java - 线程 "main"java.lang.NullPointerException 中的异常 - Json 到 Java

Question

我正在尝试通过GSON将JSON代码解析为JAVA。我之前做过一个类似的例子，它运行得很好，但是当我尝试在我的程序中实现时，它给出了错误:

'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)

' 真的很纠结这个问题，我已经做了研究，但仍然找不到明确的解决方案。所以我问你们。

我的代码:

个性主类

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);
    }
    }

用户类别

    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;
        }
    }

子类

 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();
        }

Json代码

   {   
"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
                  }
                ]
}

我搜索了很多东西，但找不到可以帮助解决这个问题的问题，我知道为什么会发生 NullPointException，但不知道它是如何解决的。是的，我看过这个例子 - What is a NullPointerException, and how do I fix it?但在这种情况下并没有真正帮助我(或者它可能可以但我不确定)。谢谢您的宝贵时间:)

Answer 1

根据您发布的代码，您的数组 ArrayList <User> child一片空白。当您第一次尝试在此行 for (User U : child) { 访问它时，这将导致 NullPointerException 。您需要初始化子数组，例如 ArrayList <User> child = new ArrayList<User>() 。 您的代码还存在许多其他问题。