Add Kotlin to an existing app | Android Developers

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href="https://developer.android.com/kotlin" class="devsite-breadcrumb-link gc-analytics-event" data-category="Site-Wide Custom Events" data-label="Breadcrumbs" data-value="3" track-type="globalNav" track-name="breadcrumb" track-metadata-position="3" track-metadata-eventdetail="Kotlin" > Kotlin </a> </li> <li class="devsite-breadcrumb-item "> <div class="devsite-breadcrumb-guillemet material-icons" aria-hidden="true"></div> <a href="https://developer.android.com/kotlin/first" class="devsite-breadcrumb-link gc-analytics-event" data-category="Site-Wide Custom Events" data-label="Breadcrumbs" data-value="4" track-type="globalNav" track-name="breadcrumb" track-metadata-position="4" track-metadata-eventdetail="" > Guides </a> </li> </ul> <devsite-thumb-rating position="header"> </devsite-thumb-rating> </div> <h1 class="devsite-page-title" tabindex="-1"> Add Kotlin to an existing app </h1> <devsite-feature-tooltip ack-key="AckCollectionsBookmarkTooltipDismiss" analytics-category="Site-Wide Custom Events" analytics-action-show="Callout Profile displayed" analytics-action-close="Callout Profile dismissed" analytics-label="Create Collection Callout" class="devsite-page-bookmark-tooltip nocontent" dismiss-button="true" id="devsite-collections-dropdown" dismiss-button-text="Dismiss" close-button-text="Got it"> <devsite-bookmark></devsite-bookmark> <span slot="popout-heading"> Stay organized with collections </span> <span slot="popout-contents"> Save and categorize content based on your preferences. </span> </devsite-feature-tooltip> <div class="devsite-page-title-meta"><devsite-view-release-notes></devsite-view-release-notes></div> <devsite-toc class="devsite-nav" depth="2" devsite-toc-embedded > </devsite-toc> <devsite-recommendations-dropdown class="nocontent"></devsite-recommendations-dropdown> <div class="devsite-article-body clearfix "> <p>Android Studio provides <a href="/kotlin">full support for Kotlin</a>, enabling you to add Kotlin files to your existing project and convert Java language code to Kotlin. You can then use all of Android Studio's existing tools with your Kotlin code, including autocomplete, lint checking, refactoring, debugging, and more.</p> <p>If you're starting a new project and want to use Kotlin, see <a href="/studio/projects/create-project">Create a project</a>.</p> <p>For samples, check out our <a href="/samples/index?language=kotlin">Kotlin code samples</a>.</p> <h2 id="add" data-text="Add Kotlin to an existing project" tabindex="-1">Add Kotlin to an existing project</h2> <p>To add Kotlin to your project, do the following:</p> <ol> <li><p>Click <strong>File > New</strong>, and choose one of the various Android templates, such as a new blank <strong>Fragment</strong>, as shown in figure 1. If you don't see the list of templates in this menu, first open the <strong>Project</strong> window, and select your app module.</p> <figure> <img src="/static/images/kotlin/choose-template.png" width="500px" alt="create a new blank fragment" /> <figcaption>Figure 1. Choose from the available templates, such as fragment or activity.</figcaption> </figure></li> <li><p>In the wizard that appears, choose <strong>Kotlin</strong> for the <strong>Source Language</strong>. Figure 2 shows the <strong>New Android Activity</strong> dialog for when you want to create a new activity.</p> <figure> <img src="/static/images/kotlin/choose-source-language.png" width="500px" alt="dialog that lets you choose Kotlin for your source language" /> <figcaption>Figure 2. A <b>New Android Activity</b> dialog where you can choose <b>Kotlin</b> as your <b>Source Language</b>.</figcaption> </figure></li> <li><p>Continue through the wizard.</p></li> </ol> <p>Alternatively, you can click <strong>File > New > Kotlin File/Class</strong> to create a basic Kotlin file. If you don't see this option, open the <strong>Project</strong> window and select the <strong>java</strong> directory. The <strong>New Kotlin File/Class</strong> window lets you define the file name and provides several choices for the file type: <strong>File</strong>, <strong>Class</strong>, <strong>Interface</strong>, <strong>Enum Class</strong>, or <strong>Object</strong>. The choice you make determines the basic scaffolding created for you in the new Kotlin file. If you choose <strong>Class</strong>, Android Studio creates a new Kotlin source file with the given name and a matching class definition. If you choose <strong>Interface</strong>, an interface is declared in the file, and so on.</p> <p>If this is the first time you have added a new Kotlin class or file to your project directly (not using the Android templates), Android Studio displays a warning that Kotlin is not configured in the project, as shown in figure 3. Configure Kotlin by clicking <strong>Configure</strong> either in the upper right corner of the editor or in the <strong>event log alert</strong> that pops up in the lower-right corner.