Android & Kotlin - The code awakens #01

Android & Kotlin
The code awakens #01

Omar Miatello
Member of GDG Milano (Italy)
Android Developer @ Satispay
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What is Kotlin?
Statically typed programming language for the JVM, Android and the browser.
(http://kotlinlang.org/)
Why Kotlin?
Concise: drastically reduce the amount of boilerplate code you need to write.
Safe: avoid entire classes of errors such as null pointer exceptions.
Interoperable: leverage existing frameworks and libraries of the JVM with 100% Java
Interoperability.
and more... http://kotlinlang.org/docs/reference/comparison-to-java.html

Kotlin vs Java - Part 1
Property
String templates
Lambdas
Lazy properties

vs
public class MyKotlinClass {
val a: Int = 1
}
public class MyJavaClass {
private final int a = 1;
public int getA() {
return a;
}
}
#1 Kotlin - Properties: val, var
http://kotlinlang.org/docs/reference/properties.html

val a: Int = 1
var b: Int = 1
}
private int b = 1;
public int getA() {
return a;
}
public int getB() {
return b;
}
public void setB(int b) {
this.b = b;
}
}
vs

val a: Int = 1
var b: Int = 1
val c = 1
var d = 1
}
private int b = 1;
private final int c = 1;
private int d = 1;
public int getA() { return a; }
public int getB() { return b; }
public void setB(int b) { this.b = b; }
public int getC() { return c; }
public int getD() { return d; }
public void setD(int d) { this.d = d; }
}
vs

class MyKotlinClass {
val name = "Omar"
val surname = "Miatello"
}
class MyJavaClass {
final String getName() {
return "Omar";
}
final String getSurname() {
return "Miatello";
}
}
#2 Kotlin - String templates
http://kotlinlang.org/docs/reference/basic-syntax.html#using-string-templates
vs

val name = "Omar"
val surname = "Miatello"
val example = "My name is $name $surname"
}
class MyJavaClass {
final String getName() {
return "Omar";
}
final String getSurname() {
return "Miatello";
}
final String getExample() {
return String.format("My name is %s %s",
getName(), getSurname()); }
}
#2 Kotlin - String templates
http://kotlinlang.org/docs/reference/basic-syntax.html#using-string-templates
vs

class MyActivity : Activity() {
fun example() {
val view = findViewById(R.id.button)
view.setOnClickListener {
}
}
}
class MyActivity extends Activity {
void example() {
View view = findViewById(R.id.button);
view.setOnClickListener(
);
}
}
#3 Kotlin - Lambdas
http://kotlinlang.org/docs/reference/coding-conventions.html#lambdas
vs

class MyActivity : Activity() {
fun example() {
val view = findViewById(R.id.button)
view.setOnClickListener {
Log.d("TAG", "Item clicked!")
}
}
}
class MyActivity extends Activity {
void example() {
View view = findViewById(R.id.button);
view.setOnClickListener(
new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.d("TAG", "Item clicked!");
}
}
);
}
}
#3 Kotlin - Lambdas
http://kotlinlang.org/docs/reference/coding-conventions.html#lambdas
vs

class MyJavaClass {
class MyItem { }
MyItem item;
}
#4 Kotlin - Delegated Properties: lazy (as example)
http://kotlinlang.org/docs/reference/delegated-properties.html
vs

class MyJavaClass {
class MyItem { }
MyItem item;
final MyItem getItem() {
if (item == null) {
item = new MyItem();
}
return item;
}
}
vs

class MyItem
val item by lazy { MyItem() }
}
// Simplified: in Kotlin is synchronized
class MyJavaClass {
class MyItem { }
MyItem item;
final MyItem getItem() {
if (item == null) {
item = new MyItem();
}
return item;
}
}
vs

Install Kotlin in Android Studio
Part 1 / 2

1. Open “Preferences”

2. Choose “Plugins” and “Install
JetBrains plugin…”

3. Install “Kotlin” and “Kotlin
Extensions For Android”

4. Restart Android Studio, and
open (or create) a project

5. Create a new “Kotlin class”

Android Example
From Java to Kotlin! (step1)
https://github.com/jacklt/KotlinExample/tree/java-version

dummy/HeroItem.java
public class HeroItem {
public final String id;
public final String content;
public final String details;
public HeroItem(String id, String content, String details) {
this.id = id;
this.content = content;
this.details = details;
}
@Override
public String toString() {
return content;
}
}

dummy/HeroItem.java
public class HeroItem {
public final String id;
public final String content;
public final String details;
public HeroItem(String id, String content, String details) {
this.id = id;
this.content = content;
this.details = details;
}
@Override
public String toString() {
return content;
}
}
“Convert Java File to Kotlin File”
CTRL + ALT + SHIFT + K
(or CMD + ALT + SHIFT + K)

dummy/HeroItem.kt
class HeroItem(val id: String, val content: String, val details: String) {
override fun toString(): String {
return content
}
}

dummy/HeroItem.kt (optimized)
data class HeroItem(val id: String, val content: String, val details: String)

dummy/Items.kt (... a new hope)
data class HeroItem(val id: String, val content: String, val details: String)
data class VillanItem(val name: String, val power: String)
data class SpacecraftItem(val armaments: String, val defenses: String)
data class TinyItem(val name: String)
// ...

