如何将记录数组存储到每个字段中而不必定义数组位置？

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。但我目前坚持如何将每个字段存储到一个记录数组中，而无需定义数组的位置。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，将会出现空指针异常错误。

这是我的代码

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

一个片段是有，为什么你不能只用一个ArrayList具体原因是什么？读取整个文件一次看起来非常浪费，只是为了确定数组的大小，然后再次读取它以获取记录的实际值。

Answer 2

如果您要通过数据搜索关键字，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); 
    } 

} 

或者您当然可以使用ArrayList。这是如何。

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); 
    } 

}