A.工程的根目录配置build.gradle

buildscript {
   **ext.kotlin_version = '1.3.50'**
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.5.0'
      **classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"**
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

B.moudle中配置build.gradle

apply plugin: 'com.android.application'
**apply plugin: 'kotlin-android'**
**apply plugin: 'kotlin-android-extensions'**

....

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
  **implementation"org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"**
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.core:core-ktx:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    implementation 'androidx.lifecycle:lifecycle-extensions:2.0.0'
    implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.0.0'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test:runner:1.2.0'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
}

C.比较Activity

kotlin 代码

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.main_activity)
        if (savedInstanceState == null) {
            supportFragmentManager.beginTransaction()
                .replace(R.id.container, MainFragment.newInstance())
                .commitNow()
        }
    }
}

java 代码

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_activity);
        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .replace(R.id.container, MainFragment.newInstance())
                    .commitNow();
        }
    }
}

D.比较Fragment

kotlin 代码

class MainFragment : Fragment() {
    companion object {
        fun newInstance() = MainFragment()
    }

    private lateinit var viewModel: MainViewModel

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        return inflater.inflate(R.layout.main_fragment, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        viewModel = ViewModelProviders.of(this).get(MainViewModel::class.java)
        // TODO: Use the ViewModel
    }
}

java 代码

public class MainFragment extends Fragment {
    private MainViewModel mViewModel;

    public static MainFragment newInstance() {
        return new MainFragment();
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.main_fragment, container, false);
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        mViewModel = ViewModelProviders.of(this).get(MainViewModel.class);
        // TODO: Use the ViewModel
    }
}

区别：

1. 类的修饰符
2. 继承方式
3. 方法修饰符
4. 方法参数(onCreate)
5. 一行代码结束（;）
6. 单例的定义（MainFragment.newInstance()）
7. 方法返回值（onCreateView方法）