java中的随机访问文件 [英] random access file in java

问题描述

我有以下字段：

• 库存控制（16字节记录）
  
  
  
  
  • 产品ID代码（int - 4字节）
  
  
  • 库存数量（整数 - 4字节）
  
  
  • 价格（双 - 8字节）
  
  

  如何使用以下方法创建固定长度的随机访问文件以上长度？我在网上尝试了一些例子，但是当我尝试访问它们时，我得到了一个EOF异常或随机地址值。

  How do I create a fixed length random access file using the above lengths? I tried some examples online, but I either get an EOF exception or random address values when I try to access them.

  我尝试了一些更多的例子而无法理解概念非常好。我正在尝试一个项目，并将尝试探索更多。

  I tried some more examples and couldn't understand the concept very well. I'm trying a project with it and will try to explore more on it.

  这是一些示例数据。数据中可能存在漏洞否。有货可能 23 == 023 。

  Here is some example data. There might be holes in the data where No. in stock could be 23 == 023.

            Quantity
  ID. No.   In Stock   Price
  
  -------   --------   ------
   1001       476      $28.35
   1002       240      $32.56
   1003       517      $51.27
   1004       284      $23.75
   1005       165      $32.25
  

  谢谢你帮助。

  推荐答案

  java.io.RandomAccessFile是您正在寻找的类。这是一个示例实现（您可能想要编写一些单元测试，因为我没有：）

  java.io.RandomAccessFile is the class you're looking for. Here's an example implementation (you'll probably want to write some unit tests, as I haven't :)

  package test;
  
  import java.io.IOException;
  import java.io.RandomAccessFile;
  
  public class Raf {
      private static class Record{
          private final double price;
          private final int id;
          private final int stock;
  
          public Record(int id, int stock, double price){
              this.id = id;
              this.stock = stock;
              this.price = price;
          }
  
          public void pack(int n, int offset, byte[] array){
              array[offset + 0] = (byte)(n & 0xff);
              array[offset + 1] = (byte)((n >> 8) & 0xff);
              array[offset + 2] = (byte)((n >> 16) & 0xff);
              array[offset + 3] = (byte)((n >> 24) & 0xff);
          }
  
          public void pack(double n, int offset, byte[] array){
              long bytes = Double.doubleToRawLongBits(n);
              pack((int) (bytes & 0xffffffff), offset, array);
              pack((int) ((bytes >> 32) & 0xffffffff), offset + 4, array);
          }
  
          public byte[] getBytes() {
              byte[] record = new byte[16];
              pack(id, 0, record);
              pack(stock, 4, record);
              pack(price, 8, record);
              return record;
          }
      }
  
      private static final int RECORD_SIZE = 16;
      private static final int N_RECORDS = 1024;
  
      /**
       * @param args
       * @throws IOException 
       */
      public static void main(String[] args) throws IOException {
          RandomAccessFile raf = new RandomAccessFile(args[0], "rw");
          try{
              raf.seek(RECORD_SIZE * N_RECORDS);
  
              raf.seek(0);
  
              raf.write(new Record(1001, 476, 28.35).getBytes());
              raf.write(new Record(1002, 240, 32.56).getBytes());
          } finally {
              raf.close();
          }
      }
  }
  

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