Kotlin is a Concise, Safe and Statically typed programming language focused on Interoperability with Java.

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }
}

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val toolbar = findViewById(R.id.toolbar) as Toolbar
        setSupportActionBar(toolbar)
        val fab = findViewById(R.id.fab) as FloatingActionButton
        fab.setOnClickListener { view -> Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG).setAction("Action", null).show() }
    }
}

Java (19 lines)

Kotlin (12 lines)

Extend and Implement

The words “extends” and “implement” were replaced by a colon “:”

Fun - ctions

class MainActivity : AppCompatActivity()

We don’t have our classic “public void methodName()” structure.

But where is the type returned from our “onCreate” method?

override fun onCreate(savedInstanceState: Bundle?) : Unit {

Bye Semicolon;

In Kotlin you don’t have to put the semicolon at the end of a sentences but still you can do it if you want (don’t do it).

Values and Variable

val price = 100        // Int
price = 30             // don't compile! it's a constant
var total = price * 3  // Int
val name = "Juancho"   // String

val lastname : String = "Keddit" // explicit type definition
var size : Double = 30.0
var time : Float = 15f

Properties and Fields

resources.getString(R.string.id)
// vs
getResources().getString(R.string.id) // still allowed

Safe?:Safe!

val a : String = null  // don't compile!
var b : Int          // neither as must be initialized or abstract.
val ok : String? = null // OK :)

everything in Kotlin is not nullable unless you specifically declare it in this way

Safe call

val context : Context? = null
val res = context?.getResources() // not crash, res will be null

Smart cast

val context : Context? = null
if (context != null) {
    val res = context.getResources()    // Don't need '?' anymore
    val appName = res.getString(R.string.app_name)
    val shortName = appName.substring(0, 2)
}
val context : Context? = null
context?.let {
    val res = context.getResources()    // Don't need '?' anymore
    val appName = res.getString(R.string.app_name)
    val shortName = appName.substring(0, 2)
}

NewsFragment.kt: Extensions Function, Android Extensions and more…

NewsFragment.kt

Delegate Adapters & Data Classes with Kotlin

NewsAdapter

Observable

fun getNews(): Observable<List<RedditNewsItem>> {
    ...
}

onNext(item: T)

onCompleted()

onError(e: Throwable)

Subscribe>Subscription>Observer

val subscription = newsManager.getNews().subscribe (
        { retrievedNews ->
            ...
        },
        { e ->
            ...
        }

onNext

onError

SubscribeOn

val subscription = newsManager.getNews()
        .subscribeOn(Schedulers.io())
        .subscribe (...)

ObserveOn in the main thread

val subscription = newsManager.getNews()
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe ( ...
)

AndroidSchedulers.mainThread()

Kotlin & Dagger 2

...and finally....

What happened after this?

Builder Design Pattern

Factory Design Pattern

Base elements (annotations) of Dagger 2

Graph generated by daggraph

To generate this dependency we need two modules:

• ApplicationModule 

• Activity Module

and their respective components:

• ApplicationComponent

• ActivityComponent

@Module
public class ApplicationModule {
    private final Application mApplication;
    public ApplicationModule(Application app) {
        mApplication = app;
    }
    @Provides
    @ApplicationContext
    Context provideContext() {
        return mApplication;
    }
    @Provides
    Application provideApplication() {
        return mApplication;
    }
   ...

    @Provides
    @DatabaseInfo
    String provideDatabaseName() {
        return "demo-dagger.db";
    }
    @Provides
    @DatabaseInfo
    Integer provideDatabaseVersion() {
        return 2;
    }
    @Provides
    SharedPreferences provideSharedPrefs() {
        return mApplication.getSharedPreferences("demo-prefs", Context.MODE_PRIVATE);
    }
}

@Module
public class ActivityModule {
    private Activity mActivity;
    public ActivityModule(Activity activity) {
        mActivity = activity;
    }
    @Provides
    @ActivityContext
    Context provideContext() {
        return mActivity;
    }
    @Provides
    Activity provideActivity() {
        return mActivity;
    }
}

@Singleton
@Component(modules = ApplicationModule.class)
public interface ApplicationComponent {
    void inject(DemoApplication demoApplication);
    @ApplicationContext
    Context getContext();
    Application getApplication();
    DataManager getDataManager();
    SharedPrefsHelper getPreferenceHelper();
    DbHelper getDbHelper();
}

@PerActivity
@Component(dependencies = ApplicationComponent.class, modules = ActivityModule.class)
public interface ActivityComponent {
    void inject(MainActivity mainActivity);
}