How to get human-readable error text for assertions when comparing complex data objects

Problem statement

This repository explores implementations of the following requirement:

As automation QA engineer
Given I have two objects that I need to compare
And these objects have these properties:

• fields may be non-primitive POJOs
• fields may be non-primitive lists of POJOs
• both top level and nested fields of each POJO may have a set of fields that I don't want to use for comparison (ignored fields)
• all POJOs may have a common ancestor (BaseDto) that may implement some helpful interfaces / instance methods

When I compare two objects in assertion
If objects are not equal
I want to see an error message that:

• does not show ignored fields
• shows only mismatching fields and values of each object recursively
• shows the "path" to mismatching field in object hierarchy (including collection index)
• shows the difference between equality type (mismatching field, value or collection size)
• outputs difference only between primitive/simple/single-field types (or else recurse deeper)
• groups fields of the same object together

Stretch goals:

• uses single source of truth for ignored fields for both Object.equals and assertion message
• will possibly have to resort to reflection for this
• have method to get and use index-like property (id,uuid) in assertion output to make mismatching object identification easier
• have option sort fields in assertion message in the same order as declared data object or alphabetically

This is how the result error message in assertion should look like:

Mismatch between List<User> objects:

[0].name: mismatching value
expected: Sherlock 
actual: Moriarty

[0].address[1].city: mismatching value
expected: London
actual: null

[0].address[1].building: mismatching value
expected: null
actual: 221B

[1].address: mismatching collection size
expected: 2
actual: 0

Implementation details

It is possible to implement this using any assertion framework that allows to provide custom assertion message. For instance, we could collect the data ourselves, see e.g. ReflectionAssertionsTest. This should work regardless of which assertions we use.

But we will use AssertJ since it is almost perfect for our use case and requires only minimal customization and has many features that we need already implemented.

Solution(s)

Check it out. We still had to override the assertion message, but at least the object comparison could be implemented in a nice and extensible way.