Java Order by与不同的子对象类型一对一关系

问题

如果id是ExamObject，则需要根据ExamTask排序ExamObjects；如果是questionNumber，则需要根据ExamQuestion对其排序。我怎样才能做到这一点？

重要

考试只有一组ExamTask或ExamQuestion。换句话说，一个考试不能同时包含ExamTask和ExamQuestion。

背景信息

我有一个名为Exam的Entity类，该类可以包含一个或多个ExamObject实体。

@Entity
public class Exam {
   @OneToMany(mappedBy = "exam" ...)
   @OrderBy("id") //I need to order this by question number if its ExamQuestion
   private Set<ExamObject> objects;
   ...
}

ExamObject可以使用JOINED分为以下两种类型

ExamTask扩展了ExamObject

ExamQuestion扩展了ExamObject并具有称为questionNumber的列

ExamObject

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public class ExamObject {
    @Id
    private Long id;
    ...

ExamTask

@Entity
@PrimaryKeyJoinColumn(name = "id", referencedColumnName = "id")
public class ExamTask extends ExamObject{
   ...

ExamQuestion

@Entity
@PrimaryKeyJoinColumn(name = "id", referencedColumnName = "id")
public class ExamQuestion extends ExamObject{
   @Column(name = "question_number")
   private Integer questionNumber;
   ...

最佳答案

通过声明集合为TreeSet类型并向构造函数提供Comparator，插入的项目将自动排序。

new TreeSet<ExamObject>(new Comparator<ExamObject>(){

    @Override
    public int compare(ExamObject a, ExamObject b){
        int q1 = 0, q2 = 0;
        if(a instanceof ExamQuestion){
            q1 = ((ExamQuestion)a).questionNumber;
            if(b instanceof ExamQuestion){
                q2 = ((ExamQuestion)b).questionNumber;
            }else{
                q2 = ((ExamTask)b).id;
            }
        }else{
            q1 = ((ExamTask)a).id;
            if(b instanceof ExamQuestion){
                q2 = ((ExamQuestion)b).questionNumber;
            }else{
                q2 = ((ExamTask)b).id;
            }
        }
        if(q1 == q2) return 0;
        else return q1 < q2  ? -1 : 1;
    }
});