Android & Kotlin: The code awakens (episode 02) - 2/3

Kotlin vs Java - Part 2
Null-safety
Elvis Operator
Smart-cast
Collections

Transcript

1. Android & Kotlin The code awakens #02
2. Omar Miatello Member of GDG Milano (Italy) Android Developer @ Satispay Personal profile google.com/+OmarMiatello Google+ Community: Kotlin for Android goo.gl/mUKF1w Google Presentation #01 goo.gl/0jHLmE #02 goo.gl/h3uG8M #03 goo.gl/hnwvqu Google Photo #01 goo.gl/photos/cKP9L6zqZcxDRGzQ8 #02 goo.gl/photos/sXdpkbihCi5xAAnx7 #03 goo.gl/photos/P6kGhLE8yrWYnhAW6
3. Previously on Android & Kotlin #01: goo.gl/0jHLmE • Properties val a: Int = 1 // val = READ ONLY (getter) var b: Int = 1 // var = READ/WRITE (getter/setter) • String templates "My name is $name $surname" • Lambdas view.setOnClickListener { Log.d("TAG", "Item clicked!") } • Delegated properties (example: lazy) val item by lazy { MyItem() }
4. dummy/HeroAdapter.java public class HeroAdapter extends RecyclerView.Adapter<HeroAdapter.ViewHolder> { 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 public void onBindViewHolder(final ViewHolder holder, int position) { HeroItem item = mValues.get(position); “Convert Java File to Kotlin File” CTRL + ALT + SHIFT + K (or CMD + ALT + SHIFT + K)
5. Kotlin vs Java - Part 2 Null-safety Elvis Operator Smart-cast Collections
6. class MyKotlinClass { val a: String = "ciao" val b: String = null // Error at compile time } #5 Kotlin - Null Safety http://kotlinlang.org/docs/reference/null-safety.html vs
7. class MyKotlinClass { val a: String = "ciao" val b: String = null // Error at compile time val c: String? = null val d: String? = "ok" } #5 Kotlin - Null Safety http://kotlinlang.org/docs/reference/null-safety.html vs
8. class MyKotlinClass { val a: String = "ciao" val b: String = null // Error at compile time val c: String? = null val d: String? = "ok" fun example(e: String, f: String?) { e.length() f.length() // Error at compile time } } static class MyUtils { void example(String e, String f) { e.length(); // throw NullPointerException? f.length(); // throw NullPointerException? } } #5 Kotlin - Null Safety http://kotlinlang.org/docs/reference/null-safety.html vs
9. class MyKotlinClass { val a: String = "ciao" val b: String = null // Error at compile time val c: String? = null val d: String? = "ok" fun example(e: String, f: String?) { e.length() f.length() // Error at compile time f?.length() } } static class MyUtils { void example(String e, String f) { e.length(); // throw NullPointerException? f.length(); // throw NullPointerException? } } #5 Kotlin - Null Safety http://kotlinlang.org/docs/reference/null-safety.html vs
10. class MyKotlinClass { val a: String = "ciao" val b: String = null // Error at compile time val c: String? = null val d: String? = "ok" fun example(e: String, f: String?) { e.length() f.length() // Error at compile time f?.length() if (f != null) { f.length() } } } static class MyUtils { void example(String e, String f) { e.length(); // throw NullPointerException? f.length(); // throw NullPointerException? if (e != null) { e.length(); } if (f != null) { e.length(); } } } #5 Kotlin - Null Safety http://kotlinlang.org/docs/reference/null-safety.html vs
11. val map = mapOf( "dog" to "woof", "cat" to "meow", "bird" to "tweet", "mouse" to "squeek") fun sound(animal: String): String? { return map.get(animal) } class MyUtils { static Map<String, String> map = // ... static String sound(String animal) { return map.get(animal); } } #6 Kotlin - Elvis Operator http://kotlinlang.org/docs/reference/idioms.html#if-not-null-and-else-shorthand vs
12. val map = mapOf( "dog" to "woof", "cat" to "meow", "bird" to "tweet", "mouse" to "squeek") fun sound(animal: String): String? { return map.get(animal) } fun example() { val foxSay = sound("fox") ?: "No one knows" } class MyUtils { static Map<String, String> map = // ... static String sound(String animal) { return map.get(animal); } static void example() { String s = sound("fox"); String foxSay = s != null ? s : "No one knows"; } } #6 Kotlin - Elvis Operator http://kotlinlang.org/docs/reference/idioms.html#if-not-null-and-else-shorthand vs
13. val map = mapOf( "dog" to "woof", "cat" to "meow", "bird" to "tweet", "mouse" to "squeek") fun sound(animal: String): String? { return map.get(animal) } fun example() { val foxSay = sound("fox") ?: "No one knows" } class MyUtils { static Map<String, String> map = // ... static String sound(String animal) { return map.get(animal); } static void example() { String s = sound("fox"); String foxSay = s != null ? s : "No one knows"; } } #6 Kotlin - Elvis Operator http://kotlinlang.org/docs/reference/idioms.html#if-not-null-and-else-shorthand vs
