Problem with equals() method in Hibernate

Question

I am developing an application in Hibernate where I have model classes like these:

public class Employee
{
    private int ID;
    private String name;
    private Department department;
    //other properties
    //constructors, getters and setters
}

Note that the ID is not a value populated by the user and is populated using GenerationType.Identity as the strategy.

Also I have another class Department as follows:

public class Department
{
    private int ID;
    private String name;

    private Set<Employee> employees; //this is actually a HashSet

    //other implementations
}

There is a ManyToOne bi-directional relationship between an Employee and a Department.

So to add a new Employee to an existing Department, I do the following

Department existingDepartment = ...;
Employee newEmployee = ...;

existingDepartment.addEmployee(newEmployee);
employee.setDepartent(existinDepartment);

session.save(newEmployee);

Now conceptually two Employee objects are the same if they have the same ID. So my equals() method in the Employee class looks like this:

public boolean equals(Object o)
{
    if(!(o instanceOf Employee))
    {
        return false;
    }

    Employee other = (Employee)o;

    if(this.ID == o.etID())
    {
        return true;
    }

    return false;
}

Now the problem is when I create a new Employee(); I do not have its ID, since it will be assigned when it will be persisted. So when I say

existingDepartment.addEmployee(newEmployee);

the internal HashSet of the Department object is effectively using an equals() method which is broken [since it uses a member variable to determine equality that as not been initialized properly].

This seems like a very basic problem but, how do I solve it? Or am I designing my classes totally wrong? Or should my equals method be re-written to compare other values instead of ID, which I guess would be absurd.

score 7 · Accepted Answer · edited May 23 '17 at 09:59

This seems like a very basic problem but, how do I solve it? Or am I designing my classes totally wrong? Or should my equals method be re-written to compare other values instead of ID, which I guess would be absurd.

There are two different philosophies concerning this.

a) equals() / hashCode() based on DB id

Drawback: you can't compare persistent and non-persistent objects

b) equals() / hashCode() based on contents

Drawback: two objects with the same id may turn out to be non-equal.

I prefer the second approach, it makes more sense from a Java point of view (although admittedly not from a DB point of view). The only thing I'd want to make sure is that you never mix the approaches.

This has been discussed many times before, btw:

Should I write equals() methods in JPA entities?

What is the best practice when implementing equals() for entities with generated ids

JPA : not overriding equals() and hashCode() in the entities?
(etc.)

+1 with a note that the second approach is the only way to do it that won't break hashing collections by mutating a property of a contained object upon which equals() and hashCode() are dependent. — Ryan Stewart, Aug 29 '11 at 15:06

score 1 · Answer 2 · answered Aug 29 '11 at 14:55

1

Rewrite your equals method, so that it returns false, when o is null:

@Override
public boolean equals(Object obj) {
    if (obj == null) {
        return false;
    }
    if (getClass() != obj.getClass()) {
        return false;
    }
    final Employee other = (Employee) obj;
    if (this.id != other.id && (this.id == null || !this.id.equals(other.id))) {
        return false;
    }
    return true;
}

answered Aug 29 '11 at 14:55

chzbrgla

5,158
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My `ID`s are `int`s, primitives. :-) – user801138 Aug 29 '11 at 15:03
"My IDs are ints, primitives." That's usually a very bad idea. – Sean Patrick Floyd Aug 29 '11 at 15:15
@Sean, you shouldn't use boxed primitives unless you have to. This might be a good case to use them but if you use all boxed ints all the time then that's horrible practice and you lose a lot of performance too. – Amir Raminfar Aug 29 '11 at 15:32
@Amir in general, I agree (especially when you are doing arithmetics, of course). But for persistent entities, wrappers are the easiest way to distinguish between set and unset IDs. – Sean Patrick Floyd Aug 29 '11 at 15:34
Agreed on explaining why and where to use wrappers. – Amir Raminfar Aug 29 '11 at 15:36
oops. kind of eluded me that you're using int primitives :) sry – chzbrgla Aug 29 '11 at 18:46

score 0 · Answer 3 · answered Aug 29 '11 at 14:55

0

In hibernate usually you can tell it to use a value when it hasn't been saved to db. For example, I have used -1 for an ID which hasn't been stored yet.

You should initialize your ids like this to make sure you get a consistent behavior.

private int ID = -1;

answered Aug 29 '11 at 14:55

Amir Raminfar

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you are referring to the `unsaved-value` attribute http://docs.jboss.org/hibernate/core/3.3/reference/en/html/mapping.html#mapping-declaration-id – mcabral Aug 29 '11 at 14:57

stefan bachert · Answer 4 · 2011-08-29T15:39:46.250

You may add an transient field with an different non-persistent id. (maybe you should upgrade to "long" id). Something like this for Example

public class Employee {
  private static int lastTID = 0;
  private int ID = -1;
  private transient int tID;
 ..
 public Employee () {
    synchronized (getClass()) {
      tId = -- lastTID;
    }
 }
 public boolean equals(Object o) {
 ..
   Employee other = (Employee)o;
 ..
   if (ID != -1) {
    return ID == other.ID;
   } else {
    return other.ID == -1 && tID == other.tID;
   }
 }

In any case you have to assure that there are not saved and unsaved Employee in use.

Another strategy is to save Employee first and then to add it to Department

Problem with equals() method in Hibernate

4 Answers4