Java8Stream的flatmap方法使用

// 老师
public class Teacher {
    private String id;
    private String name;
    private String subject;
}
// 学生
public class Student {
    private String id;
    private String name;
    private String techId;
    // 重写hashCode及equals方法(id及name相同就为同一个学生)
}
// 家长
public class Parents {
    private String id;
    private String name;
    private String chirldId;
    private String email;
}
// 课程
public enum Subject {
    private String value;
    private String desc;
    Subject(String value, String desc) {
        this.value = value;
        this.desc = desc;
    }
    Math("1", "数学"),
    Chinese("2", "汉语"),
    Music("3", "音乐"),
    English("4", "英语");
}

public class Test {
    // 模拟数据
    public static Student s1 = new Student("1", "zhangsan", "001");
    public static Student s2 = new Student("2", "lisi", "001");
    public static Student s3 = new Student("3", "wangwu", "001");
    public static Student s4 = new Student("4", "zhaoliu", "001");
    public static Student s5 = new Student("6", "tianqi", "001");
    public static Student s6 = new Student("6", "tianqi", "002");
    public static Student s7 = new Student("6", "tianqi", "003");
    public static Teacher t1 = new Teacher("001", "xiaoming", Subject.Math.getValue());
    public static Teacher t2 = new Teacher("002", "lihua", Subject.Music.getValue());
    public static Teacher t3 = new Teacher("003", "hanmeimei", Subject.Math.getValue());
    public static Teacher t4 = new Teacher("004", "lihua", Subject.English.getValue());
    public static List<Student> stus = new ArrayList<>();
    public static List<Teacher> teacs = new ArrayList<>();
    static {
        stus.add(s1);
        stus.add(s2);
        stus.add(s3);
        stus.add(s4);
        stus.add(s5);
        stus.add(s6);
        stus.add(s7);
        teacs.add(t1);
        teacs.add(t2);
        teacs.add(t3);
        teacs.add(t4);
    }
 
 
    public static void main(String[] args) {
        // 找到所有数学老师的学生的家长的电话,并找他们开家长会
        List<Parents> collect = teacs.stream()
                // 过滤数学老师
                .filter(t -> Subject.Math.getValue().equals(t.getSubject()))
                // 通过老师找学生
                .flatMap(t -> stus.stream().filter(s -> s.getTechId().equals(t.getId())))
                // 过滤重复的学生(使用student的equals和hashCode方法)
                .distinct()
                // 通过学生找家长(这里就简化为创建家长对象)
                .map(s -> {
                    Parents p = new Parents();
                    p.setId(UUID.randomUUID().toString());
                    p.setChirldId(s.getId());
                    p.setName(s.getName().toUpperCase() + "‘s Parent");
                    p.setEmail((int) (Math.random() * 1000000) + "@qq.com");
                    return p;
                })
                .collect(Collectors.toList());
        // 打印到控制台看看
        collect.stream()
                .forEach(System.out::println);
    }
}

Parents{id=‘3d9312eb-0df5-4ec6-998f-94a32c2253b4‘, name=‘LISI‘s Parent‘, chirldId=‘2‘, telNo=‘844668@qq.com‘}
Parents{id=‘7f0b92f5-872d-4671-982d-ef1b48840ce3‘, name=‘WANGWU‘s Parent‘, chirldId=‘3‘, telNo=‘563932@qq.com‘}
Parents{id=‘c318bffd-8c6d-4849-8109-9c686c97fb77‘, name=‘ZHAOLIU‘s Parent‘, chirldId=‘4‘, telNo=‘108022@qq.com‘}
Parents{id=‘a4ff1bbc-c9b6-4ad2-872c-f4df670c7bb6‘, name=‘TIANQI‘s Parent‘, chirldId=‘6‘, telNo=‘658956@qq.com‘}

public class Test {
    public static void main(String[] args) {
        List<Parents> pars = new ArrayList<>();
        Set<Student> targetStudents = new HashSet<>();
        for (Teacher t : teacs) {
            if (t.getSubject().equals(Subject.Math.getValue())) {
                for (Student s : stus) {
                    if (s.getTechId().equals(t.getId())) {
                        targetStudents.add(s);
                    }
                }
            }
        }
        for (Student s : targetStudents) {
            Parents p = new Parents();
            p.setId(UUID.randomUUID().toString());
            p.setChirldId(s.getId());
            p.setName(s.getName().toUpperCase() + "‘s Parent");
            p.setEmail((int) (Math.random() * 1000000) + "@qq.com");
            pars.add(p);
        }
        for (Parents p : pars) {
            System.out.println(p);
        }
    }
}