dummy/HeroAdapter.java (before HeroItem Conversion)
public class HeroAdapter extends RecyclerView.Adapter<HeroAdapter.ViewHolder> {
// ...
@Override
public void onBindViewHolder(final ViewHolder holder, int position) {
HeroItem item = mValues.get(position);
holder.item = item;
holder.idView.setText(item.id);
holder.contentView.setText(item.content);
// ...
}
// ...
}

dummy/HeroAdapter.java (after HeroItem Conversion)
// ...
@Override
holder.item = item;
holder.idView.setText(item.getId());
holder.contentView.setText(item.getContent());
// ...
}
// ...
}

Part 2 / 2

6. Choose from menu “Tools” >
“Kotlin” > “Configure Kotlin in
Project”

6. Choose from menu “Tools” >
“Kotlin” > “Configure Kotlin in
Project”
dependencies {
// other dependencies ...
compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
}
buildscript {
ext.kotlin_version = '1.0.0-beta-2423'
repositories {
mavenCentral()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
}
}
repositories {
mavenCentral()
}

Android Example
HeroAdapter

dummy/HeroAdapter.java
private final List<HeroItem> mValues;
private final HeroOnClickListener mListener;
public HeroAdapter(List<HeroItem> items, HeroOnClickListener listener) {
mValues = items;
mListener = listener;
}
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_hero, parent, false);
return new ViewHolder(view);
}
@Override

dummy/HeroAdapter.kt (need manual fix)
// ...
inner class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
val idView: TextView
val contentView: TextView
var item: HeroItem
init {
idView = view.findViewById(R.id.id) as TextView
contentView = view.findViewById(R.id.content) as TextView
}
}
// ...

dummy/HeroAdapter.kt (fixed!)
// ...
inner class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
val idView: TextView
val contentView: TextView
var item: HeroItem? = null
init {
idView = view.findViewById(R.id.id) as TextView
contentView = view.findViewById(R.id.content) as TextView
}
}
// ...

dummy/HeroAdapter.kt (optimized)
// OPTIMIZATION 1: In class constructor
val mListener: HeroOnClickListener?
// can be replaced with
val mListener: Function1<HeroItem, Unit>?
// OPTIMIZATION 2: Method definition
override fun getItemCount(): Int {
return mValues.size
}
// can be replaced with
override fun getItemCount() = mValues.size

Android Example
MainActivity
with Kotlin Android Extension

build.gradle (add kotlin-android-extensions)
// ..
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
classpath "org.jetbrains.kotlin:kotlin-android-extensions:$kotlin_version"
}
// ..

MainActivity.java
public class MainActivity extends AppCompatActivity {
private Toolbar toolbar;
private FloatingActionButton fab;
private RecyclerView recyclerView;
private HeroAdapter adapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
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)

MainActivity.kt (optimized, add import)
import kotlinx.android.synthetic.activity_main.*

MainActivity.kt (optimized, use lazy for adapter)
class MainActivity : AppCompatActivity() {
private val adapter by lazy {
HeroAdapter(HeroDummyContent.ITEMS) { Snackbar.make(fab, "Tap on: " + it, Snackbar.LENGTH_SHORT).show() }
}

MainActivity.kt (optimized, remove all unused property)
class MainActivity : AppCompatActivity() {
private val adapter by lazy {
HeroAdapter(HeroDummyContent.ITEMS) { Snackbar.make(fab, "Tap on: " + it, Snackbar.LENGTH_SHORT).show() }
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
setSupportActionBar(toolbar)
fab.setOnClickListener { Snackbar.make(it, "...", Snackbar.LENGTH_LONG).setAction("Action", null).show() }
recyclerView.layoutManager = LinearLayoutManager(this)
recyclerView.adapter = adapter
}
}

Questions?
Developers playground - #EX1
- Add properties to HeroItem: name, gender (String), power (Int)
- Show item in list like: “$name (Power: $power)”
- Choose (and keep in memory) hero “onClick”
- Fight with second “onClick” (show a Snackbar)
Hint: Add in MainActivity “selectedHero” property (in Kotlin, or use field in Java)
Start with: https://github.com/jacklt/KotlinExample/tree/java-version or https://github.com/jacklt/KotlinExample/tree/kotlin-
version
Solution: https://github.com/jacklt/KotlinExample/tree/ex1

THANKS!
Omar Miatello, Member of GDG Milano (Italy)
Android Developer @ Satispay

Android & Kotlin - The code awakens #01

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