你为什么应该学习 Kotlin ？

/*
 Create a POJO with getters, setters, `equals()`, `hashCode()`, `toString()` and `copy()` in a single line:
*/
​
data class Customer(val name: String, val email: String, val company: String)
​
// Or filter a list using a lambda expression:
​
val positiveNumbers = list.filter { it > 0 }
​
// Want a singleton? Create an object:
​
object ThisIsASingleton {
    val companyName: String = "JetBrains"
}

/*
 Get rid of those pesky NullPointerExceptions, you know, The Billion Dollar Mistake
*/
​
var output: String
output = null   // Compilation error
​
// Kotlin protects you from mistakenly operating on nullable types
​
val name: String? = null    // Nullable type
println(name.length())      // Compilation error
​
// And if you check a type is right, the compiler will auto-cast it for you
​
fun calculateTotal(obj: Any) {
    if (obj is Invoice)
        obj.calculateTotal()
}

/*
 Use any existing library on the JVM, as there’s 100% compatibility, including SAM support.
*/
​
import io.reactivex.Flowable
import io.reactivex.schedulers.Schedulers
​
Flowable
    .fromCallable {
        Thread.sleep(1000) //  imitate expensive computation
        "Done"
    }
    .subscribeOn(Schedulers.io())
    .observeOn(Schedulers.single())
    .subscribe(::println, Throwable::printStackTrace)
// Target either the JVM or JavaScript. Write code in Kotlin and decide where you want to deploy to
​
import kotlin.browser.window
​
fun onLoad() {
    window.document.body!!.innerHTML += "<br/>Hello, Kotlin!"
}