准备工作

引入pom

<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.78</version>
</dependency>

<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.18.22</version>
 </dependency>

实体bean

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class User {
   
    private Integer id;
    //姓名
    private String name;
}

fastjson

1.JSONObject获取所有的key

技巧：
JSONObject获取key：↓
    JSONObject obj；
    for (Map.Entry<String, Object> entry : cutReceiveAlarmMessageObject.entrySet()) {
   
           String s = entry.getKey();
    }

2.集合中实体对象转换 list中Enrey转Dto

list中Enrey转Dto：↓
    List<WarningNoticeDto> warningNoticeDtoList = warningNoticeList.getInfo().getList().stream().map(this::getEntryToDto).collect(Collectors.toList());
    /**
     * entry转DTO
     * @param warningNotice entry
     * @return dto
     */
    private WarningNoticeDto getEntryToDto(WarningNotice warningNotice) {
   
        WarningNoticeDto warningNoticeDto = new WarningNoticeDto();
        BeanUtils.copyProperties(warningNotice, warningNoticeDto);
        return warningNoticeDto;
    }

3.字符串转List

import com.alibaba.fastjson.TypeReference;
import com.alibaba.fastjson.JSONObject;

String str = "[
  {
    "id": 5,
    "nodeIdArr": "[\"221\",\"222\"]",
    "nodeNameArr": "[\"enb_221\",\"2222\"]",
    "upperLimitOfTheBusyTimeThreshold": 9,
    "lowerLimitOfTheBusyTimeThreshold": 2,
    "dateRangeBeginTime": 1701648000000,
    "dateRangeEndTime": 1701682200000,
    "createTime": 1701676594000,
    "updateTime": 1701737385000,
    "activeState": "1"
  },
  {
    "id": 6,
    "nodeIdArr": "[\"2003\",\"501\",\"10010\"]",
    "nodeNameArr": "[\"CityA\",\"501\",\"Vir1\"]",
    "upperLimitOfTheBusyTimeThreshold": 9,
    "lowerLimitOfTheBusyTimeThreshold": 2,
    "dateRangeBeginTime": 1701648000000,
    "dateRangeEndTime": 1701682200000,
    "createTime": 1701676641000,
    "updateTime": 1701737382000,
    "activeState": "1"
  }]"
List<BusyTimeIndicatorAlarmThreshold> busyTimeIndicatorAlarmThresholdList = new ArrayList<>();
busyTimeIndicatorAlarmThresholdList = JSONObject.parseObject(str, new TypeReference<List<BusyTimeIndicatorAlarmThreshold>>() {});

方式一、List busyTimeIndicatorAlarmThresholdList = new ArrayList<>();
busyTimeIndicatorAlarmThresholdList = JSONObject.parseObject(str, new TypeReference>() {});

方式二、List userList = JSONArray.parseArray(str, User.class);

4.json字符串转JSONObject

@Test
public void jsonStrConverJSONObject(){
   
    String str = "{\"id\":1,\"name\":\"tom\"}";
    JSONObject jsonObject = JSONObject.parseObject(str);
    System.out.println(jsonObject);     
}

输出：{"name":"tom","id":1}

5.list根据ids数组过滤list

@Test
public void listFilter() {
   
    List<User> list = new ArrayList<>();
    list.add(new User(1, "a"));
    list.add(new User(2, "b"));
    list.add(new User(3, "c"));
    list.add(new User(4, "d"));
    list.add(new User(5, "e"));
    list.add(new User(6, "f"));
    list.add(new User(7, "g"));
    list.add(new User(8, "h"));
    list.add(new User(9, "i"));
    list.add(new User(10, "j"));

    //注意：数组类型必须使用Integer才可以，使用int会判断失败
    Integer[] arr = new Integer[]{
   1,2,5,6,9};
    List<User> filterList = list.stream().filter(item -> Arrays.asList(arr).contains(item.getId())).collect(Collectors.toList());
    filterList.stream().forEach(System.out::println);
}

6.json字符串转JavaBean对象

@Test
public void jsonStrConverBean(){
   
    String str = "{\"id\":1,\"name\":\"tom\"}";
    User user = JSONObject.parseObject(str, User.class);
    System.out.println(user);   
}

输出：User(id=1, name=tom)

7.json对象转javabean

@Test
public void JSONObjectConverBean(){
   
    JSONObject jsonObject = new JSONObject();
    jsonObject.put("id", 1);
    jsonObject.put("name", "tom");
    User user = JSONObject.toJavaObject(jsonObject, User.class);
    System.out.println(user);   
}

输出：User(id=1, name=tom)

8.jsonObject转map

@Test
public void JSONObjectConverMap(){
   
    JSONObject jsonObject = new JSONObject();
    jsonObject.put("id", 1);
    jsonObject.put("name", "tom");
    Map<String,String> map = JSONObject.parseObject(jsonObject.toJSONString(), Map.class);
    System.out.println(map);   
}

输出：{name=tom, id=1}

9.List\转jsonArray

@Test
public void listConverjJsonArray(){
   
    List<User> list = new ArrayList<>();
    list.add(new User(1, "a"));
    list.add(new User(2, "b"));
    //错误写法，因为list.toString()输出[User(id=1, name=a), User(id=2, name=b)]。这东西无法json解析，会报错：com.alibaba.fastjson.JSONException: syntax error, pos 2, line 1, column 3[User(id=1, name=a), User(id=2, name=b)]
    //        JSONArray jsonArray =JSONArray.parseArray(list.toString());
    //正确写法，简写方式
    JSONArray jsonArray =JSONArray.parseArray(JSONObject.toJSONString(list));

