Comment stocker un tableau d'enregistrements dans chaque champ sans avoir à définir la position du tableau?

Question

J'essaie d'inviter l'utilisateur pour l'information et l'écrire dans un fichier texte, puis le programme lit à partir du fichier texte et il y a un compteur qui détermine la longueur de l'enregistrement du tableau. Mais je suis actuellement bloqué sur la façon dont je peux stocker chaque champ dans un tableau d'enregistrement sans définir la position du tableau. Jusqu'à présent, il réussit à lire à partir du fichier texte, mais quand j'écris dans ce fichier texte, il y aura une erreur d'erreur de pointeur nul.

C'est un extrait de mon code

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

Y at-il une raison particulière pour laquelle vous ne pouvez pas utiliser un ArrayList? Il semble assez inutile de lire le fichier entier une seule fois, juste pour déterminer la taille du tableau, puis de le lire à nouveau pour obtenir les valeurs réelles des enregistrements.

Answer 2

Si vous allez chercher avec une clé à travers les données, alors je pense que vous devriez utiliser une carte. Vous devez changer les méthodes "createPublisher" et "printPublisher".

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); 
    } 

} 

Ou vous pouvez utiliser ArrayList bien sûr. Voici comment.

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); 
    } 

}