(#) Method is missing the @CheckReturnValue annotation !!! WARNING: Method is missing the @CheckReturnValue annotation This is a warning. Id : `RxJava2MethodMissingCheckReturnValue` Summary : Method is missing the @CheckReturnValue annotation Severity : Warning Category : Correctness Platform : Any Vendor : vanniktech/lint-rules/ Feedback : https://github.com/vanniktech/lint-rules/issues Min : Lint 8.0 and 8.1 Compiled : Lint 8.0 and 8.1 Artifact : [com.vanniktech:lint-rules-rxjava2](com_vanniktech_lint-rules-rxjava2.md.html) Since : 0.6.0 Affects : Kotlin and Java files Editing : This check runs on the fly in the IDE editor Implementation : [Source Code](https://github.com/vanniktech/lint-rules/tree/master/lint-rules-rxjava2-lint/src/main/kotlin/com/vanniktech/lintrules/rxjava2/RxJava2MethodMissingCheckReturnValueDetector.kt) Tests : [Source Code](https://github.com/vanniktech/lint-rules/tree/master/lint-rules-rxjava2-lint/src/test/kotlin/com/vanniktech/lintrules/rxjava2/RxJava2MethodMissingCheckReturnValueDetectorTest.kt) Methods returning RxJava Reactive Types should be annotated with the @CheckReturnValue annotation. Static analyze tools such as Lint or ErrorProne can detect when the return value of a method is not used. This is usually an indication of a bug. If this is done on purpose (e.g. fire & forget) it should be stated explicitly. !!! Tip This lint check has an associated quickfix available in the IDE. (##) Example Here is an example of lint warnings produced by this check: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text src/foo/Example.java:15:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] private Observable<Object> observable() { ---------- src/foo/Example.java:19:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] protected Flowable<Object> flowable() { -------- src/foo/Example.java:23:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] protected Single<Object> single() { ------ src/foo/Example.java:27:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] protected Maybe<Object> single() { ------ src/foo/Example.java:31:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] public Completable completable() { ----------- src/foo/Example.java:35:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] public Disposable disposable() { ---------- src/foo/Example.java:39:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] public CompositeDisposable compositeDisposable() { ------------------- src/foo/Example.java:43:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] public TestObserver testObserver() { ------------ src/foo/Example.java:47:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] public TestSubscriber testSubscriber() { -------------- src/foo/Example.java:51:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] public Scheduler scheduler() { --------- src/foo/Example.java:55:Warning: Method should have @CheckReturnValue annotation [RxJava2MethodMissingCheckReturnValue] private Observable<List<Object>> observableList() { -------------- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is the source file referenced above: `src/foo/Example.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package foo; import io.reactivex.Observable; import io.reactivex.Flowable; import io.reactivex.Single; import io.reactivex.Maybe; import io.reactivex.Completable; import io.reactivex.disposables.Disposable; import io.reactivex.disposables.CompositeDisposable; import io.reactivex.observers.TestObserver; import io.reactivex.subscribers.TestSubscriber; import io.reactivex.Scheduler; class Example { private Observable observable() { return null; } protected Flowable flowable() { return null; } protected Single single() { return null; } protected Maybe single() { return null; } public Completable completable() { return null; } public Disposable disposable() { return null; } public CompositeDisposable compositeDisposable() { return null; } public TestObserver testObserver() { return null; } public TestSubscriber testSubscriber() { return null; } public Scheduler scheduler() { return null; } private Observable> observableList() { return null; } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You can also visit the [source code](https://github.com/vanniktech/lint-rules/tree/master/lint-rules-rxjava2-lint/src/test/kotlin/com/vanniktech/lintrules/rxjava2/RxJava2MethodMissingCheckReturnValueDetectorTest.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, `RxJava2MethodMissingCheckReturnValueDetector.methodMissingCheckReturnValue`. To report a problem with this extracted sample, visit https://github.com/vanniktech/lint-rules/issues. (##) Including !!! This is not a built-in check. To include it, add the below dependency to your project. This lint check is included in the lint documentation, but the Android team may or may not agree with its recommendations. ``` // build.gradle.kts lintChecks("com.vanniktech:lint-rules-rxjava2:0.25.0") // build.gradle lintChecks 'com.vanniktech:lint-rules-rxjava2:0.25.0' // build.gradle.kts with version catalogs: lintChecks(libs.lint.rules.rxjava2) # libs.versions.toml [versions] lint-rules-rxjava2 = "0.25.0" [libraries] # For clarity and text wrapping purposes the following declaration is # shown split up across lines, but in TOML it needs to be on a single # line (see https://github.com/toml-lang/toml/issues/516) so adjust # when pasting into libs.versions.toml: lint-rules-rxjava2 = { module = "com.vanniktech:lint-rules-rxjava2", version.ref = "lint-rules-rxjava2" } ``` 0.25.0 is the version this documentation was generated from; there may be newer versions available. [Additional details about com.vanniktech:lint-rules-rxjava2](com_vanniktech_lint-rules-rxjava2.md.html). (##) 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("RxJava2MethodMissingCheckReturnValue") fun method() { problematicStatement() } ``` or ```java // Java @SuppressWarnings("RxJava2MethodMissingCheckReturnValue") void method() { problematicStatement(); } ``` * Using a suppression comment like this on the line above: ```kt //noinspection RxJava2MethodMissingCheckReturnValue 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="RxJava2MethodMissingCheckReturnValue" 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 'RxJava2MethodMissingCheckReturnValue' } ``` In Android projects this should be nested inside an `android { }` block. * For manual invocations of `lint`, using the `--ignore` flag: ``` $ lint --ignore RxJava2MethodMissingCheckReturnValue ...` ``` * Last, but not least, using baselines, as discussed [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).