Convert an existing Flutter Kotlin project to Flutter Java project

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.

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'
   }