Checking for deep equality in JUnit tests

Question

I am writing unit tests for objects that are cloned, serialized, and/or written to an XML file. In all three cases I would like to verify that the resulting object is the "same" as the original one. I have gone through several iterations in my approach and having found fault with all of them, was wondering what other people did.

My first idea was to manually implement the equals method in all the classes, and use assertEquals. I abandoned this this approach after deciding that overriding equals to perform a deep compare on mutable objects is a bad thing, as you almost always want collections to use reference equality for mutable objects they contain[1].

Then I figured I could just rename the method to contentEquals or something. However, after thinking more, I realized this wouldn't help me find the sort of regressions I was looking for. If a programmer adds a new (mutable) field, and forgets to add it to the clone method, then he will probably forget to add it to the contentEquals method too, and all these regression tests I'm writing will be worthless.

I then wrote a nifty assertContentEquals function that uses reflection to check the value of all the (non-transient) members of an object, recursively if necessary. This avoids the problems with the manual compare method above since it assumes by default that all fields must be preserved and the programmer must explicitly declare fields to skip. However, there are legitimate cases when a field really shouldn't be the same after cloning[2]. I put in an extra parameter toassertContentEquals that lists which fields to ignore, but since this list is declared in the unit test, it gets real ugly real fast in the case of recursive checking.

So I am now thinking of moving back to including a contentEquals method in each class being tested, but this time implemented using a helper function similar to the assertContentsEquals described above. This way when operating recursively, the exemptions will be defined in each individual class.

Any comments? How have you approached this issue in the past?

Edited to expound on my thoughts:

[1]I got the rational for not overriding equals on mutable classes from this article. Once you stick a mutable object in a Set/Map, if a field changes then its hash will change but its bucket will not, breaking things. So the options are to not override equals/getHash on mutable objects or have a policy of never changing a mutable object once it has been put into a collection.

I didn't mention that I am implementing these regression test on an existing codebase. In this context, the idea of changing the definition of equals, and then having to find all instances where it could change the behavior of the software frightens to me. I feel like I could easily break more than I fix.

[2]One example in our code base is a graph structure, where each node needs a unique identifier to use to link the nodes XML when eventually written to XML. When we clone these objects we want the identifier to be different, but everything else to remain the same. After ruminating about it more, it seems like the questions "is this object already in this collection" and "are these objects defined the same", use fundamentally different concepts of equality in this context. The first is asking about identity and I would want the ID included if doing a deep compare, while the second is asking about similarity and I don't want the ID included. This is making me lean more against implementing the equals method.

Do you guys agree with this decision, or do you think that implementing equals is the better way to go?

Answer 1

I would go with the reflection approach and define a custom Annotation with RetentionPolicy.RUNTIME to allow the implementers of the tested classes to mark the fields that are expected to change after cloning. You can then check the annotation with reflection and skip the marked fields.

This way you can keep your test code generic and simple and have a convenient means to mark exceptions directly in the code without affecting the design or runtime behavior of the code that needs to be tested.

The annotation could look like this:

import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD})
public @interface ChangesOnClone
{
}

This is how it can be used in the code that is to be tested:

class ABC
{
     private String name;

     @ChangesOnClone
     private Cache cache;
}

And finally the relevant part of the test code:

for ( Field field : fields )
{
    if( field.getAnnotation( ChangesOnClone.class ) )
        continue;
    // else test it
}