使用 Streams 展平 Java 对象

Posted 2023-02-23

【中文标题】使用 Streams 展平 Java 对象

【英文标题】：Flatten Java Object using Streams

【发布时间】：2019-12-04 07:24:11

【问题描述】：

我有这样的课：

class Student 
    String name;
    String age;
    List<Course> Courses;


class Course 
    String courseName;
    String teacher;

我有：

List<Student> students

我想得到：

List<Object> objects

具有像这样的值：

Kevin  22  CS   Rob
Kevin  22  MT   Chris
Mary   21  AT   Harry
Mary   21  AP   Henry
Mary   21  PP   Joe
Prak   25  null null

使用 Java 8 Streams 最简单的方法是什么？ 可以flatMap帮忙吗？

看看最后一个。即使Prak 没有参加任何课程（即List<Course> 为空），我也想获得记录，只有Student 以及其他课程

【参考方案1】：

请看下面的代码，我还在底部粘贴了输出。在这里，我们可以使用具有四个字符串实例变量的某个对象的 List 而不是使用 List。

List c1= Arrays.asList(new Course("CS", "Rob"), new Course("MT", "Chris"));

    List<Course> c2= Arrays.asList(new Course("AT", "Harry"), new Course("AP","Henry"),  new Course("PP", "Joe"));

    List<Course> c3= Arrays.asList(new Course());
    List<Student> s1= Arrays.asList(new Student("Kevin", "22", c1), new Student("Mary", "21", c2), new Student("Prak", "25", c3));

   List<String> list= s1.stream().flatMap(s-> s.getCourses().stream().map(d1-> s.getName()+" "+s.getAge()+" "+""+d1.getCourseName()+" "+d1.getTeacher()) ).collect(Collectors.toList());
   list.forEach( System.out::println);

输出：

凯文 22 CS Rob 凯文 22 MT 克里斯 玛丽 21 在哈利 玛丽 21 AP 亨利 玛丽 21 PP 乔 Prak 25 null null

【参考方案2】：

如果你想要一个新的对象扁平化，是的，flatMap 和 map 的组合可以工作，但是检查 List 是否为空：

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Test1 

    public static void main(String[] args) 
        Course course1 = new Course("course 1", "teacher 1");
        Course course2 = new Course("course 2", "teacher 2");
        Course course3 = new Course("course 3", "teacher 3");
        Course course4 = new Course("course 4", "teacher 4");
        Student student1 = new Student("name 1", "15", Arrays.asList(course1, course2, course3, course4));
        Student student2 = new Student("name 2", "16", new ArrayList<>());
        Student student3 = new Student("name 2", "16", Arrays.asList(course1, course3));
        Student student4 = new Student("name 4", "17", new ArrayList<>());

        List<Student> students = Arrays.asList(student1, student2, student3, student4);

    List<Objects> objects = students.stream().flatMap(student -> 
        student.getCourses().isEmpty() ?
            Stream.of(new Objects(student.getName(), student.getAge(), null, null)) :
            student.getCourses().stream().map(course -> new Objects(student.getName(), student.getAge(), course.getCourseName(), course.getTeacher()))
    ).collect(Collectors.toList());

        objects.forEach(System.out::println);
    


class Course 
    String courseName;
    String teacher;

    public Course(String courseName, String teacher) 
        this.courseName = courseName;
        this.teacher = teacher;
    

    public String getCourseName() 
        return courseName;
    

    public void setCourseName(String courseName) 
        this.courseName = courseName;
    

    public String getTeacher() 
        return teacher;
    

    public void setTeacher(String teacher) 
        this.teacher = teacher;
    


class Student 
    String name;
    String age;
    List<Course> courses;

    public Student(String name, String age, List<Course> courses) 
        this.name = name;
        this.age = age;
        this.courses = courses;
    

    public String getName() 
        return name;
    

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

    public String getAge() 
        return age;
    

    public void setAge(String age) 
        this.age = age;
    

    public List<Course> getCourses() 
        return courses;
    

    public void setCourses(List<Course> courses) 
        this.courses = courses;
    


class Objects
    String name;
    String age;
    String courseName;
    String teacher;

    public Objects(String name, String age, String courseName, String teacher) 
        this.name = name;
        this.age = age;
        this.courseName = courseName;
        this.teacher = teacher;
    

    public String getName() 
        return name;
    

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

    public String getAge() 
        return age;
    

    public void setAge(String age) 
        this.age = age;
    

    public String getCourseName() 
        return courseName;
    

    public void setCourseName(String courseName) 
        this.courseName = courseName;
    

    public String getTeacher() 
        return teacher;
    

    public void setTeacher(String teacher) 
        this.teacher = teacher;
    

    @Override
    public String toString() 
        return "name: " + this.name + " age: " + this.age + " course name: " + this.getCourseName() + " teacher: " + this.getTeacher();
    

输出

name: name 1 age: 15 course name: course 1 teacher: teacher 1
name: name 1 age: 15 course name: course 2 teacher: teacher 2
name: name 1 age: 15 course name: course 3 teacher: teacher 3
name: name 1 age: 15 course name: course 4 teacher: teacher 4
name: name 2 age: 16 course name: null teacher: null
name: name 2 age: 16 course name: course 1 teacher: teacher 1
name: name 2 age: 16 course name: course 3 teacher: teacher 3
name: name 4 age: 17 course name: null teacher: null