How to use Java and Kotlin together in android 

1. Download Kotlin plugin

2. Add dependencies of "kotlin-gradle-plugin" in build.gradle (Project level)

dependencies {
    classpath "com.android.tools.build:gradle:$agp_version"
    classpath 'com.google.gms:google-services:4.3.5'
    // NOTE: Do not place your application dependencies here; they belong
    // in the individual module build.gradle files

    // Add the Crashlytics Gradle plugin
    classpath 'com.google.firebase:firebase-crashlytics-gradle:2.7.1'

    //kotlin
    classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.6.0'    //here
}

3. Add kotlin plugin id in build.gradle (App level)

plugins {
    id 'com.android.application'
    id 'kotlin-android'
}

4.  Add sourceSets for kotlin file

android {
    compileSdkVersion 34
    buildToolsVersion "30.0.3"

    defaultConfig {
        applicationId "org.techtown.volleyball"
        minSdkVersion 21
        targetSdkVersion 34
        multiDexEnabled true
        versionCode 16
        versionName "1.2.11"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_11
        targetCompatibility JavaVersion.VERSION_11
    }
    buildFeatures {
        viewBinding true
    }
    
    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'    //here
    }
}

5. Add kotlin dependencies in build.gradle (App level)

dependencies {
    //kotlin
    implementation 'org.jetbrains.kotlin:kotlin-stdlib:1.6.0'

}

6. Sync gradle file and wrie kotlin code.

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