line Code Quality Improvement Techniques Part 22 (opens in new tab)
The post argues that developers should avoid overriding the `equals` method to compare only a subset of an object’s properties, as this violates the fundamental principles of identity and structural equivalence. Implementing "partial equality" often leads to subtle, hard-to-trace bugs in reactive programming environments where UI updates depend on detecting changes through equality checks. To ensure system reliability, `equals` must strictly represent either referential identity or total structural equivalence. ### Risks of Partial Equality in Reactive UI * Reactive frameworks such as Kotlin’s `StateFlow`, `Flow`, and Android’s `LiveData` utilize `distinctUntilChanged` logic to optimize performance. * These "observable" patterns compare the new object instance with the previous one using `equals`; if the result is `true`, the update is ignored to prevent unnecessary re-rendering. * If a `UserProfileViewData` object only compares a `userId` field, the UI will fail to reflect changes to a user's nickname or profile image because the framework incorrectly assumes the data has not changed. * To avoid this, any comparison logic that only checks specific fields should be moved to a uniquely named function, such as `hasSameIdWith()`, instead of hijacking the standard `equals` method. ### Defining Identity vs. Equivalence * **Identity (Referential Equality):** This indicates that two references point to the exact same object instance, which is the default behavior of `Object.equals()` in Java or `Any.equals()` in Kotlin. * **Equivalence (Structural Equality):** This indicates that two objects are logically the same because all their properties match. In Kotlin, `data class` implementations provide this by default for all parameters defined in the primary constructor. * Proper implementation of equivalence requires that all fields within the object also have clearly defined equality logic. ### Nuances and Implementation Exceptions * **Kotlin Data Class Limitations:** Only properties declared in the primary constructor are included in the compiler-generated `equals` and `hashCode` methods; properties declared in the class body are ignored by default. * **Calculated Caches:** It is acceptable to exclude certain fields from an equality check if they do not change the logical state of the object, such as a `cachedValue` used to store the results of a heavy mathematical operation. * **Context-Dependent Equality:** The definition of equality can change based on the model's purpose. For example, a mathematical model might treat 1/2 and 2/4 as equal, whereas a UI display model might treat them as different because they represent different strings of text. When implementing `equals`, prioritize full structural equivalence to prevent data-stale bugs in reactive systems. If you only need to compare a unique identifier, create a dedicated method instead of repurposing the standard equality check.