14. class MyUtils { static void example(View myView) { if (myView instanceof ImageView) { } } } #7 Kotlin - Smart Cast http://kotlinlang.org/docs/reference/typecasts.html#smart-casts vs
15. class MyUtils { static void example(View myView) { if (myView instanceof ImageView) { ImageView imageView = (ImageView) myView; } } } #7 Kotlin - Smart Cast http://kotlinlang.org/docs/reference/typecasts.html#smart-casts vs
16. class MyUtils { static void example(View myView) { if (myView instanceof ImageView) { ImageView imageView = (ImageView) myView; imageView.setImageAlpha(10); } } } #7 Kotlin - Smart Cast http://kotlinlang.org/docs/reference/typecasts.html#smart-casts vs
17. class MyUtils { static void example(View myView) { if (myView instanceof ImageView) { ImageView imageView = (ImageView) myView; imageView.setImageAlpha(10); } else if (myView instanceof TextView) { TextView textView = (TextView) myView; textView.setText("Ciao"); } } } #7 Kotlin - Smart Cast http://kotlinlang.org/docs/reference/typecasts.html#smart-casts vs
18. fun example1(myView: View) { if (myView is ImageView) { myView.setImageAlpha(10) } else if (myView is TextView) { myView.setText("Ciao") } } class MyUtils { static void example(View myView) { if (myView instanceof ImageView) { ImageView imageView = (ImageView) myView; imageView.setImageAlpha(10); } else if (myView instanceof TextView) { TextView textView = (TextView) myView; textView.setText("Ciao"); } } } #7 Kotlin - Smart Cast http://kotlinlang.org/docs/reference/typecasts.html#smart-casts vs
19. fun example1(myView: View) { if (myView is ImageView) { myView.setImageAlpha(10) } else if (myView is TextView) { myView.setText("Ciao") } } fun example2(myView: View) { when (myView) { is ImageView -> myView.imageAlpha = 10 is TextView -> myView.text = "Ciao" } } class MyUtils { static void example(View myView) { if (myView instanceof ImageView) { ImageView imageView = (ImageView) myView; imageView.setImageAlpha(10); } else if (myView instanceof TextView) { TextView textView = (TextView) myView; textView.setText("Ciao"); } } } #7 Kotlin - Smart Cast http://kotlinlang.org/docs/reference/typecasts.html#smart-casts vs
20. fun example() { val os = listOf("Android", "iOS", null, "Windows Phone") } class MyUtils { static void example() { List<String> os = Arrays.asList("Android", "iOS", null, "Windows Phone"); } } #8 Kotlin - Using collections http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections vs
21. fun example() { val os = listOf("Android", "iOS", null, "Windows Phone") os.filterNotNull() } class MyUtils { static void example() { List<String> os = Arrays.asList("Android", "iOS", null, "Windows Phone"); List<String> osNotNull = new ArrayList<>(); for (String name : os) { if (name != null) osNotNull.add(name); } } } #8 Kotlin - Using collections http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections vs
22. fun example() { val os = listOf("Android", "iOS", null, "Windows Phone") os.filterNotNull().sortedBy { it.length() } } class MyUtils { static void example() { List<String> os = Arrays.asList("Android", "iOS", null, "Windows Phone"); List<String> osNotNull = new ArrayList<>(); for (String name : os) { if (name != null) osNotNull.add(name); } Collections.sort(osNotNull, new Comparator<String>() { @Override public int compare(String l, String r) { return r.length() - l.length(); } }); } } #8 Kotlin - Using collections http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections vs
23. fun example() { val os = listOf("Android", "iOS", null, "Windows Phone") os.filterNotNull().sortedBy { it.length() } .map { it.toUpperCase() } } class MyUtils { static void example() { List<String> os = Arrays.asList("Android", "iOS", null, "Windows Phone"); List<String> osNotNull = new ArrayList<>(); for (String name : os) { if (name != null) osNotNull.add(name); } Collections.sort(osNotNull, new Comparator<String>() { @Override public int compare(String l, String r) { return l.length() - r.length(); } }); for (String name : osNotNull) { String value = name.toUpperCase(); } } } #8 Kotlin - Using collections http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections vs
24. fun example() { val os = listOf("Android", "iOS", null, "Windows Phone") os.filterNotNull().sortedBy { it.length() } .map { it.toUpperCase() } .forEach { print(it) } } class MyUtils { static void example() { List<String> os = Arrays.asList("Android", "iOS", null, "Windows Phone"); List<String> osNotNull = new ArrayList<>(); for (String name : os) { if (name != null) osNotNull.add(name); } Collections.sort(osNotNull, new Comparator<String>() { @Override public int compare(String l, String r) { return l.length() - r.length(); } }); for (String name : osNotNull) { String value = name.toUpperCase(); print(value); } } } #8 Kotlin - Using collections http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections vs
