()) in the while loop consumes the first valid input. How to correct this? thanks a lot

Scanner sc = new Scanner(System.in);

System.out.println("Type a double-type number:");
while (!sc.hasNextDouble())
{
    System.out.println("Invalid input\n Type the double-type number:");
    sc.next();
}
userInput = sc.nextDouble();    // need to check the data type?

推荐答案

由于您可能不获得双输入，最好读取一个字符串，然后尝试将其转换为双倍。标准模式是：

Since you may not get a double entered, best to read in a String, then attempt to convert it to a double. The standard pattern is:

Scanner sc = new Scanner(System.in);
double userInput = 0;
while (true) {
    System.out.println("Type a double-type number:");
    try {
        userInput = Double.parseDouble(sc.next());
        break; // will only get to here if input was a double
    } catch (NumberFormatException ignore) {
        System.out.println("Invalid input");
    }
}

循环不能退出，直到双已经输入，之后 userInput 将保存该值。

The loop can't exit until a double has been entered, after which userInput will hold that value.

还要注意如何将提示放在循环中，你可以避免无效输入的代码重复。

Note also how by putting the prompt inside the loop, you can avoid code duplication on invalid input.

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