//轉換器
        GsonBuilder builder = new GsonBuilder(); 
        // 不轉換沒有 @Expose 註解的欄位 
        builder.excludeFieldsWithoutExposeAnnotation();
        Gson gson = builder.create(); 
        
        //1、物件轉string
        Student stu = new Student();
        stu.setStudentId(333);
        stu.setStudentName("qqq");
        String stuStr = gson.toJson(stu);
        System.out.println(stuStr); //{"studentName":"qqq","studentId":333}
        
        
        //2、string轉物件
        Student user2 = gson.fromJson(stuStr, Student.class); 
        System.out.println(user2); 
        String stuTemp = "{\"studentName\":\"qqq2\",\"studentId\":3335}";
        Student user4 = gson.fromJson(stuTemp, Student.class); 
        System.out.println(user4); 

        //3、物件List轉string
        List<Student> testBeanList = new ArrayList<Student>(); 
        Student testBean = new Student(); 
        testBean.setStudentId(555);
        testBean.setStudentName("552");
        testBeanList.add(testBean); 
        
        //Gson gsonList = new Gson(); 
        Type type = new TypeToken<List<Student>>(){}.getType();  //指定集合物件屬性
        String beanListToJson = gson.toJson(testBeanList, type); 
        System.out.println(beanListToJson); //[{"studentName":"552","studentId":555}] 

        //集合string轉物件list
        List<Student> testBeanListFromJson = gson.fromJson(beanListToJson, type); 
        System.out.println(testBeanListFromJson); //[555:552]

        //4、集合如果不指定型別 預設為String
        List<String> testList = new ArrayList<String>(); 
        testList.add("first"); 
        testList.add("second"); 
        String listToJson = gson.toJson(testList); 
        System.out.println(listToJson); //["first","second"] 
        
        //5、集合字串轉回來需要指定型別
        List<String> testList2 = (List<String>) gson.fromJson(listToJson, 
          new TypeToken<List<String>>() { 
          }.getType()); 
        System.out.println(testList2); 

        //6、 將HashMap字串轉換為 JSON 
        Map<String, String> testMap = new HashMap<String, String>(); 
        testMap.put("id", "id.first"); 
        testMap.put("name", "name.second"); 
        String mapToJson = gson.toJson(testMap); 
        System.out.println(mapToJson); //{"id":"id.first","name":"name.second"}
        //7、stringMap轉物件
        Map<String, String> userMap2 = (Map<String, String>) gson.fromJson(mapToJson, 
                new TypeToken<Map<String, String>>() { 
            }.getType()); 
        System.out.println(userMap2); //{id=id.first, name=name.second} 

        //8、物件含有普通物件、集合、map情況
        Student user1 = new Student(); 
        user1.setStudentId(1001); 
        user1.setStudentName("張三"); 
        
        Student user3 = new Student(); 
        user3.setStudentId(1002); 
        user3.setStudentName("李四"); 
        
        Map<String, Student> userMap = new HashMap<String, Student>(); 
        userMap.put("user1", user1); 
        userMap.put("user3", user3); 
        
        List<Student> userList = new ArrayList<Student>(); 
        userList.add(user1); 
        userList.add(user3); 
        
        Teacher groupBean = new Teacher(); 
        groupBean.setStudent(user1);
        groupBean.setStus(userList);
        groupBean.setMap((HashMap)userMap);
        //groupBean.setUserList(userList); 
        Gson gsonGroup = new Gson(); 

        String sGroupBean = gsonGroup.toJson(groupBean, new TypeToken<Teacher>() { 
            }.getType()); 
        System.out.println(sGroupBean); 
        /*{"stus":[{"studentName":"張三","studentId":1001},{"studentName":"李四","studentId":1002}],"student":{"studentName":"張三","studentId":1001},"map":{"user3":{"studentName":"李四","studentId":1002},"user1":{"studentName":"張三","studentId":1001}},"id":0,"age":0}*/

        //9、複雜物件string轉物件
        Teacher groupBean2 = (Teacher) gson.fromJson(sGroupBean, 
              new TypeToken<Teacher>() { 
              }.getType()); 
        System.out.println(groupBean2); 

package com.andtools;

import com.google.gson.annotations.Expose;

public class Student {
    @Expose
    private String studentName;
    @Expose
    private int studentId;
    public Student(){}
    public Student(int studentId,String studentName){
        this.setStudentId(studentId);
        this.setStudentName(studentName);
    }
    public String toString(){
        return this.getStudentId() + ":" + this.getStudentName();
    }
    public String getStudentName() {
        return studentName;
    }
    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }
    public int getStudentId() {
        return studentId;
    }
    public void setStudentId(int studentId) {
        this.studentId = studentId;
    }
    
}

package com.andtools;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

import com.google.gson.annotations.Expose;

public class Teacher {
    @Expose
    private int id;
    @Expose
    private String name;
    @Expose
    private int age;
    @Expose
    private Student student;
    @Expose
    private List stus;
    @Expose
    private HashMap map;
    public String toString(){
        return this.getId()+":"+this.getName()+":"+this.getAge() +":"+ this.getStudent().toString() + ":" + this.getStus() + ":" + this.getMap();
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public Student getStudent() {
        return student;
    }
    public void setStudent(Student student) {
        this.student = student;
    }
    public List getStus() {
        return stus;
    }
    public void setStus(List stus) {
        this.stus = stus;
    }
    public HashMap getMap() {
        return map;
    }
    public void setMap(HashMap map) {
        this.map = map;
    }
    
}