Java Stream流遍历处理 Map 的详细用法

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Java Stream流处理Map 详细用法

我们在使用Stream的时候难免要使用处理流后要自定义生成自己的Map, 应该如何处理? 下面我们用代码来讲解一下 我们要如何生成Map

1. 固定结构 stream().collect(Collectors.toMap(Person::getName,Person::getId))
2. 固定结构, 相同value处理 stream().collect(Collectors.toMap(Person::getName, Person::getId, (v1, v2) -> v1))
3. 分组group操作 stream().collect(Collectors.groupingBy(Person::getSex))
4. 自定义Map结构 Map<String, Set> gradeAgesMap = list.stream() .collect(Collectors.groupingBy(Person::getGrade, Collectors.mapping(Person::getAge, Collectors.toSet())));
5. 自定义Map结构 Map<String, List> gradePIdListMap = list.stream() .collect(Collectors.groupingBy(Person::getGrade, Collectors.mapping(Person::getId, Collectors.toList())));

下面我们用代码实战一下, 看一下结果

实体类

package com.jzj.model;

/**
 * 当前描述：
 *
 * @author: jiazijie
 * @since: 2021/7/23 下午4:06
 */
public class Person {
    private Integer id;
    private String name;
    private Integer age;
    private String grade;

    public Person(Integer id, String name, Integer age, String grade) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.grade = grade;
    }

    public String getName() {
        return name;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getGrade() {
        return grade;
    }

    public void setGrade(String grade) {
        this.grade = grade;
    }
}

Stream方法

package com.jzj.util;

import com.alibaba.fastjson.JSONObject;
import com.xrxs.jzj.model.Person;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collectors;

import static java.util.stream.Collectors.toSet;

/**
 * 当前描述：
 *
 * @author: jiazijie
 * @since: 2021/7/23 下午3:26
 */
public class Test {

    public static void main(String[] args) {
        List<Person> list = new ArrayList<>();
        Person p1 = new Person(1, "a", 18, "A");
        Person p2 = new Person(2, "b", 19, "B");
        Person p3 = new Person(3, "c", 18, "C");
        Person p4 = new Person(4, "a", 21, "A");
        Person p5 = new Person(5, "a", 18, "B");

        list.add(p1);
        list.add(p2);
        list.add(p3);
        list.add(p4);
        list.add(p5);

        /**
         * 获取 每个ID对应的 Person Map
         */
        Map<Integer, Person> mapObj = list.stream().collect(Collectors.toMap(Person::getId, Function.identity(), (v1, v2) -> v1));
        // {1:{"age":18,"grade":"A","id":1,"name":"a"},2:{"age":19,"grade":"B","id":2,"name":"b"},3:{"age":18,"grade":"C","id":3,"name":"c"},4:{"age":21,"grade":"A","id":4,"name":"a"},5:{"age":18,"grade":"B","id":5,"name":"a"}}
        System.out.println(JSONObject.toJSONString(mapObj));

        // 获取 每个名字 对应的ID， 如果有重复的名字，就会报错!!!!!Duplicate key 1 比如 a-1,a-4,a-5 这三个同样的名字对应不同的id
        // Map<String, Integer> mapNameId =list.stream().collect(Collectors.toMap(Person::getName,Person::getId));
        // System.out.println(JSONObject.toJSONString(mapNameId));

        /**
         * 为了避免这种情况，要制定 value (v1,v2)->v1 用第一次出现的value还是最后一次出现的value v1 用第一次出现的
         * v2用最后一次出现的
         */
        Map<String, Integer> mapNameId1 = list.stream().collect(Collectors.toMap(Person::getName, Person::getId, (v1, v2) -> v1));
        Map<String, Integer> mapNameId2 = list.stream().collect(Collectors.toMap(Person::getName, Person::getId, (v1, v2) -> v2));
        // {"a":1,"b":2,"c":3}
        System.out.println(JSONObject.toJSONString(mapNameId1));
        // {"a":5,"b":2,"c":3}
        System.out.println(JSONObject.toJSONString(mapNameId2));

        /**
         * 获取 每个班级的 人员集合，以 班级为分组 group
         */
        Map<String, List<Person>> gradePersonsMap = list.stream().collect(Collectors.groupingBy(Person::getGrade));
        // {"A":[{"age":18,"grade":"A","id":1,"name":"a"},{"age":21,"grade":"A","id":4,"name":"a"}],"B":[{"age":19,"grade":"B","id":2,"name":"b"},{"age":18,"grade":"B","id":5,"name":"a"}],"C":[{"age":18,"grade":"C","id":3,"name":"c"}]}
        System.out.println(JSONObject.toJSONString(gradePersonsMap));

        /**
         * 获取 每个班级的 人员年龄的Set集合 以班级为分组，年龄去重
         */
        Map<String, Set<Integer>> gradeAgesMap = list.stream()
                .collect(Collectors.groupingBy(Person::getGrade, Collectors.mapping(Person::getAge, Collectors.toSet())));
        // {"A":[18,21],"B":[18,19],"C":[18]}
        System.out.println(JSONObject.toJSONString(gradeAgesMap));

        /**
         * 获取 每个班级的 人员ID的List集合 以班级为分组，人员ID
         */
        Map<String, List<Integer>> gradePIdListMap = list.stream()
                .collect(Collectors.groupingBy(Person::getGrade, Collectors.mapping(Person::getId, Collectors.toList())));
        // {"A":[1,4],"B":[2,5],"C":[3]}
        System.out.println(JSONObject.toJSONString(gradePIdListMap));

        /**
         * 获取 每个班级的 人员集合，以 班级为分组 group
         */
        Map<String, List<Person>> gradePersons = list.stream().collect(Collectors.groupingBy(Person::getGrade));

        //// 以Map开始遍历 获取班级 中年龄的 去重集合
        Map<String, Set<Integer>> gradeAgeSetMap = gradePersons.entrySet().stream().collect(
                Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue().stream().map(Person::getAge).collect(toSet()), (v1, v2) -> v1));
        // {"A":[18,21],"B":[18,19],"C":[18]}
        System.out.println(JsonUtil.toJson(gradeAgeSetMap));
    }

}