从文件读取双精度时输入不匹配 [英] Input mismatch when reading double from file

问题描述

我必须编写一个程序，它将从文本文件balances.txt中读取名称和余额，并组织成一个报告，然后将余额汇总为总计。这是文件包含的内容：

I have to write a program that will read the names and balances from text file "balances.txt" and organize into a report that will then sum up the balances into a total. This is what the file contains:

JAKIE JOHNSON,2051.59
SAMUEL PAUL SMITH,10842.23
ELISE ELLISON,720.54

我原本写的代码给了我我想要的东西，但被告知不使用循环，数组或 parseDouble 。我现在尝试了以下操作，但每次使用 nextDouble 时，我都会收到错误。代码：

I had originally written the code which gave me exactly what I wanted, but was told not use loops, arrays, or parseDouble. I've now tried the following, but I keep getting an error every time I used nextDouble. The code:

import java.io.File;
import java.text.NumberFormat;
import java.text.DecimalFormat;
import java.io.FileInputStream ;
import java.io.FileNotFoundException ;
import java.io.IOException ;
import java.util.Scanner ;

public class BankFile {
    public static void main(String[] args) throws IOException {
        Scanner fileIn = null;

        try {
            String filename = "balances.txt" ; 
            File newFile = new File(filename);
            Scanner in = new Scanner(newFile);

            in.useDelimiter(",");

            String name = in.next();
            System.out.println(name);
            // trying to see if first name will display

            double money = in.nextDouble();
            System.out.println(money);
            // trying to see if first double will display
        } catch (FileNotFoundException e) {
            System.out.println("File not found.");
            System.exit(0);
        }
    }
}

这是输出和异常stacktrace：

This is the output and exception stacktrace:

JAKIE JOHNSON
Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Scanner.java:864)
    at java.util.Scanner.next(Scanner.java:1485)
    at java.util.Scanner.nextDouble(Scanner.java:2413)
    at Lab2.main(BankFile.java:52) `

推荐答案

如果你看看 Javadoc ：

useDelimiter

useDelimiter

public Sc​​anner useDelimiter（String pattern）

将此扫描仪的分隔模式设置为由指定字符串构造的模式。

Sets this scanner's delimiting pattern to a pattern constructed from the specified String.

现在，如果你看看在怎么样你做你的：

Now if you take a look at how you do yours:

in.useDelimiter(",");

这将使用逗号作为分隔符，现在让我们来看看你的文本文件：

This will use commas as the delimiter, now let's take a look at your text file:

JAKIE JOHNSON,2051.59
SAMUEL PAUL SMITH,10842.23
ELISE ELLISON,720.54

起初看起来似乎逗号很好，但是既然已经设置了分隔符，就会发生这种情况：

At first it may seem that commas are fine, but since you've set the delimiter, this is what happens:

首先你打电话给 in.next（）返回：

JAKIE JOHNSON,2051.59
^^^^^^^^^^^^^

没关系，但是当你打电话给 in.nextDouble（）时，会发生以下情况：

That's fine, but when you then call in.nextDouble(), the below happens:

JAKIE JOHNSON,2051.59
              ^^^^^^^
SAMUEL PAUL SMITH,10842.23
^^^^^^^^^^^^^^^^^

如您所见，下一行也会被选中双，这不是一个有效的双。这会导致Java报告 InputMismatchException ，因为预期的输入不是你得到的 - 一个字符串。要解决此问题，请使用正则表达式来分隔换行符：

As you can see, the next line is also selected along with the double, which isn't a valid double. This causes Java to report an InputMismatchException, as the expected input isn't what you get - a string. To combat this, use a regular expression to also delimit newlines:

in.useDelimiter(",|\n");

这将匹配新行和逗号，因此它将正确分隔。管道（ | ）表示它将界定 。这将正确输出：

This will match new-lines and commas, so it will delimit correctly. The pipe (|) means that it will delimit either. This will correctly output:

JAKIE JOHNSON
2051.59

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