我哪里做错了?

Question

 1 package per.swwfourteen.fourteen;
 2 
 3 public class Student {
 4     private String id;
 5 
 6     private String name;
 7 
 8     private String password;
 9 
10     private String age;
11 
12     public void setId(String id) {
13         this.id = id;
14     }
15 
16     public String getId() {
17         return this.id;
18     }
19 
20     public void setName(String name) {
21         this.name = name;
22     }
23 
24     public String getName() {
25         return this.name;
26     }
27 
28     public void setPassword(String password) {
29         this.password = password;
30     }
31 
32     public String getPassword() {
33         return this.password;
34     }
35 
36     public void setAge(String age) {
37         this.age = age;
38     }
39 
40     public String getAge() {
41         return this.age;
42     }
43 
44 }

 1 package per.swwfourteen.fourteen;
 2 
 3 public class Teacher {
 4     private String id;
 5 
 6     private String name;
 7 
 8     private String password;
 9 
10     private String age;
11 
12     private Car car;
13 
14     public void setId(String id) {
15         this.id = id;
16     }
17 
18     public String getId() {
19         return this.id;
20     }
21 
22     public void setName(String name) {
23         this.name = name;
24     }
25 
26     public String getName() {
27         return this.name;
28     }
29 
30     public void setPassword(String password) {
31         this.password = password;
32     }
33 
34     public String getPassword() {
35         return this.password;
36     }
37 
38     public void setAge(String age) {
39         this.age = age;
40     }
41 
42     public String getAge() {
43         return this.age;
44     }
45 
46     public void setCar(Car car) {
47         this.car = car;
48     }
49 
50     public Car getCar() {
51         return this.car;
52     }
53 
54 }

 1 package per.swwfourteen.fourteen;
 2 
 3 public class Car {
 4     private String num;
 5 
 6     public void setNum(String num) {
 7         this.num = num;
 8     }
 9 
10     public String getNum() {
11         return this.num;
12     }
13 
14 }

 1 package per.swwfourteen.fourteen;
 2 
 3 import java.util.List;
 4 
 5 public class Root_List {
 6     private List<Student> student;
 7 
 8     private List<Teacher> teacher;
 9 
10     public void setStudent(List<Student> student) {
11         this.student = student;
12     }
13 
14     public List<Student> getStudent() {
15         return this.student;
16     }
17 
18     public void setTeacher(List<Teacher> teacher) {
19         this.teacher = teacher;
20     }
21 
22     public List<Teacher> getTeacher() {
23         return this.teacher;
24     }
25 
26 }

 1 package per.swwfourteen.fourteen;
 2 
 3 import java.io.BufferedReader;
 4 import java.io.File;
 5 import java.io.FileReader;
 6 import java.io.IOException;
 7 
 8 import com.alibaba.fastjson.JSONObject;
 9 
10 public class JsonRoot {
11     public static void main(String[] args)throws Exception{
12         Root_List root=JSONObject.parseObject(ReadRoot("D:"+File.separator+"1.json"),Root_List.class);
13         for(Student s:root.getStudent()){
14             System.out.println(s.getId());
15             System.out.println(s.getName());
16             System.out.println(s.getAge());
17             System.out.println(s.getPassword());
18         }
19         for(Teacher t:root.getTeacher()){
20             System.out.println(t.getId());
21             System.out.println(t.getName());
22             System.out.println(t.getAge());
23             System.out.println(t.getPassword());
24             System.out.println(t.getCar());
25         }
26     }
27     
28     public static String ReadRoot(String rer)throws IOException{
29         StringBuffer sb=new StringBuffer();
30         BufferedReader br=new BufferedReader(new FileReader(rer));
31         char[] it=new char[1024];
32         int sum=0;
33         while((sum=br.read(it))!=-1){
34             String s=String.valueOf(it, 0, sum);
35             sb.append(s);
36         }
37         br.close();
38         return sb.toString();
39     }
40 }

运行后提示13行报错,

Exception in thread "main" java.lang.NullPointerException
at per.swwfourteen.fourteen.JsonRoot.main(JsonRoot.java:13)

 1 {
 2   "Root": {
 3     "student": [
 4       {
 5         "id": "001",
 6         "name": "student1",
 7         "password": "123",
 8         "age": "20"
 9       },
10       {
11         "id": "002",
12         "name": "student2",
13         "password": "456",
14         "age": "21"
15       },
16       {
17         "id": "003",
18         "name": "student3",
19         "password": "123",
20         "age": "21"
21       }
22     ],
23     "teacher": [
24       {
25         "id": "001",
26         "name": "teacher1",
27         "password": "123",
28         "age": "20",
29         "car": { "num": "098" }
30       },
31       {
32         "id": "002",
33         "name": "teacher2",
34         "password": "123",
35         "age": "20",
36         "car": { "num": "098" }
37       }
38     ]
39   }
40 }

 

Answer 1

json转成对象后，root是个map类吧？你用root.get("student")试试，或者遍历keySet集合

Answer 2

public List<Student> getStudent() { if(this.student==null)return new List<Student>(); return this.student; }  

Answer 3

new List<Student>(); 这块儿报错..

Answer 4

@略知闻风雨: .我又不会写Java代码..只是说思路. 问题是因为你对null 循环了.只要返回一个空数组出去就好了.

Answer 5

空对象，你调试一下，root中没对象，读json可能有问题

Answer 6

d一个for循环都进不去

Answer 7

@略知闻风雨: 肯定啊，空对象，就是你的root中集合为空，取不到student对象，我是让你看第十二行 Root_List root=JSONObject.parseObject(ReadRoot("D:"+File.separator+"1.json"),Root_List.class); 这里面的root中有数据没，应该是没有，解析json获得数据为空，你看下是不是

Answer 8

@博客园乄小光: 是的,student和teacher都是null

Answer 9

@略知闻风雨: 还是读json，你看一下路径，读json的逻辑是不是有问题，往上一步调试，进到ReadRoot方法里面，一步一步找

Answer 10

@博客园乄小光: 纳闷了,我student类里是有东西的啊,怎么就null了呢