Download pdf - Kotlin: Let's Make Android Great Again

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ExtendedSeoul

ExtendedSeoul

Statically typed programming language for the JVM, Android and the browser

100% interoperable with Java™

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ExtendedSeoul

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MyActivity.java

btnDoIt = (Button) findViewById(R.id.btn_do_it);

MyActivity.java

btnDoIt = (Button) findViewById(R.id.btn_do_it); btnDoIt.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { // TODO } });

MyActivity.java

btnDoIt = (Button) findViewById(R.id.btn_do_it); btnDoIt.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { // TODO } });

WithRxJava.java

Observable.fromArray(1, 2, 3, 4, 5) .map(new Function<Integer, String>() { @Override public String apply(Integer param) { return "A" + param; } });

WithRxJava.java

Observable.fromArray(1, 2, 3, 4, 5) .map(new Function<Integer, String>() { @Override public String apply(Integer param) { return "A" + param; } });

WithRxJava.java

Observable.fromArray("1", "2", "3", "4", "5") .map(new Function<Integer, String>() { @Override public String apply(Integer param) { return "A" + param; } });

WithRxJava.java

Observable.fromArray("1", "2", "3", "4", "5") .map(new Function<Integer, String>() { @Override public String apply(Integer param) { return "A" + param; } });

WithRxJava.java

Observable.fromArray("1", "2", "3", "4", "5") .map(new Function<String, String>() { @Override public String apply(String param) { return "A" + param; } });

User.java

String getAddress() { ... }

User user = getCurrentUser(); user.getAddress().toUpperCase();

ExtendedSeoul

I call it my billion-dollar mistake. This has led to innumerable errors … probably caused a billion dollars of pain and damage in the last forty years.

ExtendedSeoul

- Tony Hoare

User.java

String getAddress() { return this.address; }


User.java

String getAddress() { return null == this.address ? "" : this.address; }


User.java

@Nullable String getAddress() { return this.address; }


User.java

@NonNull String getAddress() { return null == this.address ? "" : this.address; }


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MyActivity.java

Button btnFoo; TextView tvBar;

@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main);

btnFoo = (Button) findViewById(R.id.btn_foo); tvBar = (TextView) findViewById(R.id.tv_bar); ... }

MyActivity.java




Intents.java

Intent intent = new Intent(this, OtherActivity.class); intent.putExtra("id", 10L); intent.setFlag(Intent.FLAG_ACTIVITY_SINGLE_TOP); startActivity(intent);


MyActivity.java

btnDoIt.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { // TODO } });

MyActivity.kt

btnDoIt.setOnClickListener(object : View.OnClickListener() { override fun onClick(v: View) { // TODO } })

MyActivity.kt

btnDoIt.setOnClickListener({ v: View -> // TODO })

MyActivity.kt

btnDoIt.setOnClickListener({ v -> // TODO })

MyActivity.kt

btnDoIt.setOnClickListener({ it.context // View.context })

MyActivity.kt

btnDoIt.setOnClickListener { it.context // View.context }

StandardLibrary.kt

val people : List<String> = ..

people.filter { it.startsWith('S') } .filter { it.length < 10 } .onEach { it.toUpperCase() } .forEach { println(it) }

StandardLibrary.kt

val people : List<String> = ..

people.filter { it.startsWith('S') } .filter { it.length < 10 } .onEach { String::toUpperCase() } .forEach { ::println }

StandardLibrary.kt

val people : List<String> = .. people.stream() .filter { it.startsWith('S') } .filter { it.length < 10 } .map(String::toUpperCase) .forEach(::println)

User.java

@Nullable String getAddress() { return this.address; }


User.java

// class User var address: String? = null

//// val user : User = getCurrentUser() user.address?.toUpperCase()

User.java



User.java



User.java


//// val user = getCurrentUser() user.address?.toUpperCase()

User.java

@NonNull String getAddress() { return null == this.address ? "" : this.address; }


User.java

// class User var address: String? = null get() = field ?: ""

//// val user = getCurrentUser() user.address.toUpperCase()

User.java



User.java



MyActivity.java




MyActivity.kt

override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main)

btn_foo // (Button) findViewById(R.id.btn_foo) tv_bar // (TextView) findViewById(R.id.tv_bar) ... }

Intents.java

Intent intent = new Intent(this, OtherActivity.class); intent.putExtra("id", 10L); intent.setFlag(Intent.FLAG_ACTIVITY_SINGLE_TOP); startActivity(intent);

Intents.kt

val intent = Intent(this, OtherActivity::class.java) intent.putExtra("id", 10L) intent.setFlag(Intent.FLAG_ACTIVITY_SINGLE_TOP) startActivity(intent)

Intents.kt

startActivity( intentFor<SomeOtherActivity>("id" to 10L).singleTop())

Collections.kt

val mutableList : MutableList<String> = mutableListOf(“foo", "bar", "baz") mutableList.add("fizz")

val immutableList : List<String> = listOf("foo", "bar", "baz") immutableList.add("fizz")

Collections.kt

val mutableList : MutableList<String> = mutableListOf(“foo", "bar", "baz") mutableList.add("fizz")

val immutableList : List<String> = listOf("foo", "bar", "baz") immutableList.add("fizz")

Collections.kt

val emptyStringList = listOf<String>() val cities = listOf("Seoul", "Busan") val mutableCities = mutableListOf("Seoul, Busan")

val emptyStringSet = setOf<String>() val cities = setOf("Seoul", "Busan") val mutableCities = mutableSetOf("Seoul, Busan")

Collections.kt

val pair : Pair<String, String> = Pair("Seoul", "SEO")

Collections.kt

val pair : Pair<String, String> = Pair("Seoul", "SEO")

// with stdlib val pair : Pair<String, String> = "Seoul" to "SEO"

Person.java

public class Person { String name; String address;

Person(String name, String address) { this.name = name; this.address = address; }

public String getAddress() { return address; }

.. }

Person.javapublic class Person {

String name;

String address;

Person(String name, String address) { this.name = name; this.address = address; }

public String getAddress() { return address; }

public void setAddress(String address) { this.address = address; }

@Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false;

Person person = (Person) o;

if (name != null ? !name.equals(person.name) : person.name != null) return false; return address != null ? address.equals(person.address) : person.address == null;

}

@Override public int hashCode() { int result = name != null ? name.hashCode() : 0; result = 31 * result + (address != null ? address.hashCode() : 0); return result; } }

Person.kt

data class Person(val name: String, val address: String)

MyToast.kt

Toast.makeText(applicationContext, "Hello, Kotlin!", Toast.LENGTH_SHORT).show()

MyToast.kt

// Define an extension function on Context fun Context.toast(message: String) { Toast.makeText(this.applicationContext, message, Toast.LENGTH_SHORT).show() }

// available in class Context and its descendants toast("Hello, Kotlin!")

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