</p> <figure> <img src="/static/images/kotlin/configure-kotlin.png" width="400px" alt="warning dialog that prompts you to configure Kotlin for your project" /> <figcaption>Figure 3. Android Studio displays a warning dialog when Kotlin isn't configured for your project.</figcaption> </figure> <p>Choose the option to configure Kotlin for <strong>All modules containing Kotlin files</strong> when prompted, as shown in figure 4:</p> <figure> <img src="/static/images/kotlin/all-modules.png" width="400px" alt="choose to configure Kotlin for all modules that contain Kotlin code" /> <figcaption>Figure 4. Choose to configure Kotlin for all modules that contain Kotlin code.</figcaption> </figure> <p>Once you click <strong>OK</strong>, Android Studio adds Kotlin to your project classpath and applies the Kotlin Android plugin to each module that contains Kotlin files. Your <code translate="no" dir="ltr">build.gradle</code> files should look similar to the examples below:</p> <div> <div class="ds-selector-tabs"><section><h3 id="groovy" data-text="Groovy" tabindex="-1">Groovy</h3> <pre class="prettyprint lang-groovy" translate="no" dir="ltr"> // Project build.gradle file. buildscript { ext.kotlin_version = '1.4.10' ... dependencies { classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version" } } </pre> </section><section><h3 id="kts" data-text="Kotlin" tabindex="-1">Kotlin</h3> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"> // Project build.gradle.kts file. buildscript { extra["kotlin_version"] = "1.4.10" ... dependencies { classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version") } } </pre> </section></div> </div> <div> <div class="ds-selector-tabs"><section><h3 id="groovy" data-text="Groovy" tabindex="-1">Groovy</h3> <pre class="prettyprint lang-groovy" translate="no" dir="ltr"> // Inside each module using kotlin plugins { ... id 'kotlin-android' } ... dependencies { implementation 'androidx.core:core-ktx:1.3.2' implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version" } </pre> </section><section><h3 id="kts" data-text="Kotlin" tabindex="-1">Kotlin</h3> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"> // Inside each module using kotlin plugins { ... kotlin("android") } ... val kotlin_version: String by rootProject.extra dependencies { implementation("androidx.core:core-ktx:1.3.2") implementation("org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version") } </pre> </section></div> </div> <h2 id="source" data-text="Source organization" tabindex="-1">Source organization</h2> <p>By default, new Kotlin files are saved in <code translate="no" dir="ltr">src/main/java/</code>, which makes it easy to see both Kotlin and Java files in one location. If you'd prefer to separate your Kotlin files from your Java files, you can put Kotlin files under <code translate="no" dir="ltr">src/main/kotlin/</code> instead. If you do this, then you also need to include this directory in your <a href="/studio/build#sourcesets"><code translate="no" dir="ltr">sourceSets</code></a> configuration, as shown below:</p> <div> <div class="ds-selector-tabs"><section><h3 id="groovy" data-text="Groovy" tabindex="-1">Groovy</h3> <pre class="prettyprint lang-groovy" translate="no" dir="ltr"> android { sourceSets { main.java.srcDirs += 'src/main/kotlin' } } </pre> </section><section><h3 id="kts" data-text="Kotlin" tabindex="-1">Kotlin</h3> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"> android { sourceSets { getByName("main") { java.srcDir("src/main/kotlin") } } } </pre> </section></div> </div> <h2 id="convert" data-text="Convert existing Java code to Kotlin code" tabindex="-1">Convert existing Java code to Kotlin code</h2> <p>To convert Java code to Kotlin, open the Java file in Android Studio, and select <strong>Code > Convert Java File to Kotlin File</strong>. Alternatively, create a new Kotlin file (<strong>File > New > Kotlin File/Class</strong>), and then paste your Java code into that file. Android Studio then displays a prompt and offers to convert your code to Kotlin, as shown in figure 5. Click <strong>Yes</strong> to convert. You can optionally check <strong>Don't show this dialog next time</strong>, which makes future conversions automatic.</p> <figure> <img src="/static/images/kotlin/convert-code.png" width="400px" alt="choose to configure Kotlin for all modules that contain Kotlin code" /> <figcaption>Figure 5. Android Studio can convert Java code to Kotlin.