Convert an existing Flutter Kotlin project to Flutter Java project

I had the same problem, for me this solution works.

1. Move folder com.example.test_app (any name you have) from android/app/src/main/kotlin -> android/app/src/main/java

2. Replace MainActivity.kt with Java version, or copy down here

   package com.example.test_app;import androidx.annotation.NonNull;import io.flutter.embedding.android.FlutterActivity;import io.flutter.embedding.engine.FlutterEngine;import io.flutter.plugins.GeneratedPluginRegistrant;public class MainActivity extends FlutterActivity { @Override public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) { GeneratedPluginRegistrant.registerWith(flutterEngine); }}

3. Remove following code android/app/build.grandle

   ...apply plugin: 'kotlin-android'...sourceSets {    main.java.srcDirs += 'src/main/kotlin'}

4. In the same place replace following:

   dependencies {    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"    testImplementation 'junit:junit:4.12'    androidTestImplementation 'androidx.test:runner:1.1.0'    androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.0'}

   to

   dependencies {    testImplementation 'junit:junit:4.12'    androidTestImplementation 'com.android.support.test:runner:1.0.2'    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'}

By default flutter template supports writing Android code using Kotlin, or iOS code using Swift. To use Java or Objective-C, use the -i and/or -a flags:

In a terminal run: flutter create -i objc -a java your_project_name.

If you wanna change your existing app platform language choice, as a workaround you can delete the android/ directory and run flutter create -a java to get the directory re-created for the new language choice (same for ios/ and Swift).You need to re-apply custom changes though.

run flutter create -a java . inside your project directory

flutter create: If run on a project that already exists, this will repair the project, recreating any files that are missing.

delete the kotlin directory in android/src/main if it only consists of the generated example code