代码演示：

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class JavaJson {
    public static void main(String[] args) {
        Gson gson = new Gson();
        
        // 1.将javabean转成 json字符串
        Book book = new Book(100, "三国");
        String strBook = gson.toJson(book);
        System.out.println(strBook);

        // 2.将json字符串转成 javabean
        Book book2 = gson.fromJson(strBook, Book.class);
        System.out.println(book2);

        // 3.将list对象转成 json字符串
        List<Book> bookList = new ArrayList<>();
        bookList.add(new Book(200, "宝莲灯"));
        bookList.add(new Book(200, "西游记"));
        String strBookList = gson.toJson(bookList);
        System.out.println(strBookList);

        // 4.把json字符串转成 list对象
        Type type = new TypeToken<List<Book>>() {
        }.getType(); // 返回类型的完整路径
        List<Book> booksList2 = gson.fromJson(strBookList, type);
        System.out.println(booksList2);

        // 5.把map对象转成 json字符串
        Map<String, Book> bookMap = new HashMap<>();
        bookMap.put("k1", new Book(400, "哈利波特1"));
        bookMap.put("k2", new Book(500, "哈利波特2"));
        String strBookMap = gson.toJson(bookMap);
        System.out.println(strBookMap);

        // 6.把json字符串转成 map对象
        Map<String, Book> bookMap2 = gson.fromJson(strBookMap, new TypeToken<Map<String, Book>>() {
        }.getType());
        System.out.println(bookMap2);
    }
}