如何检索已添加到集合中的最新记录？

Question

I have a class called MyClass which keeps a set of student objects as member.

Currently I can pass the id of required student and retrieve the object but not sure how to retrieve the latest one who has been added.

I am wondering how I can change the code to retrieve the object of last student that was added to the class?

"I know set does not maintain order, is there any method to retrieve the latest one using hibernate or by comparing the enrolledDate members of student class?"

MyClass

import java.util.Set

@Entity
public class MyClass {

   @Id
   @GeneratedValue
   private long id;

   @OneToMany
   @LazyCollection(LazyCollectionOption.FALSE)
   private Set<Student> students;

   ....
}

Student class

@Entity
public class Student {

   @Id
   @GeneratedValue
   private long id;

   private String name;

   @Temporal(javax.persistence.TemporalType.DATE)
   private Date enrolledDate; //I may need to use this date as it is enrollment date but not sure how to retrieve the latest one.
}

Hibernate Code

 MyClass myClass = (MyClass) session.get(MyClass.class, id);

 Iterator<MyClass> it = myClass.getStudents().iterator();

 Student stu = new Student();

 while (it.hasNext()) {
            stu = it.next();
            if (stu.getId() == stuId) {
                        break;
            }
 }

System.err.println("Student name is:" + stu.getName());

Answer 1

assuming that the id of student is auto increment you can use TreeSet so that the recent Student will be right most element in the TreeSet (write your compareTo() on top of id ).You can use last() of TreeSet which will give you the latest.

Even instead of id you can use anything that is ascending order (write compareTo() accordingly ),and put student into TreeSet and fetch last() ie recent student object.

Hope it helps.

Answer 2

Okay a Set (or some other collection) in Hibernate can be ordered in one of two ways: using @Sort (in-memory sort) or using @OrderBy (database sort).

As @OrderBy cannot be used on nested properties (eg myclass.student.dateEnrolled) you will need to use @Sort.

http://docs.jboss.org/hibernate/core/3.5/javadocs/org/hibernate/annotations/Sort.html

http://eubauer.de/kingsware/2011/03/15/ordering-collections-with-jpa/

All you need to do then is:

@OneToMany
@Sort(...)
private Set<Student> students; //note leave as Set

If the types in the Set implement Comparable and this is the Order you want you can specify SortType.NATURAL. Otherwise you can specify a Comparator to handle the sorting.

Hibernate will ensure the Set passed in is sorted. If we assume this Set implements the NavigableSet Interface (and you would need to check this) then you can use the methods of that Interface to find the latest:

http://docs.oracle.com/javase/6/docs/api/java/util/NavigableSet.html

Note that if there were a large number of students and you only ever wanted the latest then this is a sub-optimal solution as all Students will be loaded.

Answer 3

Set不能维持顺序，您可以尝试其他Collections，例如LinkedHashSet，它以插入顺序存储元素，或者可以通过PriorityQueue从两端进行访问。