如何在不必定义数组位置的情况下将记录数组存储到每个字段中？

Question

我试图提示用户输入信息并将其写入文本文件，然后程序从文本文件中读取，并且有一个计数器确定数组记录的长度。但我目前被困在如何在不定义数组位置的情况下将每个字段存储到记录数组中。到目前为止，它成功地从文本文件中读取，但是当我写入该文本文件时，会出现空指针异常错误。

这是我的代码的 sn-p

static class publisherDetails
{
    String pubID;
    String publisher;
    String pubAddress;
}

public static void publisherDetails() throws IOException
{
    File publisherMain = new File("publisherMain.txt");
    Scanner readpublisher = new Scanner(publisherMain);
    publisherDetails [] publisherList = new 
    publisherDetails[countPubLines(0, publisherMain)];
    createPublisher(publisherList, publisherMain, readpublisher);
    readpublisher.close();
    printPublisher(publisherList);
}

public static int countPubLines(int count, File publisherMain) throws IOException
{
  Scanner countingLines = new Scanner(publisherMain);

  while (countingLines.hasNextLine())
  {
     String reading = countingLines.nextLine();
     count++;
  } 

  countingLines.close();
  count = count/3;
  return count;
}

public static void createPublisher(publisherDetails[] publisherList, File publisherMain, Scanner readpublisher) throws IOException
{
while ( readpublisher.hasNextLine() )
    {
       for (int i=0; i< publisherList.length; i++) 
       publisherList[i] = new publisherDetails();
        {
          publisherList[0].pubID = readpublisher.nextLine();
          publisherList[0].publisher =readpublisher.nextLine();
          publisherList[0].pubAddress =readpublisher.nextLine();
          publisherList[1].pubID =readpublisher.nextLine();
          publisherList[1].publisher =readpublisher.nextLine();
          publisherList[1].pubAddress =readpublisher.nextLine();
          publisherList[2].pubID =readpublisher.nextLine();
          publisherList[2].publisher =readpublisher.nextLine();
          publisherList[2].pubAddress =readpublisher.nextLine();
          publisherList[3].pubID =readpublisher.nextLine();
          publisherList[3].publisher =readpublisher.nextLine();
          publisherList[3].pubAddress =readpublisher.nextLine();
        }

    }
    readpublisher.close();
}

public static void printPublisher(publisherDetails[] publisherList)  
{
for (int j=0 ;j<publisherList.length; j++)
     {
       output("Publisher ID : " + publisherList[j].pubID);
       output("Publisher : " + publisherList[j].publisher);
       output("Publisher Address :  " + publisherList[j].pubAddress);
     }

}  


public static void addPublisher() throws IOException
{
   FileWriter inputPublisher = new FileWriter(new File("publisherMain.txt",true);
   newPublisher(inputPublisher);
}


public static void newPublisher(FileWriter inputPublisher) throws IOException
  {
    Scanner scan = new Scanner(System.in);
    output("Please enter publisher ID");
    String ID = scan.nextLine();
    inputPublisher.write("\n"+ID);

    output("Please enter Publisher ");
    String publisher = scan.nextLine();
    inputPublisher.write("\n"+publisher);

    output("Please enter publisher Address");
    String pubAddress = scan.nextLine();
    inputPublisher.write("\n"+pubAddress);

    scan.close();
    inputPublisher.close();
  }

Answer 1

您不能只使用ArrayList 有什么具体原因吗？一次读取整个文件似乎很浪费，只是为了确定数组的大小，然后再次读取它以获得记录的实际值。

Answer 2

如果您要使用关键字搜索数据，那么我认为您应该使用地图。您必须更改“createPublisher”和“printPublisher”方法。

public static Map<String, publisherDetails> createPublisher(File publisherMain, Scanner readpublisher)
        throws IOException {

    Map<String, publisherDetails> publisherMap = new HashMap<String, publisherDetails>();
    publisherDetails newPublisher = new publisherDetails();
    while (readpublisher.hasNextLine()) {

        newPublisher.pubID = readpublisher.nextLine();
        newPublisher.publisher = readpublisher.nextLine();
        newPublisher.pubAddress = readpublisher.nextLine();
        // Fill the map 
        publisherMap.put(newPublisher.pubID, newPublisher);
        readpublisher.nextLine();
    }
    readpublisher.close();
    return publisherMap;
}

public static void printPublisher(Map<String, publisherDetails> publisherMap) {
    // loop through the whole data
    // or you can search with key like publisherMap.get(pubId); 
    for(Map.Entry<String,publisherDetails> entry : publisherMap.entrySet()){

        output("Publisher ID : "       + entry.getValue().pubID);
        output("Publisher : "          + entry.getValue().publisher);
        output("Publisher Address :  " + entry.getValue().pubAddress);
    }

}

当然，您也可以使用 ArrayList。方法如下。

public static List<publisherDetails> createPublisher(File publisherMain, Scanner readpublisher)
        throws IOException {

    List<publisherDetails> publisherList = new ArrayList<publisherDetails>();
    publisherDetails newPublisher = new publisherDetails();
    while (readpublisher.hasNextLine()) {
        readpublisher.nextLine();
        newPublisher.pubID = readpublisher.nextLine();
        newPublisher.publisher = readpublisher.nextLine();
        newPublisher.pubAddress = readpublisher.nextLine();
        // Fill the list
        publisherList.add(newPublisher);

    }
    readpublisher.close();
    return publisherList;
}

public static void printPublisher(List<publisherDetails> publisherList) {
    // loop through the whole data
    for(publisherDetails publisher : publisherList){

        output("Publisher ID : "       + publisher.pubID);
        output("Publisher : "          + publisher.publisher);
        output("Publisher Address :  " + publisher.pubAddress);
    }

}