Java8 Stream flatmap中间操作用法解析

// 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);
  }
}

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);
    }
  }
}