Problem with inheritance?

[bfeater]

I tried out JaVers (version 1.1.0), but encountered a problem. Comparing a property with a value of different subclasses leads to an IllegalArgumentException. Is this intended behaviour?

Example to reproduce the exception:

   class Example {
      class Holder {
         A a;
      }

      class A {}

      class B0 extends A {
         String b0 = "asdf";
      }

      class B1 extends A {
         String b1 = "qwer";
      }

      public void doesNotWork() {
         Javers javers = JaversBuilder.javers().build();

         Holder first = new Holder();
         first.a = new B0();
         Holder second = new Holder();
         second.a = new B1();

         Diff diff = javers.compare(first, second);
      }
   }

[bartoszwalacik]

Looks like a bug, will check it out

[bartoszwalacik]

There is no bug here, but indeed, error message is misleading. It will be fixed in the next release to TYPE_MISMATCH error

What you are doing here is comparing objects with different types.
You cannot compare B0 instance with B1 instance because they have different fields.

Field String b0 is not connected to String b1 despite the same type (String).
It will work only if you abstract the field definition to A class

[bfeater]

Do you really mean that? Perhaps another example will make the problem clear

class Store {
List bicycles;
}

class Bicycle {
int speed;
}

class Mountenbike {
int seatHeight;
}

...
Store store0 = new Store();
store0.bicycles = new ArrayList<>();
store0.bicycles.add(new Bicycle());

Store store1 = new Store();
store1.bicycles = new ArrayList<>();
store0.bicycles.add(new Mountenbike());

Comparing store0 with store1, I would expect a ListChange with a removal
of a Bicycle and an addition of a Mountenbike. But in the current
implementation of JaVers I get an exception.

Am 20.03.2015 um 23:08 schrieb Bartosz Walacik:

There is no bug here, but indeed, error message is misleading. It will be fixed in the next release to TYPE_MISMATCH error

What you are doing here is comparing objects with different types.
You cannot compare B0 instance with B1 instance because they have different fields.

Field String b0 is not connected to String b1 despite the same type (String).
It will work only if you abstract the field definition to A class

---

Reply to this email directly or view it on GitHub:
#127 (comment)

Sebastian Gebhardt
Email: sebastian.gebhardt@bfeater.de
PGP-Public Key: http://www.bfeater.de/bfeater_pubkey.asc

[bartoszwalacik]

Could you please push a mini-project to github with a failing test?

[bfeater]

I have created a small example. In the test I have added a comment with my expected result.

https://github.com/bfeater/javers_inheritance_test

[bartoszwalacik]

ok, will take a look

[bartoszwalacik]

in the 1.1.2-SNAPSHOT, you will get better exception desc:

JaversException: TYPE_MISMATCH JaVers runtime error - Looks like you are comparing two objects with different types. There is no property 'seatHeight' in type 'Bicycle'.

the clue is the same, we don't know how to compare Bicycle with Mountainbike.
If the number of gears (common field) is different, that is clear that objects are different.
But what if the number of gears would be equal? Does it mean that objects are equal? Not sure ...

[bartoszwalacik]

In fact, when you start adding fields to a subclass, it's not possible to write valid equals(), without breaking Liskov Principle. Try to write a valid equals() for both classes (Bicycle with Mountainbike), I mean reflexive, symmetric, transitive and consistent. Imho it's not possible.