(#) Fragment not instantiatable !!! ERROR: Fragment not instantiatable This is an error. Id : `ValidFragment` Summary : Fragment not instantiatable Severity : Error Category : Correctness Platform : Android Vendor : Android Open Source Project Feedback : https://issuetracker.google.com/issues/new?component=192708 Since : Initial Affects : Kotlin and Java files Editing : This check runs on the fly in the IDE editor See : https://developer.android.com/reference/android/app/Fragment.html#Fragment() 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/FragmentDetector.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/FragmentDetectorTest.kt) Copyright Year : 2012 From the Fragment documentation: **Every** fragment must have an empty constructor, so it can be instantiated when restoring its activity's state. It is strongly recommended that subclasses do not have other constructors with parameters, since these constructors will not be called when the fragment is re-instantiated; instead, arguments can be supplied by the caller with `setArguments(Bundle)` and later retrieved by the Fragment with `getArguments()`. Note that this is no longer true when you are using `androidx.fragment.app.Fragment`; with the `FragmentFactory` you can supply any arguments you want (as of version androidx version 1.1). (##) Example Here is an example of lint warnings produced by this check: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text src/test/pkg/FragmentTest.java:10:Error: This fragment class should be public (test.pkg.FragmentTest.Fragment1) [ValidFragment] private static class Fragment1 extends Fragment { --------- src/test/pkg/FragmentTest.java:15:Error: This fragment inner class should be static (test.pkg.FragmentTest.Fragment2) [ValidFragment] public class Fragment2 extends Fragment { --------- src/test/pkg/FragmentTest.java:21:Error: The default constructor must be public [ValidFragment] private Fragment3() { --------- src/test/pkg/FragmentTest.java:26:Error: This fragment should provide a default constructor (a public constructor with no arguments) (test.pkg.FragmentTest.Fragment4) [ValidFragment] public static class Fragment4 extends Fragment { --------- src/test/pkg/FragmentTest.java:27:Error: Avoid non-default constructors in fragments: use a default constructor plus Fragment#setArguments(Bundle) instead [ValidFragment] private Fragment4(int sample) { --------- src/test/pkg/FragmentTest.java:36:Error: Avoid non-default constructors in fragments: use a default constructor plus Fragment#setArguments(Bundle) instead [ValidFragment] public Fragment5(int sample) { --------- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is the source file referenced above: `src/test/pkg/FragmentTest.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package test.pkg; import android.annotation.SuppressLint; import android.app.Fragment; @SuppressWarnings("unused") public class FragmentTest { // Should be public private static class Fragment1 extends Fragment { } // Should be static public class Fragment2 extends Fragment { } // Should have a public constructor public static class Fragment3 extends Fragment { private Fragment3() { } } // Should have a public constructor with no arguments public static class Fragment4 extends Fragment { private Fragment4(int sample) { } } // Should *only* have the default constructor, not the // multi-argument one public static class Fragment5 extends Fragment { public Fragment5() { } public Fragment5(int sample) { } } // Suppressed @SuppressLint("ValidFragment") public static class Fragment6 extends Fragment { private Fragment6() { } } public static class ValidFragment1 extends Fragment { public ValidFragment1() { } } // (Not a fragment) private class NotAFragment { } // Ok: Has implicit constructor public static class Fragment7 extends Fragment { } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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/FragmentDetectorTest.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, `FragmentDetector.testBasic`. 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("ValidFragment") fun method() { problematicStatement() } ``` or ```java // Java @SuppressWarnings("ValidFragment") void method() { problematicStatement(); } ``` * Using a suppression comment like this on the line above: ```kt //noinspection ValidFragment 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="ValidFragment" 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 'ValidFragment' } ``` In Android projects this should be nested inside an `android { }` block. * For manual invocations of `lint`, using the `--ignore` flag: ``` $ lint --ignore ValidFragment ...` ``` * Last, but not least, using baselines, as discussed [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).