在另一個類中調用時，此方法不起作用

Question

此代碼將X初始化爲新的格式化程序變量。

private Formatter X; 
    public void Create() { 
        try{ 
         X = new Formatter("users.txt"); 
         X.format("%1$20s %2$20s %3$20s %4$20s %5$20s %6$20s %7$20s %8$20s %9$20s \n","Firstname","Lastname","Password","ID","Addressln1","Addressln2","Addressln3","EstimatedValue","Tax Owed"); 
         System.out.println("Due to there not being any users file present one has been created."); 
        } 
        catch(Exception e){ 
         System.out.println("There is a users file present so i will not create another one."); 
        } 
       } 
      } 

這是一件我有故障碼，在X.format位，它不斷給我一個空指針錯誤，我已經竭盡所能來解決它​​。

public void WriteToFilecmd(){ 
       Scanner input = new Scanner(System.in); 
       System.out.println("Please enter your First name"); 
       String Fname = input.next(); 
       System.out.println("Please enter your Last name"); 
       String Lname = input.next(); 
       System.out.println("Please enter your Password"); 
       String Password = input.next(); 
       System.out.println("Please enter your user ID"); 
       String ID = input.next(); 
       System.out.println("Please enter the first address line of your Property"); 
       String addressln1 = input.next(); 
       System.out.println("Please enter the second address line of your Property"); 
       String addressln2 = input.next(); 
       System.out.println("Please enter the third address line of your Property"); 
       String addressln3 = input.next(); 
       System.out.println("Please enter the properties estimated market value"); 
       String EstimatedPropertyValue = input.next(); 
       System.out.println("Please enter your tax owed"); 
       String Taxowed = input.next(); 
       input.close(); 
       X.format("%1$20s %2$20s %3$20s %4$20s %5$20s %6$20s %7$20s %8$20s %9$20s \n",Fname,Lname,Password,ID,addressln1,addressln2,addressln3,EstimatedPropertyValue,Taxowed); 


    } 

Answer 1

NullPointerException就行

X.format("%1$20s %2$20s %3$20s %4$20s %5$20s %6$20s %7$20s %8$20s %9$20s \n",Fname,Lname,Password,ID,addressln1,addressln2,addressln3,EstimatedPropertyValue,Taxowed); 

意味着要麼X本身爲空，或者該方法將拋出它。通常，Java基礎類非常好地處理null參數（並且Scanner#next()不會返回null）並且不包含錯誤，所以X在使用它時最有可能是null。

如果您create方法捕獲異常（見控制檯輸出）可能發生這種情況，您覆蓋X後，或者如果你不叫在所有create方法，或者你把它爲時已晚。

另請注意，小精靈異常處理（「必須抓住它們」，catch(Exception e)）是一種不好的做法。抓住你能處理的東西，而不是你能抓住的東西。

Answer 2

密切注意這一行：

X.format("%1$20s %2$20s %3$20s %4$20s %5$20s %6$20s %7$20s %8$20s %9$20s \n",Fname,Lname,Password,ID,addressln1,addressln2,addressln3,EstimatedPropertyValue,Taxowed); 

哪裏變X從何而來？如果它在是定義的級別高於此方法（可能的級別），那麼它的值是null，並且您不能取消引用null。

Answer 3

確保X（不管它是什麼）已被初始化。

的這個例子中，如果， 你不知道這意味着什麼：

Scanner scanner; // not initialized 
Scanner scanner = new Scanner(System.in); // initialized 

你不必雖然初始化在聲明變量。這也會初始化變量：

Scanner scanner; // not initialized 
scanner = new Scanner(System.in); // good job you init'd it! 

如果您沒有使用「new」設置變量，您將得到nullpointerexception。 確保在調用WriteToFilecmd之前執行此操作。

顯然，不要將你的x變量設置爲「new Scanner（System.in）」， 調用X類型的構造函數。