Java 使用jackson通过json文件的键选择值

这可能是整个网站上最愚蠢的问题。我不熟悉Java和JSON，需要帮助。我正在使用API Jackson。程序接收一个JSON文件。从中我需要得到：

20岁至30岁的人群名单，按姓名排序

独特的城市名单

年龄间隔为0-10、11-20、21-30等的人数

目前，我已经学会了如何将json文件转换为java列表

import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.IOException;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.List;

public class Main {
    public static void main(String[] args) {

    ObjectMapper mapper = new ObjectMapper();

    try {
        List<Data> data = Arrays.asList(mapper.readValue(Paths.get("C:\\data.json").toFile(), Data[].class));


        System.out.println(data);


        } catch (JsonParseException jsonParseException) {
        jsonParseException.printStackTrace();
    } catch (JsonMappingException jsonMappingException) {
        jsonMappingException.printStackTrace();
    } catch (IOException ioException) {
        ioException.printStackTrace();
    }

}
}

Json文件如下所示

[
    {
        "id": "1",
        "firstName": "Lesley",
        "lastName": "Bryan",
        "dateOfBirth": "11/28/61",
        "city": "Southampton–Portsmouth"
    },
    {
        "id": "2",
        "firstName": "Edward",
        "lastName": "Houston",
        "dateOfBirth": "10/5/92",
        "city": "Southampton–Portsmouth"
    },
    {
        "id": "3",
        "firstName": "Donald",
        "lastName": "Ross",
        "dateOfBirth": "12/10/79",
        "city": "Glasgow"
    },
    {
        "id": "4",
        "firstName": "Peter",
        "lastName": "Kelly",
        "dateOfBirth": "3/17/04",
        "city": "Birmingham–Wolverhampton"
    },
    {
        "id": "5",
        "firstName": "Anthony",
        "lastName": "McKinney",
        "dateOfBirth": "3/6/68",
        "city": "Liverpool"
    },
    {
        "id": "6",
        "firstName": "David",
        "lastName": "Stewart",
        "dateOfBirth": "4/11/73",
        "city": "Leeds–Bradford"
    },
    {
        "id": "7",
        "firstName": "Christopher",
        "lastName": "Austin",
        "dateOfBirth": "12/28/74",
        "city": "Birmingham–Wolverhampton"
    },
    {
        "id": "8",
        "firstName": "Alvin",
        "lastName": "Hodge",
        "dateOfBirth": "11/25/58",
        "city": "Newcastle upon Tyne–Sunderland"
    },
    {
        "id": "9",
        "firstName": "Gerald",
        "lastName": "Higgins",
        "dateOfBirth": "6/28/55",
        "city": "Liverpool"
    },
    {
        "id": "10",
        "firstName": "Amos",
        "lastName": "Owens",
        "dateOfBirth": "1/16/01",
        "city": "Manchester-Salford"
    },
    {
        "id": "11",
        "firstName": "Christian",
        "lastName": "Bishop",
        "dateOfBirth": "11/14/50",
        "city": "Nottingham"
    },
    {
        "id": "12",
        "firstName": "Robert",
        "lastName": "Caldwell",
        "dateOfBirth": "12/8/80",
        "city": "Manchester-Salford"
    },
    {
        "id": "13",
        "firstName": "Brian",
        "lastName": "Heath",
        "dateOfBirth": "9/23/02",
        "city": "Newcastle upon Tyne–Sunderland"
    },
    {
        "id": "14",
        "firstName": "Mark",
        "lastName": "Anthony",
        "dateOfBirth": "1/8/92",
        "city": "London"
    },
    {
        "id": "15",
        "firstName": "Mark",
        "lastName": "Watson",
        "dateOfBirth": "7/27/91",
        "city": "Nottingham"
    },
    {
        "id": "16",
        "firstName": "Charles",
        "lastName": "Stafford",
        "dateOfBirth": "1/26/90",
        "city": "Birmingham–Wolverhampton"
    },
    {
        "id": "17",
        "firstName": "Steven",
        "lastName": "Merritt",
        "dateOfBirth": "12/4/63",
        "city": "Leeds–Bradford"
    },
    {
        "id": "18",
        "firstName": "John",
        "lastName": "Holmes",
        "dateOfBirth": "4/22/52",
        "city": "Southampton–Portsmouth"
    },
    {
        "id": "19",
        "firstName": "Mervin",
        "lastName": "Lewis",
        "dateOfBirth": "10/27/95",
        "city": "Birmingham–Wolverhampton"
    },
    {
        "id": "20",
        "firstName": "Peter",
        "lastName": "Marsh",
        "dateOfBirth": "12/10/63",
        "city": "Glasgow"
    },
    {
        "id": "21",
        "firstName": "Piers",
        "lastName": "Harrington",
        "dateOfBirth": "4/27/85",
        "city": "London"
    },
    {
        "id": "22",
        "firstName": "Matthew",
        "lastName": "O’Brien’",
        "dateOfBirth": "1/19/59",
        "city": "Manchester-Salford"
    }
]

