修復Java中的nullpointerexception

Question

我有一個2d數組正從另一個方法寫入文件。我已經在我的目擊方法中定義了2d數組的內容，但是當它傳遞給save方法時，內容（至少在block [1] [1]中）變爲null。我如何去確保價值的定義？ 我迄今爲止代碼：

（目擊法）

public void Sighting() 
{ 
    Scanner input = new Scanner (System.in); 
    String MigChoice; //initiates Migrant Choice variable to be stored 
    String Trail; //initiates Trail to be stored 
    String NumberSeen; //initiates number to be stored 
    String Species; //initiates species to be stored 
    String Date; //initiates date to be stored 
    String[][] EntryList; 
    EntryList = new String [500][5]; 
    System.out.print("What species of bird was observed?\n"); 
    Species = input.nextLine(); 
    System.out.print("What trail did this spotting take place on?\nDirectory:\n"); 
    System.out.println("1). Alligator Alley"); 
    System.out.println("2). Eagle Roost"); 
    System.out.println("3). Heron Hideout"); 
    System.out.println("4). Lost Bridge Trail"); 
    System.out.println("5). Marsh Rabbit Run"); 
    System.out.println("6). Otter"); 
    System.out.println("7). Shady Oak"); 
    System.out.println("8). Wading Bird Way"); 
    System.out.println("9). Windmill Whisper"); 
    Trail = input.nextLine(); 
    System.out.println("The species is:\n1.)Migrant\n2.)Residential"); 
    System.out.print("Please enter Migrant or Residential: "); 
    MigChoice = input.next(); 
    System.out.print("What was the time of this sighting (in mm/dd/yyyy format)?\n"); 
    Date = input.next(); 
    System.out.print("Finally, how many birds were observed?\n"); 
    NumberSeen = input.next(); 
    EntryList [0][0] = Species; 
    EntryList [0][1] = Trail; 
    EntryList [0][2] = MigChoice; 
    EntryList [0][3] = Date; 
    EntryList [0][4] = NumberSeen; 
    Save(EntryList); 
    System.out.print("Thank you for adding an entry!"); 
    System.out.println("Returning to main menu"); 
    Menu(); 
} 

（保存方法）

public void Save(String[][] EntryList) { 
    try { 
     String[][] content = EntryList; 
     File file = new File("CBB.dat"); 

     // if file doesnt exists, then create it 
     if (!file.exists()) { 
      file.createNewFile(); 
     } 
     if (EntryList[0][0] != null) { 
      DataInputStream instream; 
      DataOutputStream outstream; 
      instream = new DataInputStream(new BufferedInputStream(
        new FileInputStream(file))); // buffers the data stream 
      outstream = new DataOutputStream(new BufferedOutputStream(
        new FileOutputStream(file))); 
      FileWriter fw = new FileWriter("CBB.dat", true); 
      BufferedWriter writer = new BufferedWriter(fw); 
      for (int row = 0; row < EntryList.length; row++) { 
       outstream.writeUTF(EntryList[row][0]); 

       outstream.writeUTF(EntryList[row][1]); 

       outstream.writeUTF(EntryList[row][2]); 

       outstream.writeUTF(EntryList[row][3]); 

       outstream.writeUTF(EntryList[row][4]); 
      } 
      outstream.close(); 
     } else 
      System.out.print("Something is wrong"); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } 
} 

錯誤消息：

java.lang.NullPointerException 
at java.io.DataOutputStream.writeUTF(DataOutputStream.java:330) 
at java.io.DataOutputStream.writeUTF(DataOutputStream.java:306) 
at Dossier.Save(Dossier.java:158) 
at Dossier.Sighting(Dossier.java:133) 

Answer 1

的writeUTF將是，如果你扔NullPointerException傳遞一個空對象作爲參數。

將表示長度信息到輸出流的兩個字節，接着是字符串 S IN的每一個字符的修飾的UTF-8表示 。如果s爲null，則拋出NullPointerException。 中的每個字符將根據字符的值將字符串s轉換爲一組一個，兩個或三個字節，即 。

您可以添加空檢查，如果你的循環：

for (int row = 0; row < EntryList.length; row++) 
    { 
      for(int col = 0; col < EntryList[row].length;col++) { 
       if(EntryList[row][col] != null) 
        outstream.writeUTF(EntryList[row][col]); 
      } 
    }