java - 扫描仪双值 - InputMismatchException

Question

我尝试以最简单的方式使用扫描仪:

代码:

double gas, efficiency, distance, cost;
Scanner scanner = new Scanner(System.in);

System.out.print("Enter the number of gallons of gas in the tank: ");
gas = scanner.nextDouble();
System.out.print("Enter the fuel efficiency: ");
efficiency = scanner.nextDouble();

但是在第一次输入 5.1 后它会抛出:

Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Scanner.java:909)
    at java.util.Scanner.next(Scanner.java:1530)
    at java.util.Scanner.nextDouble(Scanner.java:2456)
    at udacity.MileagePrinter.main(MileagePrinter.java:59)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

JavaDocs 状态:

Thrown by a Scanner to indicate that the token retrieved does not match the
pattern for the expected type, or that the token is out of range for the expected type.

但在我看来，一切看起来都是正确的，并且应该可以正常工作。

问题:

• 为什么会出现这种情况？
• 如何避免这个麻烦？

Answer 1

您应该为您的扫描仪精确设置区域设置。

Scanner scanner = new Scanner(System.in).useLocale(Locale.US);

来自doc :

An instance of this class is capable of scanning numbers in the standard formats as well as in the formats of the scanner's locale. A scanner's initial locale is the value returned by the Locale.getDefault() method; it may be changed via the useLocale(java.util.Locale) method

The localized formats are defined in terms of the following parameters, which for a particular locale are taken from that locale's DecimalFormat object, df, and its and DecimalFormatSymbols object, dfs.

因此，您的默认区域设置肯定使用 DecimalFormat，它期望逗号作为十进制分隔符而不是点。