Introduction To Kotlin in SUSI Android App

Lately, we wrote some of the code of SUSI Android App in Kotlin. Kotlin is a very similar language to Java but with much more advantages than Java. It is easy to adapt and learn. There is no doubt that Kotlin is better than Java but with the announcement of Kotlin Support in Google IO’17 for Android development, Kotlin seems a decent way to write code for an Android App.

Advantages of Kotlin over Java

    1. Reduce Boilerplate Code: It helps making development of app faster as it reduces more than 20 percent of boilerplate code. Writing long statements again and again is a headache for developers. Kotlin comes to rescue in that situation.
    2. Removes Null Pointer Exception: Once a large company faced millions of dollars of loss due to null pointer exception. It causes crashes of apps more often than anything else. Thus Kotlin helps in Null checks and makes app free from Null pointer Exceptions.
    3. Interoperable with Java: Kotlin code and Java code are interoperable. Which means you can write half your code in kotlin and half in Java and it will work like a charm. You can call java methods from Kotlin code and vice versa. So, you can simply move your existing Java based app to Kotlin slowly making your app always running.
    4. Lambda and Inline functions: Yes, Kotlin also has functionalities from functional programming languages. Mainly and most widely used feature of those languages is Lambda functions.
    5. Direct Reference of Views by Id: You do not need to write findViewById(R.id.view_name) or use any other library like Butterknife for view binding. You can simply use the view by its id.
    6. No semicolon:  Last but not the least, you do not need to add a semicolon after each statement. In fact, you do not need to add semicolon at all.

Setting up Android Studio to work with Kotlin

If you have latest Android Studio Canary Version, there is already a build support for Kotlin in it. You need not do anything in that case. But if you don’t have the Canary version, you can add Kotlin Plugin in your Android Studio. Follow the below steps to do that.

  1. Install the Kotlin Plugin:

Android Studio → Preferences… →Plugins → Browse Repository → type “Kotlin” in search box → install

  1. Restart your Android Studio and Rebuild the project. Everything else is already set up in SUSI Android App but if you want to do it for your other apps, follow this link.

Implementation in SUSI Android App

So, I am not going to give unnecessary code but will point out specific things where Kotlin helped a lot to reduce unnecessary code and made the code compact.

1. Listeners:

Earlier with Java

Button signup = (Button) findViewById(R.id.sign_up);

signup.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
               startActivity(new Intent(LoginActivity.this, SignUpActivity.class));
            }
        });

Now, with Kotlin

fun signUp() {
   sign_up.setOnClickListener { startActivity(Intent(this@LoginActivity, SignUpActivity::class.java)) }
}

2. Models

With Java

public class MapData {

    private double latitude;
    private double longitude;
    private double zoom;

    public MapData(double latitude, double longitude, double zoom) {
        this.latitude = latitude;
        this.longitude = longitude;
        this.zoom = zoom;
    }

    public double getLatitude() {
        return latitude;
    }

    public void setLatitude(double latitude) {
        this.latitude = latitude;
    }

    public double getLongitude() {
        return longitude;
    }

    public void setLongitude(double longitude) {
        this.longitude = longitude;
    }

    public double getZoom() {
        return zoom;
    }

    public void setZoom(double zoom) {
        this.zoom = zoom;
    }
}

With Kotlin

class MapData (var latitude: Double, var longitude: Double, var zoom: Double) 

3. Constructor

With Java

public class LoginPresenter {
    private LoginActivity loginActivity;
    public LoginPresenter(loginActivity: LoginActivity){
        this.loginActivity = loginActivity;
    }
}

With Kotlin

class LoginPresenter(loginActivity: LoginActivity) {
}

Summary

So, this blog was to give you an idea about Kotlin programming language, it’s advantages over java and information how you can set it up on your Android Studio so as to help you a little in understanding the codebase of SUSI Android App a little more.

Resources

  1. Official Kotlin Guide for Syntax Reference and further learning  https://kotlinlang.org/docs/reference/
  2. Blog by Elye on Setting up Kotlin on Android Studio https://android.jlelse.eu/setup-kotlin-for-android-studio-1bffdf1362e8
  3. Youtube Video tutorial by Derek Banas on Kotlin https://www.youtube.com/watch?v=H_oGi8uuDpA

Shifting from Java to Kotlin in SUSI Android

SUSI Android (https://github.com/fossasia/susi_android) is written in Java. After the announcement of Google to officially support Kotlin as a first class language for Android development we decided to shift to Kotlin as it is more robust and code friendly than Java.

Advantages of Kotlin over Java

  1. Kotlin is a null safe language. It changes all the instances used in the code to non nullable type thus it ensures that the developer don’t get any nullPointerException.
  2. Kotlin provides the way to declare Extensive function similar to that of C#. We can use this function in the same way as we use the member functions of our class.
  3. Kotlin also provides support for Lambda function and other high order functions.

For more details refer to this link.

After seeing the above points it is now clear that Kotlin is much more effective than Java and there is harm in switching the code from Java to Kotlin. Lets now see the implementation in Susi Android.

Implementation in Susi Android

In the Susi Android App we are implementing the MVP design with Kotlin. We are converting the code by one activity each time from java to Kotlin. The advantage here with Kotlin is that it is totally compatible with java at any time. Thus allowing the developer to change the code bit by bit instead of all at once.Let’s now look at SignUp Activity implementation in Susi Android.

