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 thestandard formats as well as in the formats of the scanner's locale. Ascanner's initial locale is the value returned by theLocale.getDefault() method; it may be changed via theuseLocale(java.util.Locale) method

The localized formats are defined in terms of the followingparameters, which for a particular locale are taken from that locale'sDecimalFormat 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。