如何使用 if 语句检查用户输入是否为特定变量类型？

Question

我正在创建一个数学程序，用户可以在其中输入答案。我想显示一条消息 "All solutions must be entered as decimal numbers"。我将如何确保用户输入双精度，如果不显示该消息。到目前为止我已经试过了：

if(userLetter.equalsIgnoreCase("a")) {
    System.out.print("What is the solution to the problem:" + " " + ran1Shift + " " + "+" + " " + ran2Shift + " = ");
    double userNum = input.nextDouble();
        if(userNum == additionAnswer){
            System.out.println("That's correct!");
        }
        else{
            System.out.println("The correct solution is: " + additionAnswer);
        }
    }

所以基本上我现在将其设置为显示答案是对还是错，但是如果用户输入非双精度，我怎么能制作另一个部分呢？感谢您的帮助:)

Answer 1

假设input是一个Scanner，那么你可以调用Scanner.hasNextDouble() which returns true if the next token in this scanner's input can be interpreted as a double value using the nextDouble()方法。像，

if (input.hasNextDouble()) {
    double userNum = input.nextDouble();
    if (userNum == additionAnswer) {
        System.out.println("That's correct!");
    } else {
        System.out.println("The correct solution is: " + additionAnswer);
    }
} else {
    System.out.println("All solutions must be entered as decimal numbers");
}

Answer 2

nextDouble 保证它只会扫描 看起来 像来自标准输入的双精度标记。既然你想捕获无效输入，你将不得不做更多的工作。

下面是可重入的；它将允许用户继续添加输入，直到引入有效输入。我将用户的提示留作 reader.

的练习

boolean valid = false;
do {
    try {
        double userNum = Double.parseDouble(input.nextLine());
        // other code to use `userNum` here
        valid = true;
    } catch(NumberFormatException e) {
        System.out.println("Not a valid double - please try again.");
    }
} while(!valid);

Answer 3

您可以使用循环让用户重新输入数字，以防他输入错误。

System.out.println("Enter a double number:");
while (!input.hasNextDouble())
{
    System.out.println("Invalid input\n Enter a double number:");
    input.next();
}
double userInput = input.nextDouble();    
System.out.println("You entered: " + userInput);

如何使用 if 语句检查用户输入是否为特定变量类型？

How to use an if statement to check if user input is a certain variable type?

java

double

if-statement

user-input