Java List转Map

Java List转Map教程

平时工作中，在很多场景下面会用到将 List 集合转换成 Map 对象的格式，提高效率。我们可以使用 Collectors.groupingBy 对指定的属性进行分组，转换成 Map 格式。

数据准备

学生类

package com.haicoder.net.array;
public class Student {
    /**
     * 班级ID
     */
    private Integer calssId;
    /**
     * 学生学号
     */
    private String stuNo;
    /**
     * 学生姓名
     */
    private String stuName;
    /**
     * 学生年龄
     */
    private Integer age;
    public Student(Integer calssId, String stuNo, String stuName, Integer age) {
        this.calssId = calssId;
        this.stuNo = stuNo;
        this.stuName = stuName;
        this.age = age;
    }
   @Override
   public String toString() {
        return "Student{" +
                "calssId=" + calssId +
                ", stuNo='" + stuNo + '\'' +
                ", stuName='" + stuName + '\'' +
                ", age=" + age +
                '}';
   }
  //.... 省略 get set 方法
}

构造集合数据

List<Student> studentList = new ArrayList<>();
studentList.add(new Student(1, "001", "小明",20));
studentList.add(new Student(1, "002", "小军",18));
studentList.add(new Student(1, "003", "小杰",19));
studentList.add(new Student(2, "001", "小亮",20));
studentList.add(new Student(3, "001", "小珍",18));
studentList.add(new Student(2, "002", "小琦",19));
studentList.add(new Student(3, "002", "小甲",15));

案例

根据某个字段进行分组

按照班级分组，以班级 Id 为 key，将数据转换成 Map

Map<Integer, List<Student>> classIdAndStudentMap = studentList.stream().collect(Collectors.groupingBy(Student::getCalssId));

按照属性拼接进行分组

将班级和年龄进行拼接分组

Map<String, List<Student>> classIdAndStudentMap = studentList.stream().collect(Collectors.groupingBy(student -> student.getCalssId() + "-" + student.getAge()));

根据不同条件分组

年龄小于 19 岁分一个组，大于等于 19 岁分另外一个组

Map<String, List<Student>> classIdAndStudentMap = studentList.stream().collect(Collectors.groupingBy(student -> {
            if (student.getAge() < 19) {
                return "未成年";
            } else {
                return "成年";
            }
        }));

求对应key下面的数据总数

获取每个班级下面的学生总数

Map<Integer, Long> classIdAndStudentMap = studentList.stream().collect(Collectors.groupingBy(Student::getCalssId, Collectors.counting()));

完整代码

按照属性拼接进行分组

package com.haicoder.net.array;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class StudentTest {
    public static void main(String[] args) {
        System.out.println("嗨客网(www.haicoder.net)");
        List<Student> studentList = new ArrayList<>();
        studentList.add(new Student(1, "001", "小明", 20));
        studentList.add(new Student(1, "002", "小军", 18));
        studentList.add(new Student(1, "003", "小杰", 19));
        studentList.add(new Student(2, "001", "小亮", 20));
        studentList.add(new Student(3, "001", "小珍", 18));
        studentList.add(new Student(2, "002", "小琦", 19));
        studentList.add(new Student(3, "002", "小甲", 15));
        Map<Integer, List<Student>> classIdAndStudentMap = studentList.stream().collect(Collectors.groupingBy(Student::getCalssId));
        classIdAndStudentMap.entrySet().forEach(tmp -> {
            System.out.println("班级ID：" + tmp.getKey());
            System.out.println("班级学生：" + tmp.getValue().toString());
        });
    }
}

运行结果如下：

按照属性拼接进行分组

package com.haicoder.net.array;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class StudentTest {
    public static void main(String[] args) {
        System.out.println("嗨客网(www.haicoder.net)");
        List<Student> studentList = new ArrayList<>();
        studentList.add(new Student(1, "001", "小明", 20));
        studentList.add(new Student(1, "002", "小军", 18));
        studentList.add(new Student(1, "003", "小杰", 19));
        studentList.add(new Student(2, "001", "小亮", 20));
        studentList.add(new Student(3, "001", "小珍", 18));
        studentList.add(new Student(2, "002", "小琦", 19));
        studentList.add(new Student(3, "002", "小甲", 15));
        Map<String, List<Student>> classIdAndStudentMap = studentList.stream().collect(Collectors.groupingBy(student -> student.getCalssId() + "-" + student.getAge()));
        classIdAndStudentMap.entrySet().forEach(tmp -> {
            System.out.println("key 值：" + tmp.getKey());
            System.out.println("班级学生：" + tmp.getValue().toString());
        });
    }
}

运行结果如下

根据不同条件分组

package com.haicoder.net.array;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class StudentTest {
    public static void main(String[] args) {
        System.out.println("嗨客网(www.haicoder.net)");
        List<Student> studentList = new ArrayList<>();
        studentList.add(new Student(1, "001", "小明", 20));
        studentList.add(new Student(1, "002", "小军", 18));
        studentList.add(new Student(1, "003", "小杰", 19));
        studentList.add(new Student(2, "001", "小亮", 20));
        studentList.add(new Student(3, "001", "小珍", 18));
        studentList.add(new Student(2, "002", "小琦", 19));
        studentList.add(new Student(3, "002", "小甲", 15));
        Map<String, List<Student>> classIdAndStudentMap = studentList.stream().collect(Collectors.groupingBy(student -> {
            if (student.getAge() < 19) {
                return "未成年";
            } else {
                return "成年";
            }
        }));
        classIdAndStudentMap.entrySet().forEach(tmp -> {
            System.out.println("key 值：" + tmp.getKey());
            System.out.println("value 值：" + tmp.getValue().toString());
        });
    }
}

运行结果如下

获取每个班级下面的学生总数

package com.haicoder.net.array;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class StudentTest {
    public static void main(String[] args) {
        System.out.println("嗨客网(www.haicoder.net)");
        List<Student> studentList = new ArrayList<>();
        studentList.add(new Student(1, "001", "小明", 20));
        studentList.add(new Student(1, "002", "小军", 18));
        studentList.add(new Student(1, "003", "小杰", 19));
        studentList.add(new Student(2, "001", "小亮", 20));
        studentList.add(new Student(3, "001", "小珍", 18));
        studentList.add(new Student(2, "002", "小琦", 19));
        studentList.add(new Student(3, "002", "小甲", 15));
        Map<Integer, Long> classIdAndStudentMap = studentList.stream().collect(Collectors.groupingBy(Student::getCalssId, Collectors.counting()));
        classIdAndStudentMap.entrySet().forEach(tmp -> {
            System.out.println("班级ID 值：" + tmp.getKey() + "====》" + "班级学生数量:" + tmp.getValue());
        });
    }
}

运行结果如下