如何从文本文件的下一行读取并暂停，以便稍后从该行读取？

Question

我编写了一个程序，根据这两个常量文件将随机数生成到两个文本文件中，并将随机字母生成到第三个文件中。现在我需要逐行读取每个文本文件，然后将它们放在一起。该程序是找到的建议 here 并不能真正帮助我的情况。当我尝试这种方法时，它只读取所有行直到完成，不允许我选择暂停它、转到另一个文件等。

理想情况下，我想找到一些方法来阅读下一行，然后再转到下一行。就像某种变量可以让我在阅读中占据一席之地。

public static void mergeProductCodesToFile(String prefixFile,
                                           String inlineFile,
                                           String suffixFile,
                                           String productFile) throws IOException 
{
    try (BufferedReader br = new BufferedReader(new FileReader(prefixFile))) 
    {
        String line;
        while ((line = br.readLine()) != null) 
            {
                try (PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter(productFile, true))))
                {
                        out.print(line); //This will print the next digit to the right
                }
            catch (FileNotFoundException e) 
                {
                    System.err.println("File error: " + e.getMessage());
                }  
            }
    }
}

EDIT: 正在根据以下创建的数字。基本上，常量告诉它在每一行中创建多少个数字以及要创建多少行。现在我需要将它们组合在一起而不从任何一个文本文件中删除任何内容。

public static void writeRandomCodesToFile(String codeFile, 
                                          char fromChar, char toChar,
                                          int numberOfCharactersPerCode,
                                          int numberOfCodesToGenerate) throws IOException 
{

    for (int i = 1; i <= PRODUCT_COUNT; i++)
    {
        int I = 0;


        if (codeFile == "inline.txt")
        {

            for (I = 1; I <= CHARACTERS_PER_CODE; I++)
            {
                int digit = (int)(Math.random() * 10);


                try (PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter(codeFile, true))))
                {
                        out.print(digit); //This will print the next digit to the right
                }
                catch (FileNotFoundException e) 
            {
                System.err.println("File error: " + e.getMessage());
                System.exit(1);  
            }

            }
        }

        if ((codeFile == "prefix.txt") || (codeFile == "suffix.txt"))
        {

            for (I = 1; I <= CHARACTERS_PER_CODE; I++)
            {
                Random r = new Random();
                char digit = (char)(r.nextInt(26) + 'a');
                digit = Character.toUpperCase(digit);

                try (PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter(codeFile, true))))
                {
                        out.print(digit);
                }
                catch (FileNotFoundException e) 
            {
                System.err.println("File error: " + e.getMessage());
                System.exit(1);  
            }

            }    
        }
            //This will take the text file to the next line
            if (I >= CHARACTERS_PER_CODE)
            {
                {
                Random r = new Random();
                char digit = (char)(r.nextInt(26) + 'a');

                try (PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter(codeFile, true))))
                {
                        out.println(""); //This will return a new line for the next loop
                }
                catch (FileNotFoundException e) 
            {
                System.err.println("File error: " + e.getMessage());
                System.exit(1);  
            }
                }
            }

    }
    System.out.println(codeFile + " was successfully created.");

}// end writeRandomCodesToFile()

Answer 1

尊重你的代码，它会是这样的：

public static void mergeProductCodesToFile(String prefixFile, String inlineFile, String suffixFile, String productFile) throws IOException {
    try (BufferedReader prefixReader = new BufferedReader(new FileReader(prefixFile));
        BufferedReader inlineReader = new BufferedReader(new FileReader(inlineFile));
        BufferedReader suffixReader = new BufferedReader(new FileReader(suffixFile))) {

      StringBuilder line = new StringBuilder();
      String prefix, inline, suffix;
      while ((prefix = prefixReader.readLine()) != null) {
        //assuming that nothing fails and the files are equals in # of lines.
        inline = inlineReader.readLine();
        suffix = suffixReader.readLine();
        line.append(prefix).append(inline).append(suffix).append("\r\n");
        // write it
        ...

      }
    } finally {/*close writers*/}
  }

可能会抛出一些异常。

我希望你不要用一种方法来实现它。您也可以使用迭代器，或者非常简单的 reader class （方法）。

我不会使用 List 来加载数据，至少我保证文件的大小很小并且我可以节省内存。

Answer 2

正如我们所讨论的那样，我的方法是存储数据并将其交织。正如 Sergio 在他的回答中所说，确保内存在文件大小和数据结构将使用多少内存方面不是问题。

//the main method we're working on
public static void mergeProductCodesToFile(String prefixFile,
                                       String inlineFile,
                                       String suffixFile,
                                       String productFile) throws IOException
{
    try {
        List<String> prefix = read(prefixFile);
        List<String> inline = read(inlineFile);
        List<String> suffix = read(productFile);

        String fileText = interleave(prefix, inline, suffix);
        //write the single string to file however you want
    } catch (...) {...}//do your error handling...
}

//helper methods and some static variables
private static Scanner reader;//I just prefer scanner. Use whatever you want.
private static StringBuilder sb;

private static List<String> read(String filename) throws IOException
{
    List<String> list = new ArrayList<String>;
    try (reader = new Scanner(new File(filename)))
    {
        while(reader.hasNext())
        { list.add(reader.nextLine()); }
    } catch (...) {...}//catch errors...
}

//I'm going to build the whole file in one string, but you could also have this method return one line at a time (something like an iterator) and output it to the file to avoid creating the massive string
private static String interleave(List<String> one, List<String> two, List<String> three)
{
    sb = new StringBuilder();
    for (int i = 0; i < one.size(); i++)//notice no checking on size equality of words or the lists. you might want this
    {
        sb.append(one.get(i)).append(two.get(i)).append(three.get(i)).append("\n");
    }
    return sb.toString()
}

显然在内存和性能方面还有一些不足之处；此外，还有一些方法可以使它稍微扩展到其他情况，但这是一个很好的起点。使用 c#，我可以更轻松地利用迭代器让交错一次给你一行，从而可能节省内存。只是一个不同的想法！

如何从文本文件的下一行读取并暂停，以便稍后从该行读取？

How can I read from the next line of a text file, and pause, allowing me to read from the line after that later?

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bufferedwriter

bufferedreader

printwriter