TreeSet实现自定义类的有序输出

        TreeSet是唯一有序的，如果是基本数据类型，它会按着自然顺序由小到大输出，但是涉及到自定义类型，就会涉及到各个属性比如我们自定义一个类Student，需要实现Comparable接口，实现自定义类型的排序

class Student implements Comparable<Student> {
  String name;
  int sid;
  double score;

  public Student(String name, int sid, double score) {
    super();
    this.name = name;
    this.sid = sid;
    this.score = score;
  }

  @Override
  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((name == null) ? 0 : name.hashCode());
    long temp;
    temp = Double.doubleToLongBits(score);
    result = prime * result + (int) (temp ^ (temp >>> 32));
    result = prime * result + sid;
    return result;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj)
      return true;
    if (obj == null)
      return false;
    if (getClass() != obj.getClass())
      return false;
    Student other = (Student) obj;
    if (name == null) {
      if (other.name != null)
        return false;
    } else if (!name.equals(other.name))
      return false;
    if (Double.doubleToLongBits(score) != Double
        .doubleToLongBits(other.score))
      return false;
    if (sid != other.sid)
      return false;
    return true;
  }

  @Override
  public String toString() {
    return "Student [name=" + name + ", sid=" + sid + ", score=" + score
        + "]";
  }

  public int compareTo(Student o) {
    // 按照Student类的id输出
    return this.sid-o.sid;
  }

}      

测试程序结果是否符合要求：

public class SetTest {

  public static void main(String[] args) {
    Set<Student> set = new TreeSet<Student>();
    set.add(new Student("宝玉", 1, 89));
    set.add(new Student("黛玉", 2, 87));
    set.add(new Student("袭人", 5, 87));
    set.add(new Student("宝钗", 4, 87));
    set.add(new Student("花花", 3, 89.9));
    for (Student student : set) {
      System.out.println(student);
      
    }
  }

}      

输出结果

Student [name=宝玉, sid=1, score=89.0]
Student [name=黛玉, sid=2, score=87.0]
Student [name=花花, sid=3, score=89.9]
Student [name=宝钗, sid=4, score=87.0]
Student [name=袭人, sid=5, score=87.0]