“线程中的异常”；“主要”；java.lang.NullPointerException-JSon到java

我试图通过GSON将JSON代码解析成JAVA。我以前也做过类似的例子，效果很好，但当我尝试在程序中实现时，会出现错误：

'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)

" 我已经做了研究，但仍然没有找到一个明确的解决方案。所以我在问你们

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);
    }
    }

    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;
        }
    }

 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();
        }

用户类

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);
    }
    }

    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;
        }
    }

 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();
        }

儿童班

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);
    }
    }

    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;
        }
    }

 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();
        }

---

我搜索了很多东西，但找不到有助于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，但在这方面对我帮助不大（或者它可能会，但不确定）。感谢您的时间：）

根据您发布的代码，您的数组

ArrayList子项

为空。这将在您第一次尝试访问（用户U:child）{的这一行时导致NullPointerException。您需要初始化子数组，如

ArrayList child=new ArrayList（）

。

---

您的代码还存在许多其他问题。

根据您发布的代码，您的array

ArrayList子项

为空。这将在您第一次尝试访问（用户U:child）的这一行时导致NullPointerException{。您需要初始化子数组，类似于

ArrayList child=new ArrayList（）

。

---

您的代码还有许多其他问题。

child

类中的

child

字段的名称不适合它，因为'child'的类型是User is

null

。而NullPointerException。因此，我认为您的映射有问题。我在

child

类中没有看到

children

字段

child

类中的

child

字段，其名称不适合，因为'child'的类型为User为

null

。而NullPointerException。因此，我认为您的映射有问题。我在

child

类中没有看到

children

字段。