(#) Implicit SAM Instances !!! WARNING: Implicit SAM Instances This is a warning. Id : `ImplicitSamInstance` Summary : Implicit SAM Instances Severity : Warning Category : Correctness Platform : Any 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://kotlinlang.org/docs/fun-interfaces.html#sam-conversions 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/SamDetector.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/SamDetectorTest.kt) Kotlin's support for SAM (single abstract method) interfaces lets you pass a lambda to the interface. This will create a new instance on the fly even though there is no explicit constructor call. If you pass one of these lambdas or method references into a method which (for example) stores or compares the object identity, unexpected results may happen. In particular, passing a lambda variable in as a listener, and then later attempting to remove the listener will not work because a different instance is passed in. !!! 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/test/pkg/test.kt:21:Warning: Implicit new MyInterface instance being passed to method which ends up checking instance equality; this can lead to subtle bugs [ImplicitSamInstance] handler.delete(lambda) // ERROR 1 ------ src/test/pkg/test.kt:22:Warning: Implicit new MyInterface instance being passed to method which ends up checking instance equality; this can lead to subtle bugs [ImplicitSamInstance] handler.compareIdentity1(lambda) // ERROR 2 ------ src/test/pkg/test.kt:23:Warning: Implicit new MyInterface instance being passed to method which ends up checking instance equality; this can lead to subtle bugs [ImplicitSamInstance] handler.compareIdentity2(lambda) // ERROR 3 ------ src/test/pkg/test.kt:31:Warning: Implicit new MyInterface instance being passed to method which ends up checking instance equality; this can lead to subtle bugs [ImplicitSamInstance] handler.delete(lambda2) // ERROR 4 ------- src/test/pkg/test.kt:35:Warning: Implicit new MyInterface instance being passed to method which ends up checking instance equality; this can lead to subtle bugs [ImplicitSamInstance] handler.delete(lambda3) // ERROR 5 ------- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here are the relevant source files: `src/test/pkg/test.kt`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers @file:Suppress("RedundantSamConstructor", "MoveLambdaOutsideParentheses") package test.pkg fun test(handler: MyHandler, list: List) { handler.handle(MyInterface { println("hello") }) // OK handler.handle({ println("hello") }) // OK handler.stash(MyInterface { println("hello") }, list) // OK handler.stash({ println("hello") }, list) // OK handler.store({ println("hello") }) // OK handler.delete({ println("hello") }) // OK handler.delete(MyInterface { println("hello") }) // OK handler.compareIdentity1({ println("hello") }) // OK handler.compareIdentity2({ println("hello") }) // OK handler.compareEquals1({ println("hello") }) // OK handler.compareEquals2({ println("hello") }) // OK val lambda = { println("hello") } handler.stash(lambda, list) // OK handler.store(lambda) // OK handler.delete(lambda) // ERROR 1 handler.compareIdentity1(lambda) // ERROR 2 handler.compareIdentity2(lambda) // ERROR 3 handler.compareEquals1(lambda) // OK handler.compareEquals2(lambda) // OK @Suppress("CanBeVal", "JoinDeclarationAndAssignment") var lambda2: () -> Unit lambda2 = { println("hello") } handler.stash(lambda2, list) // OK handler.delete(lambda2) // ERROR 4 val lambda3: () -> Unit = { println("hello") } handler.stash(lambda3, list) // OK handler.delete(lambda3) // ERROR 5 handler.act({ println("hello") }) // OK handler.act(::callback) // OK } fun callback() {} fun viewpost(view: android.view.View) { view.postDelayed({ println ("Hello") }, 50) } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ `src/test/pkg/JavaTest.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package test.pkg; import java.util.List; public class JavaTest { public void test(MyHandler handler, List list) { handler.handle(() -> System.out.println("hello")); // OK handler.stash(() -> System.out.println("hello"), list); // OK } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ `src/test/pkg/MyInterface.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package test.pkg; public interface MyInterface { void act(); } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ `src/test/pkg/MyHandler.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package test.pkg; import java.util.List; public class MyHandler { public void handle(MyInterface actor) { actor.act(); System.out.println(actor); MyInterface copy = actor; System.out.println(copy); } public void stash(MyInterface actor, List actors) { actors.add(actor); } public void store(MyInterface actor) { last = actor; } private MyInterface last; private void removeActor(MyInterface actor) { } public fun delete(MyInterface actor) { remove(actor); } public void compareIdentity1(MyInterface actor) { if (actor == last) { System.out.println("last"); } } public void compareIdentity2(MyInterface actor) { if (actor != last) { System.out.println("not last"); } } public void compareEquals1(MyInterface actor) { if (actor.equals(last)) { System.out.println("last"); } } public void compareEquals2(MyInterface actor) { if (last.equals(actor)) { System.out.println("last"); } } public void act(MyInterface actor) { if (actor != null) { actor.act(); } //noinspection StatementWithEmptyBody if (actor == null) { } else { actor.act(); } } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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/SamDetectorTest.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, `SamDetector.testStashingImplicitInstances`. 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("ImplicitSamInstance") fun method() { problematicStatement() } ``` or ```java // Java @SuppressWarnings("ImplicitSamInstance") void method() { problematicStatement(); } ``` * Using a suppression comment like this on the line above: ```kt //noinspection ImplicitSamInstance 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="ImplicitSamInstance" 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 'ImplicitSamInstance' } ``` In Android projects this should be nested inside an `android { }` block. * For manual invocations of `lint`, using the `--ignore` flag: ``` $ lint --ignore ImplicitSamInstance ...` ``` * Last, but not least, using baselines, as discussed [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).