实体类List中以某字段为key，获取另外一个字段的List

List转Map>

为了保证程序的性能，如果要分组查数据库，一般会用到JAVA8 中的

Collection.stream().collect(Collectors.groupingBy(Entity::getXXXX));

得到Map>,但如果想的到的list中存的是Entity里的某个字段时  就需要再次循环遍历获取，感觉有些麻烦，所以在网上找了一些简便写法，整理总结下。

假设有学生信息实体类如下：

@Data
@AllArgsConstructor
@NoArgsConstructor
class StudentInfo {
    /**
     * 姓名
     */
    private String name;
    /**
     * 班级
     */
    private Integer classNum;
}

学生信息有学生信息List如下：

StudentInfo a = new StudentInfo("张三", 1);
StudentInfo b = new StudentInfo("李四", 1);
StudentInfo c = new StudentInfo("王五", 1);
StudentInfo d = new StudentInfo("赵大", 2);
StudentInfo e = new StudentInfo("钱二", 2);
StudentInfo f = new StudentInfo("周六", 3);

List studentInfos = new ArrayList<>(Arrays.asList(a, b, c, d, e, f));

想获各个班级下的学生姓名（想要获得效果）

1 : [张三, 李四, 王五]
2 : [赵大, 钱二]
3 : [周六]

首先能想到的写法如下 JAVA8中的 Collectors.groupingBy：

public static void main(String[] args) {
	StudentInfo a = new StudentInfo("张三", 1);
	StudentInfo b = new StudentInfo("李四", 1);
	StudentInfo c = new StudentInfo("王五", 1);
	StudentInfo d = new StudentInfo("赵大", 2);
	StudentInfo e = new StudentInfo("钱二", 2);
	StudentInfo f = new StudentInfo("周六", 3);

	List studentInfos = new ArrayList<>(Arrays.asList(a, b, c, d, e, f));

	Map> collect = studentInfos.stream().collect(Collectors.groupingBy(StudentInfo::getClassNum));
	Map> class2Name = new HashMap<>();
	for (int i : collect.keySet()) {
		List collect1 = collect.get(i).stream().map(StudentInfo::getName).collect(Collectors.toList());
		class2Name.put(i, collect1);
	}
	class2Name.forEach((integer, stringList) -> System.out.println(integer + " : " + stringList));
}

总感觉应该还有高级写法，在网上寻找后，发现如下写法

使用 JAVA8 中的 computeIfAbsent

public static void main(String[] args) {
	StudentInfo a = new StudentInfo("张三", 1);
	StudentInfo b = new StudentInfo("李四", 1);
	StudentInfo c = new StudentInfo("王五", 1);
	StudentInfo d = new StudentInfo("赵大", 2);
	StudentInfo e = new StudentInfo("钱二", 2);
	StudentInfo f = new StudentInfo("周六", 3);

	List studentInfos = new ArrayList<>(Arrays.asList(a, b, c, d, e, f));

	Map> stuMap1 = new HashMap<>();
	studentInfos.forEach(studentInfo -> stuMap1.computeIfAbsent(studentInfo.getClassNum(), ArrayList::new).add(studentInfo.getName()));
	stuMap1.forEach((integer, stringList) -> System.out.println(integer + " : " + stringList));
}

使用JAVA8中的 Collectors.toMap

public static void main(String[] args) {
	StudentInfo a = new StudentInfo("张三", 1);
	StudentInfo b = new StudentInfo("李四", 1);
	StudentInfo c = new StudentInfo("王五", 1);
	StudentInfo d = new StudentInfo("赵大", 2);
	StudentInfo e = new StudentInfo("钱二", 2);
	StudentInfo f = new StudentInfo("周六", 3);

	List studentInfos = new ArrayList<>(Arrays.asList(a, b, c, d, e, f));

	Map> stuMap = studentInfos.stream()
			.collect(Collectors.toMap(StudentInfo::getClassNum, x -> Lists.newArrayList(x.getName()), (objects, objects2) -> {
				objects.addAll(objects2);
				return objects;
			}));
	stuMap.forEach((integer, stringList) -> System.out.println(integer + " : " + stringList));
	System.out.println("-----------------------------------");


	Map> stuMap2 = studentInfos.stream()
			.collect(Collectors.toMap(StudentInfo::getClassNum, x -> new ArrayList<>(Collections.singletonList(x.getName())), (objects, objects2) -> {
				objects.addAll(objects2);
				return objects;
			}));
	stuMap2.forEach((integer, stringList) -> System.out.println(integer + " : " + stringList));
}

附上整个测试代码：

package com.example.demo;

import com.alibaba.fastjson.JSON;
import com.google.common.collect.Lists;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.*;
import java.util.stream.Collectors;

public class List2MapTest {

    public static void main(String[] args) {
        StudentInfo a = new StudentInfo("张三", 1);
        StudentInfo b = new StudentInfo("李四", 1);
        StudentInfo c = new StudentInfo("王五", 1);
        StudentInfo d = new StudentInfo("赵大", 2);
        StudentInfo e = new StudentInfo("钱二", 2);
        StudentInfo f = new StudentInfo("周六", 3);

        List studentInfos = new ArrayList<>(Arrays.asList(a, b, c, d, e, f));

        /*studentInfos.add(a);
        studentInfos.add(b);
        studentInfos.add(c);
        studentInfos.add(d);
        studentInfos.add(e);
        studentInfos.add(f);*/

        Map> stuMap1 = new HashMap<>();
        studentInfos.forEach(studentInfo -> stuMap1.computeIfAbsent(studentInfo.getClassNum(), ArrayList::new).add(studentInfo.getName()));
        stuMap1.forEach((integer, stringList) -> System.out.println(integer + " : " + stringList));
        System.out.println("-----------------------------------");


        Map> stuMap = studentInfos.stream()
                .collect(Collectors.toMap(StudentInfo::getClassNum, x -> Lists.newArrayList(x.getName()), (objects, objects2) -> {
                    objects.addAll(objects2);
                    return objects;
                }));
        stuMap.forEach((integer, stringList) -> System.out.println(integer + " : " + stringList));
        System.out.println("-----------------------------------");


        Map> stuMap2 = studentInfos.stream()
                .collect(Collectors.toMap(StudentInfo::getClassNum, x -> new ArrayList<>(Collections.singletonList(x.getName())), (objects, objects2) -> {
                    objects.addAll(objects2);
                    return objects;
                }));
        stuMap2.forEach((integer, stringList) -> System.out.println(integer + " : " + stringList));
        System.out.println("-----------------------------------");


        Map> collect = studentInfos.stream().collect(Collectors.groupingBy(StudentInfo::getClassNum));
        System.out.println(JSON.toJSON(collect));

        Map> class2Name = new HashMap<>();
        for (int i : collect.keySet()) {
            List collect1 = collect.get(i).stream().map(StudentInfo::getName).collect(Collectors.toList());
            class2Name.put(i, collect1);
        }
        class2Name.forEach((integer, stringList) -> System.out.println(integer + " : " + stringList));
    }
}

@Data
@AllArgsConstructor
@NoArgsConstructor
class StudentInfo {
    /**
     * 姓名
     */
    private String name;
    /**
     * 班级
     */
    private Integer classNum;
}

 

最后再附上要用到的依赖

	org.projectlombok
	lombok
	1.16.12



	com.alibaba
	fastjson
	1.2.49



	com.google.guava
	guava
	26.0-jre