(#) Mutating an Immutable SharedPrefs Set !!! WARNING: Mutating an Immutable SharedPrefs Set This is a warning. Id : `MutatingSharedPrefs` Summary : Mutating an Immutable SharedPrefs Set Severity : Warning Category : Correctness Platform : Android Vendor : Android Open Source Project Feedback : https://issuetracker.google.com/issues/new?component=192708 Since : 4.2.0 (May 2021) Affects : Kotlin and Java files Editing : This check runs on the fly in the IDE editor 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/SharedPrefsDetector.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/SharedPrefsDetectorTest.kt) As stated in the docs for `SharedPreferences.getStringSet`, you must not modify the set returned by `getStringSet`: "Note that you must not modify the set instance returned by this call. The consistency of the stored data is not guaranteed if you do, nor is your ability to modify the instance at all." (##) Example Here is an example of lint warnings produced by this check: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text src/test/pkg/test.kt:11:Warning: Do not modify the set returned by SharedPreferences.getStringSet()` [MutatingSharedPrefs] s.removeIf { it.length < 3 } ---------------------------- src/test/pkg/test.kt:12:Warning: Do not modify the set returned by SharedPreferences.getStringSet()` [MutatingSharedPrefs] s.add("error") -------------- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is the source file referenced above: `src/test/pkg/test.kt`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers package test.pkg import android.content.SharedPreferences import android.os.Build import androidx.annotation.RequiresApi @RequiresApi(Build.VERSION_CODES.N) fun editSet(prefs: SharedPreferences) { val s = prefs.getStringSet("key", null) // ?: return s ?: return s.removeIf { it.length < 3 } s.add("error") val t = mutableSetOf() t.add("ok") } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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/SharedPrefsDetectorTest.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, `SharedPrefsDetector.testGetStringSet`. 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("MutatingSharedPrefs") fun method() { getStringSet(...) } ``` or ```java // Java @SuppressWarnings("MutatingSharedPrefs") void method() { getStringSet(...); } ``` * Using a suppression comment like this on the line above: ```kt //noinspection MutatingSharedPrefs 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="MutatingSharedPrefs" 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 'MutatingSharedPrefs' } ``` In Android projects this should be nested inside an `android { }` block. * For manual invocations of `lint`, using the `--ignore` flag: ``` $ lint --ignore MutatingSharedPrefs ...` ``` * Last, but not least, using baselines, as discussed [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).