</figcaption> </figure> <h2 id="conversion-and-nullability" data-text="Code conversion and nullability" tabindex="-1">Code conversion and nullability</h2> <p>Android Studio's conversion process produces functionally-equivalent Kotlin code that compiles and runs. However, it's likely that you need to make additional optimizations to the converted code. For example, you might want to refine how the converted code handles nullable types.</p> <p>In Android, it is common to delay initialization of <code translate="no" dir="ltr">View</code> objects and other components until the fragment or activity they are attached to reaches the appropriate lifecycle state. For example, you may have a reference to a button in one of your fragments, as demonstrated in the following snippet:</p> <pre class="prettyprint lang-java" translate="no" dir="ltr"><code translate="no" dir="ltr">public class JavaFragment extends Fragment { // Null until onCreateView. private Button button; @Override public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View root = inflater.inflate(R.layout.fragment_content, container,false); // Get a reference to the button in the view, only after the root view is inflated. button = root.findViewById(R.id.button); return root; } @Override public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) { super.onViewCreated(view, savedInstanceState); // Not null at this point of time when onViewCreated runs button.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { ... } }); } } </code></pre> <p>Even though the button variable is nullable, for all practical purposes it should never be null when used in this example. However, since its value is not assigned at the point of construction, the generated Kotlin code treats <code translate="no" dir="ltr">Button</code> as a nullable type and uses the non-null assertion operator to unwrap the button when adding a click listener, as shown below:</p> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"><code translate="no" dir="ltr">class JavaFragment : Fragment() { // Null until onCreateView. private var button: Button? = null override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? { ... // Get a reference to the button in the view, only after the root view is inflated. button = root.findViewById(R.id.button) ... } override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) // Not null at the point of time when onViewCreated fires // but force unwrapped nonetheless button!!.setOnClickListener { } } } </code></pre> <p>This conversion is less ideal than using <code translate="no" dir="ltr">lateinit</code> for this case, because you are forced to unwrap the button reference with a non-null assertion or safe-call operator in every place it is accessed.</p> <aside class="note"><strong>Note:</strong><span> This example doesn't mean that you should always avoid nullable types. Generally, this pattern of using <code translate="no" dir="ltr">lateinit</code> applies to variables which are never expected to be <code translate="no" dir="ltr">null</code> but cannot be initialized where they are defined, as is the case with <code translate="no" dir="ltr">View</code> references inside of the fragment or activity in which they live.</span></aside> <p>In other cases, where <code translate="no" dir="ltr">null</code> is a valid variable assignment based on your application's use case, using a safe-call (?.) operator with a terminating elvis operator (?:) operator may be a more appropriate way to safely unwrap the nullable object or coerce to a sensible non-null default value. Android Studio does not have enough information to make this determination during the conversion process. While it defaults to the non-null assertion, you should follow up and adjust the converted code as needed.</p> <h2 id="more-information" data-text="More information" tabindex="-1">More information</h2> <p>For more information about using both Kotlin and Java code in your project, see <a href="https://kotlinlang.org/docs/reference/java-interop.html">Calling Java code from Kotlin</a>.</p> <p>For more information about using Kotlin in enterprise scenarios, see <a href="/kotlin/adopt-for-large-teams">Adopting Kotlin for large teams</a>.</p> <p>For information about idiomatic Kotlin wrappers for existing Android APIs, see <a href="/kotlin/ktx">Android KTX</a>.</p> <devsite-hats-survey class="nocontent" hats-id="VxqvKSur40kxBYCLVTd0SSGykbno" listnr-id="5207477"></devsite-hats-survey> </div> <devsite-recommendations display="in-page" hidden yield> </devsite-recommendations> <devsite-thumb-rating position="footer"> </devsite-thumb-rating> <devsite-recommendations id="recommendations-link" yield></devsite-recommendations> <div class="devsite-floating-action-buttons"> </div> </article> <devsite-content-footer class="nocontent"> <p>Content and code samples on this page are subject to the licenses described in the <a href="/license">Content License</a>. 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