The SignUpView interface contains all the function related to the view.

interface ISignUpView {


  fun alertSuccess()

  fun alertFailure()

  fun alertError(message: String)

  fun setErrorEmail()

  fun setErrorPass()

  fun setErrorConpass(msg: String)

  fun setErrorUrl()

  fun enableSignUp(bool: Boolean)

  fun clearField()

  fun showProgress()

  fun hideProgress()

  fun passwordInvalid()

  fun emptyEmailError()

  fun emptyPasswordError()

  fun emptyConPassError()


}

The SignUpActivity implements the view interface in the following way. The view is responsible for all the interaction of user with the UI elements of the app. It does not contain any business logic related to the app.

class SignUpActivity : AppCompatActivity(), ISignUpView {


  var signUpPresenter: ISignUpPresenter? = null

  var progressDialog: ProgressDialog? = null


  override fun onCreate(savedInstanceState: Bundle?) {

      super.onCreate(savedInstanceState)

      setContentView(R.layout.activity_sign_up)

      addListeners()

      setupPasswordWatcher()


      progressDialog = ProgressDialog([email protected])

      progressDialog?.setCancelable(false)

      progressDialog?.setMessage(this.getString(R.string.signing_up))


      signUpPresenter = SignUpPresenter()

      signUpPresenter?.onAttach(this)

  }


  fun addListeners() {

      showURL()

      hideURL()

      signUp()

  }


  override fun onOptionsItemSelected(item: MenuItem): Boolean {

      if (item.itemId == android.R.id.home) {

          finish()

          return true

      }

      return super.onOptionsItemSelected(item)

  }

Now we will see the implementation of models in Susi Android in Kotlin and compare it with Java.

Lets First see the implementation in Java

public class WebSearchModel extends RealmObject {

  private String url;

  private String headline;

  private String body;

  private String imageURL;


  public WebSearchModel() {

  }


  public WebSearchModel(String url, String headline, String body, String imageUrl) {

      this.url = url;

      this.headline = headline;

      this.body = body;

      this.imageURL = imageUrl;

  }


  public void setUrl(String url) {

      this.url = url;

  }


  public void setHeadline(String headline) {

      this.headline = headline;

  }


  public void setBody(String body) {

      this.body = body;

  }


  public void setImageURL(String imageURL) {

      this.imageURL = imageURL;

  }


  public String getUrl() {

      return url;

  }


  public String getHeadline() {

      return headline;

  }


  public String getBody() {

      return body;

  }


  public String getImageURL() {

      return imageURL;

  }

}
open class WebSearchModel : RealmObject {


  var url: String? = null


  var headline: String? = null


  var body: String? = null


  var imageURL: String? = null


  constructor() {}


  constructor(url: String, headline: String, body: String, imageUrl: String) {

      this.url = url

      this.headline = headline

      this.body = body

      this.imageURL = imageUrl

  }

}

You can yourself see the difference and how easily with the help of Kotlin we can reduce the code drastically.

For diving more into the code, we can refer to the GitHub repo of Susi Android (https://github.com/fossasia/susi_android).

Resources

Porting Phimpme Android to Kotlin

As we are going ahead in Phimpme Project we are now on verge to start our account manager which deals with sharing images to many platforms right from the app. The account manager will take care of logging In the user. Saving it’s important credentials such access token, username etc as required by the API.

Google IO ‘17 just passed, and we seen tons of new features, APIs and development tools. One of the them is official support for Kotlin in Android Studio.

As stated by the developers at the conference that one best way to work on Kotlin is add today in your project. Because it is compatible with Java, we can work together on both languages in the same project. It is not mandatory for you to shift your entire code to Kotlin to build a project. So starting with the account manager we decided to bring this to our code. It helps in reducing the boilerplate code for example in Phimpme, I created a model for Realm database.

open class AccountDatabase(

 [email protected] var name: String = "",

       var username: String = "",

       var token: String = ""

) : RealmObject()

That’s all I need to create a model class, no need to create getter and setter property

This helps me to get details from getter methods and show on Account Manager Recycler View like below.

Step 1 : Upgrade to Android Studio Preview 3.0

Android Studio Preview 3.0 comes up with all new features and Kotlin support. We all upgraded to that. It has a great Android Profiler with advance features for debugging and logcat is now moved separately. This step is not mandatory, you can work on older version of Android Studio as well.

Step 2 : Configure Kotlin

It’s easy in Android Studio Preview 3.0 to configure Kotlin. Go to Tools → Kotlin → Configure Kotlin in project.

What in the configuration

  • It added a classpath dependency in project level build.gradle file
classpath"org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
  • Added Kotlin plugin
apply plugin: 'kotlin-android'
  • Added kotlin std lib in app level build.gradle
compile "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"

Step 3: How to add Kotlin files

Now your project is ready for Kotlin. In Android Studio Preview 3.0 you can create new Kotlin files from the file menu.

Also by using Activity template you can select the source language as Java or Kotlin as per your preference.

Step 4 : Work with Kotlin

There are a lot new features in Kotlin to explore. Some of them are

  • Null Safety : In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that can not (non-null references). For example, a regular variable of type String cannot hold null.
var a: String = "abc"
a = null // compilation error

To allow nulls, we can declare a variable as nullable string, written String?:

 var b: String? = "abc"
 b = null // ok
  • Val and Var are two keywords in Kotlin to declare variables. Val gives you read only variable which is same as final modifier in Java, it is not changing. In other words it is immutable Data variables. Var is mutable data variable
  • Semicolons (;) are optional
  • No switch it’s when block in Kotlin. No need to write break and case: below is snippet from phimpme app
override fun onOptionsItemSelected(item: MenuItem): Boolean {
        when (item.itemId) {
            R.id.action_add_account -> {
                val fragmentManager = fragmentManager
                val accountsPicker = AccountPickerFragment().newInstance("Accounts Picker")
                accountsPicker.show(fragmentManager, "Accounts Picker")
            }
            else -> return super.onOptionsItemSelected(item)
        }
        return true
    }


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