異常在線程「主」java.lang.NullPointerException - JSon到Java

我想通過GSON解析JSON代碼到JAVA。我以前提出過類似的例子，它完美地工作，但是當我嘗試在我的程序來實現它給人的錯誤：異常在線程「主」java.lang.NullPointerException - JSon到Java

'Exception in thread "main" java.lang.NullPointerException 
at personality.Child.toString(Child.java:25) 
at java.lang.String.valueOf(String.java:2994) 
at java.io.PrintStream.println(PrintStream.java:821) 
at personality.Personality.main(Personality.java:48)

' 很堅持這一點，我已經做了研究，但仍無法找到清晰的解決方所以我在問你們。

我的代碼：

人格主類

public class Personality { 

     public static void main(String[] args) throws IOException { 
      // TODO code application logic her 

     Gson gson = new Gson(); 

     Child C = gson.fromJson(new FileReader("user.json"), Child.class); 

     System.out.println(C); 
    } 
    }

用戶類別

public class User { 
     public String category; 
     public String id; 
     public String name; 
     public double percentage; 
     public double sampling_error; 

     //Constructor 
     public User(String category, String id, String name , double percentage, double sampling_error) { 

      this.category = category; 
      this.id = id; 
      this.name = name; 
        this.percentage = percentage; 
        this.sampling_error = sampling_error; 
     }  
     @Override 
     public String toString() {   
      return "Category " + category + " : id = " + id + " name = " + name; 
     } 
    }

子類

public class Child { 
     public ArrayList <User> child; //Assign arraylist User to Child class 


     // toString override - not related to JSON processing, just included for debugging purposes 
     @Override 
     public String toString() { 

      StringBuilder A = new StringBuilder(); 


      A.append("Number of childrens = " + child.size() + "\n"); //Error points to here 

      for (User U : child) { 

       A.append(U.toString() + "\n"); //assigns Planet To string to 
      } 

      return A.toString(); 
     }

的Json代碼

{ 
"children": [ 
        { 
        "category": "personality", 
        "id": "Adventurousness", 
        "name": "Adventurousness", 
        "percentage": 0.7871810497070462, 
        "sampling_error": 0.0535169534 
        }, 
        { 
        "category": "personality", 
        "id": "Emotionality", 
        "name": "Emotionality", 
        "percentage": 0.9806198117683198, 
        "sampling_error": 0.0500385256 
        }, 
        { 
        "category": "personality", 
        "id": "Liberalism", 
        "name": "Authority-challenging", 
        "percentage": 0.9958295966962101, 
        "sampling_error": 0.08735455339999999 
        } 
       ] 
}

我已經搜索很多東西，但無法找到的問題，可以幫助是解決這個問題，我knid知道爲什麼NullPointException發生，但不知道它是如何解決it.And是的，我看過這個例子 - What is a NullPointerException, and how do I fix it?，但在這方面並沒有真正幫助我（或者可能但不確定）。謝謝你的時間:)

來源

2016-11-15 Alia Su

'Child'類中的'child'字段名稱不適合它，因爲'child'的類型是User'null'。而NullPointerException。所以，我認爲你的映射有問題。我在'Child'類中看不到'children'字段。 – davidxxx

根據你已發佈的代碼，你的數組ArrayList <User> child爲空。當您第一次嘗試在此行上訪問它時，這將導致NullPointerException for (User U : child) {。您需要初始化子數組，如ArrayList <User> child = new ArrayList<User>()。您的代碼還有其他許多問題。

來源

2016-11-15 23:15:28 mangotang

異常在線程「主」java.lang.NullPointerException - JSon到Java

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