    //正确写法，复杂方式
    //        JSONArray jsonArray = new JSONArray();
    //        JSONObject jsonObject = null;
    //        for (User user: list) {
   
    //            jsonObject = new JSONObject();
    //            jsonObject.put("id", user.getId());
    //            jsonObject.put("name", user.getName());
    //            jsonArray.add(jsonObject);
    //        }
    System.out.println(jsonArray);
}

10.jsonArray转成String[]

@Test
public void jsonArrayConverStringArray(){
   
    JSONArray jsonArray = new JSONArray();
    jsonArray.add(0, "100");
    jsonArray.add(1, "101");
    jsonArray.add(2, "102");
    System.out.println("jsonArray:" + jsonArray);

    String[] stringArr = new String[jsonArray.size()];
    for (int i = 0; i < jsonArray.size(); i++) {
   
        stringArr[i] = jsonArray.get(i).toString();
    }
    for(String str : stringArr) {
   
        System.out.println(str);
    }
}

11.List\转int[]

List<Integer> list = xxxxxxxx;
int[] arr = list.stream().mapToInt(Integer::intValue).toArray();

12.List集合按照某个属性值又打大小排序

p2-p1降序，p1-p2升序

List<TscRegTop> neListList；
Collections.sort(neListList, new Comparator<TscRegTop>() {
            @Override
            public int compare(TscRegTop p1, TscRegTop p2) {
                return p2.getTscRegMsNums() - p1.getTscRegMsNums();
            }
        });

13.对Collections.sort排序后我想制定查询几条，比如list有10条，我只想获取前4条

neListList.subList(0, Math.min(neListList.size(), 条数))

14. Java 8 引入的 Optional 类型，它可以用来处理可能为空的值

如果user.getAge()为null那么返回0；如果不为空则返回值。

user.setXXX(Optional.ofNullable(user.getAge()).orElse(0));

15.Long类型转Integer

Long number = xxxx；
Integer item = number.intValue();

问题：为啥使用int就判断失效，而使用Integer和String都能准确判断？

/**
  * 问题：为啥使用int就判断失效，而使用Integer和String都能准确判断？
  * 答案：不能将基本数据类型转化为List列表。
*/
@Test
public void test1() {
   
    int[] arr = new int[]{
   1,2,5,6,9};
    System.out.println(Arrays.asList(arr).contains(1)); //结果为false
    Integer[] arr2 = new Integer[]{
   1,2,5,6,9};
    System.out.println(Arrays.asList(arr2).contains(1)); //结果为true
    String[] arr3 = new String[]{
   "1","2","5","6","9"};
    System.out.println(Arrays.asList(arr3).contains("1")); //结果为true

    //验证答案如下,把arr、arr2、arr3分别返回查看返回泛型，能够看出Arrays.asList(arr)返回的居然是List<int[]>，问题就出在这，说明list里面包含的是一个个的int[]，用这个判断ints.contains(1)，肯定为false
    List<int[]> ints = Arrays.asList(arr);
    List<Integer> integers = Arrays.asList(arr2);
    List<String> strings = Arrays.asList(arr3);
}

jackjson

16.字符串转List

字符串长这样

{
   
    "returnCode": "2000",
    "data": [
        {
   
            "activityId": "07e71d599d734d3f844c5014a991462b",
            "entId": null,
            "entName": "苏州芯慧和创科技发展有限公司"
        }
    ]
}

转换代码

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.JsonNode;
import java.util.List;
import java.util.ArrayList;

public class Main {
   
    public static void main(String[] args) {
   
        String json = "{\"returnCode\":\"2000\",\"data\":[{\"activityId\":\"07e71d599d734d3f844c5014a991462b\",\"entId\":null,\"entName\":\"苏州芯慧和创科技发展有限公司\"}]}";

        try {
   
            ObjectMapper objectMapper = new ObjectMapper();
            JsonNode rootNode = objectMapper.readTree(json);
            JsonNode dataArray = rootNode.get("data");

            List<User> userList = new ArrayList<>();
            for (JsonNode node : dataArray) {
   
                User user = objectMapper.treeToValue(node, User.class);
                userList.add(user);
            }

            // 打印输出
            for (User user : userList) {
   
                System.out.println(user);
            }
        } catch (Exception e) {
   
            e.printStackTrace();
        }
    }
}

本人其他文章链接

1.java小工具util系列1：日期毫秒数转日期字符串
https://blog.csdn.net/a924382407/article/details/121955349

2.java小工具util系列2：获取字符modelStr在字符串str中第count次出现时的下标
https://blog.csdn.net/a924382407/article/details/121955455

3.java小工具util系列3：正则表达式匹配:匹配不包含@特殊字符的字符串
https://blog.csdn.net/a924382407/article/details/121955737

4.java小工具util系列4：String[] 转 List＜ Integer ＞
https://blog.csdn.net/a924382407/article/details/121956201

5.java小工具util系列5：基础工具代码(Msg、PageResult、Response、常量、枚举)
https://blog.csdn.net/a924382407/article/details/120952865

6.java小工具util系列6：java执行string返回boolean结果
https://blog.csdn.net/a924382407/article/details/117124536

7.java小工具util系列7：集合中实体对象转换 list中Enrey转Dto
https://blog.csdn.net/a924382407/article/details/121957545

8.java小工具util系列8：JSONObject获取key
https://blog.csdn.net/a924382407/article/details/121957607

9.java小工具util系列9：检测一个字符串是否是时间格式
https://blog.csdn.net/a924382407/article/details/123948881

10.java小工具util系列10：时间毫秒数、时间格式字符串、日期之间相互转化
https://blog.csdn.net/a924382407/article/details/124581851