请告诉我完成任务的顺序。我再次为这么愚蠢的信息道歉。

---

注意：json文件中的所有姓名、姓氏和日期都是随机获得的。

首先，您需要更新

数据

类，以便于处理年龄和年龄组：

将

dateOfBirth

的类型更改为

LocalDate

，更新constructor/getter/provide custom setter以处理20世纪的日期

添加以年和年龄组作为字符串获取年龄的方法：

更新

toString

以打印年龄

类数据{
// ...
@JsonFormat（shape=JsonFormat.shape.STRING，pattern=“M/d/yy”）
私有本地出生日期；
public LocalDate getDateOfBirth（）{
出生返回日期；
}
公共无效setDateOfBirth（LocalDate dob）{
if（dob.isAfter（LocalDate.now（））{
出生日期=出生日期（100年）；
}
this.dateOfBirth=dob；
}
@杰索尼奥雷
公共整数getAge（）{
返回期间.between（dateOfBirth，LocalDate.now（））.getYears（）；
}
@杰索尼奥雷
公共字符串getAgeGroup（）{
int age=getAge（）；
if（年龄<11岁）{
返回“0..10”；
}
报税表（年龄/10*10+1）+.“+（（年龄/10+1）*10）；
}
@凌驾
公共字符串toString（）{
返回“[id=“+id+”，age=“+getAge（）+”，firstName=“+firstName+”，lastName=“+lastName+”，dateOfBirth=“+dateOfBirth+”，city=“+city+”]”；
}
}

然后，您可以从输入列表中获取所需的数据子集：

//按姓/名对20到30岁的人进行排序
列表年龄20到30=数据
.stream（）
.filter（p->p.getAge（）>=20&&p.getAge（）<30）
.sorted（Comparator.comparing（p->p.getLastName（）+“”+p.getFirstName（））
.collect（Collectors.toList（））；
年龄20到30.forEach（System.out:：println）；
//获取唯一城市的排序列表（使用TreeSet作为排序集）
设置城市=数据
.stream（）
.map（数据：：getCity）
.collect（收集器.toCollection（TreeSet:：new））；
cities.forEach（System.out:：println）；
//按年龄组获取统计数据
地图字节=数据
.stream（）
.collect（Collectors.groupingBy（Data:：getAgeGroup，TreeMap:：new，Collectors.counting（））；
System.out.println（byAges）；

输出

[id = 14, age=28, firstName = Mark, lastName = Anthony, dateOfBirth = 1992-01-08, city = London]
[id = 2, age=27, firstName = Edward, lastName = Houston, dateOfBirth = 1992-10-05, city = Southampton–Portsmouth]
[id = 19, age=24, firstName = Mervin, lastName = Lewis, dateOfBirth = 1995-10-27, city = Birmingham–Wolverhampton]
[id = 15, age=29, firstName = Mark, lastName = Watson, dateOfBirth = 1991-07-27, city = Nottingham]

Birmingham–Wolverhampton
Glasgow
Leeds–Bradford
Liverpool
London
Manchester-Salford
Newcastle upon Tyne–Sunderland
Nottingham
Southampton–Portsmouth

{11..20=3, 21..30=4, 31..40=3, 41..50=3, 51..60=4, 61..70=5}

---

无法构造

com.testsample.Data

（不存在与默认构造函数类似的创建者）：无法从[Source:（文件）；第3行，第3列]（通过引用链：java.lang.Object[][0]）处的对象值（无委托或基于属性的创建者）反序列化JSON文件的结构似乎存在问题。我仔细检查了上面作为字符串提供的数据在文件中的解析，结果很好。

[id = 14, age=28, firstName = Mark, lastName = Anthony, dateOfBirth = 1992-01-08, city = London]
[id = 2, age=27, firstName = Edward, lastName = Houston, dateOfBirth = 1992-10-05, city = Southampton–Portsmouth]
[id = 19, age=24, firstName = Mervin, lastName = Lewis, dateOfBirth = 1995-10-27, city = Birmingham–Wolverhampton]
[id = 15, age=29, firstName = Mark, lastName = Watson, dateOfBirth = 1991-07-27, city = Nottingham]

Birmingham–Wolverhampton
Glasgow
Leeds–Bradford
Liverpool
London
Manchester-Salford
Newcastle upon Tyne–Sunderland
Nottingham
Southampton–Portsmouth

{11..20=3, 21..30=4, 31..40=3, 41..50=3, 51..60=4, 61..70=5}