25. Android Example From Java to Kotlin! https://github.com/jacklt/KotlinExample/tree/ex1
26. dummy/Items.kt data class HeroItem(val name: String, val gender: String, val power: Int)
27. dummy/HeroDummyContent.kt object HeroDummyContent { private val MALE = "Male" private val FEMALE = "Female" val ITEMS = listOf( HeroItem("Hulk", MALE, 9), HeroItem("Iron Man", MALE, 8), HeroItem("Thor", MALE, 8), HeroItem("Jessica Jones", FEMALE, 2), HeroItem("Spiderman", MALE, 4) ) }
28. dummy/HeroDummyContent.kt > object vs class object HeroDummyContent { private val MALE = "Male" private val FEMALE = "Female" val ITEMS = listOf( HeroItem("Hulk", MALE, 9), HeroItem("Iron Man", MALE, 8), HeroItem("Thor", MALE, 8), HeroItem("Jessica Jones", FEMALE, 2), HeroItem("Spiderman", MALE, 4) ) }
29. dummy/HeroDummyContent.kt > listOf(...) object HeroDummyContent { private val MALE = "Male" private val FEMALE = "Female" val ITEMS = listOf( HeroItem("Hulk", MALE, 9), HeroItem("Iron Man", MALE, 8), HeroItem("Thor", MALE, 8), HeroItem("Jessica Jones", FEMALE, 2), HeroItem("Spiderman", MALE, 4) ) }
30. dummy/HeroAdapter.kt class HeroAdapter(val mValues: List<HeroItem>, val mListener: Function1<HeroItem, Unit>?) : RecyclerView.Adapter<HeroAdapter.ViewHolder>() { // ... override fun onBindViewHolder(holder: ViewHolder, position: Int) { val hero = mValues[position] holder.item = hero holder.nameView.text = "${hero.name} (Power: ${hero.power})" holder.genderView.text = hero.gender holder.itemView.setOnClickListener { mListener?.invoke(hero) } } // ... }
31. dummy/HeroAdapter.kt > String template class HeroAdapter(val mValues: List<HeroItem>, val mListener: Function1<HeroItem, Unit>?) : RecyclerView.Adapter<HeroAdapter.ViewHolder>() { // ... override fun onBindViewHolder(holder: ViewHolder, position: Int) { val hero = mValues[position] holder.item = hero holder.nameView.text = "${hero.name} (Power: ${hero.power})" holder.genderView.text = hero.gender holder.itemView.setOnClickListener { mListener?.invoke(hero) } } // ... }
32. MainActivity.kt class MainActivity : AppCompatActivity() { private var currentHero: HeroItem? = null private val adapter by lazy { HeroAdapter(HeroDummyContent.ITEMS) { // onItemClick ... } } // ... }
33. MainActivity.kt > adapter (lazy) HeroAdapter(HeroDummyContent.ITEMS) { // onClick val previousHero = currentHero if (previousHero != null) { // fight val conclusion = if (previousHero.power == it.power) { "It's a draw" } else if (previousHero.power > it.power) { "${previousHero.name} wins!" } else { "${it.name} wins!" } Snackbar.make(fab, "${previousHero.name} vs ${it.name}!\n$conclusion", Snackbar.LENGTH_LONG).show() currentHero = null } else { // assign current hero Snackbar.make(fab, "Choose: ${it.name}", Snackbar.LENGTH_SHORT).show() currentHero = it } }
34. MainActivity.kt > adapter (lazy) > Smart cast HeroAdapter(HeroDummyContent.ITEMS) { // onClick val previousHero = currentHero if (previousHero != null) { // fight val conclusion = if (previousHero.power == it.power) { "It's a draw" } else if (previousHero.power > it.power) { "${previousHero.name} wins!" } else { "${it.name} wins!" } Snackbar.make(fab, "${previousHero.name} vs ${it.name}!\n$conclusion", Snackbar.LENGTH_LONG).show() currentHero = null } else { // assign current hero Snackbar.make(fab, "Choose: ${it.name}", Snackbar.LENGTH_SHORT).show() currentHero = it } }
35. MainActivity.kt > adapter (lazy) > return if expression HeroAdapter(HeroDummyContent.ITEMS) { // onClick val previousHero = currentHero if (previousHero != null) { // fight val conclusion = if (previousHero.power == it.power) { "It's a draw" } else if (previousHero.power > it.power) { "${previousHero.name} wins!" } else { "${it.name} wins!" } Snackbar.make(fab, "${previousHero.name} vs ${it.name}!\n$conclusion", Snackbar.LENGTH_LONG).show() currentHero = null } else { // assign current hero Snackbar.make(fab, "Choose: ${it.name}", Snackbar.LENGTH_SHORT).show() currentHero = it } }
36. Questions? Developers playground - #EX2 - Aggiungere le proprietà a HeroItem: hair (String?), eyes (String?) - Mostrarle (se presenti) nella lista - Se vengono scelti 2 eroi di sesso opposto, viene creato un nuovo eroe - Si possono usare le proprietà dei genitori, oppure dei valori di default se mancano Start with: https://github.com/jacklt/KotlinExample/tree/ex1 Solution: https://github.com/jacklt/KotlinExample/tree/ex2
37. THANKS! Omar Miatello, Member of GDG Milano (Italy) Android Developer @ Satispay