(#) Suspicious DiffUtil Equality !!! ERROR: Suspicious DiffUtil Equality This is an error. Id : `DiffUtilEquals` Summary : Suspicious DiffUtil Equality Severity : Error Category : Correctness Platform : Android Vendor : Android Open Source Project Feedback : https://issuetracker.google.com/issues/new?component=192708 Since : 3.4.0 (April 2019) Affects : Kotlin and Java files Editing : This check runs on the fly in the IDE editor See : https://issuetracker.google.com/116789824 Implementation : [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/DiffUtilDetector.kt) Tests : [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/DiffUtilDetectorTest.kt) `areContentsTheSame` is used by `DiffUtil` to produce diffs. If the method is implemented incorrectly, such as using identity equals instead of equals, or calling equals on a class that has not implemented it, weird visual artifacts can occur. (##) Example Here is an example of lint warnings produced by this check: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text src/test/pkg/test.kt:10:Error: Suspicious equality check: Did you mean == instead of === ? [DiffUtilEquals] oldItem === newItem // ERROR --- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here are the relevant source files: `src/test/pkg/test.kt`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers package test.pkg import androidx.recyclerview.widget.DiffUtil private val diffCallback = object : DiffUtil.ItemCallback() { override fun areItemsTheSame(oldItem: Cheese, newItem: Cheese): Boolean = oldItem.id === newItem.id override fun areContentsTheSame(oldItem: Cheese, newItem: Cheese): Boolean = oldItem === newItem // ERROR } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ `src/test/pkg/MyCallback.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package test.pkg; import androidx.recyclerview.widget.DiffUtil; public class MyCallback extends DiffUtil.ItemCallback { @Override public boolean areItemsTheSame(Cheese oldItem, Cheese newItem) { return oldItem.getId() == newItem.getId(); } @Override public boolean areContentsTheSame(Cheese oldItem, Cheese newItem) { return oldItem == newItem; } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ `src/test/pkg/Cheese.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package test.pkg; public class Cheese { public String id; } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You can also visit the [source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/DiffUtilDetectorTest.kt) for the unit tests for this check to see additional scenarios. The above example was automatically extracted from the first unit test found for this lint check, `DiffUtilDetector.testIdentityOperator`. To report a problem with this extracted sample, visit https://issuetracker.google.com/issues/new?component=192708. (##) Suppressing You can suppress false positives using one of the following mechanisms: * Using a suppression annotation like this on the enclosing element: ```kt // Kotlin @Suppress("DiffUtilEquals") fun method() { problematicStatement() } ``` or ```java // Java @SuppressWarnings("DiffUtilEquals") void method() { problematicStatement(); } ``` * Using a suppression comment like this on the line above: ```kt //noinspection DiffUtilEquals problematicStatement() ``` * Using a special `lint.xml` file in the source tree which turns off the check in that folder and any sub folder. A simple file might look like this: ```xml <?xml version="1.0" encoding="UTF-8"?> <lint> <issue id="DiffUtilEquals" severity="ignore" /> </lint> ``` Instead of `ignore` you can also change the severity here, for example from `error` to `warning`. You can find additional documentation on how to filter issues by path, regular expression and so on [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html). * In Gradle projects, using the DSL syntax to configure lint. For example, you can use something like ```gradle lintOptions { disable 'DiffUtilEquals' } ``` In Android projects this should be nested inside an `android { }` block. * For manual invocations of `lint`, using the `--ignore` flag: ``` $ lint --ignore DiffUtilEquals ...` ``` * Last, but not least, using baselines, as discussed [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).