大家好，我是William。今天是Koan第八關，Nullable types，空指針類型。

闖關鏈接：
https://try.kotlinlang.org/#/Kotlin%20Koans/Introduction/Nullable%20types/Task.kt

Introduction

Nullable types

Read about null safety and safe calls in Kotlin and rewrite the following Java code using only one if expression:

public void sendMessageToClient( 
    @Nullable Client client, 
    @Nullable String message,
    @NotNull Mailer mailer
) {
    if (client == null || message == null) return;

    PersonalInfo personalInfo = client.getPersonalInfo(); 
    if (personalInfo == null) return; 

    String email = personalInfo.getEmail(); 
    if (email == null) return; 

    mailer.sendMessage(email, message);
}

解：
空指針處理是Kotlin一大特色，這里就不展開說了，直接看題目的超鏈接，先了解一下。個人覺得Safe Call作用不大，但是Elvis Operator就相當實用了，起碼優(yōu)化了一下惡心的三目運算符。!!操作符要少用?？罩羔樚幚聿皇钦f你以后不用寫類似if(XXX==null)這種判斷了，而是Kotlin讓所有潛在空指針異常在編譯階段就已經發(fā)現(xiàn)了。所以Kotlin編譯出來的程序是沒有無感知空指針異常的情況的，除非手賤用了!!操作符或者手動拋出空指針異常。

答：
本人答案，居然就過了，當然標準答案更加優(yōu)雅。

fun sendMessageToClient(
    client: Client?, message: String?, mailer: Mailer
){
    if(client==null||message==null) return
    
    val personalInfo:PersonalInfo? = client?.personalInfo
    if(personalInfo==null) return
    
    val email = personalInfo?.email
    if(email==null) return
    
    mailer.sendMessage(email, message)
}

class Client (val personalInfo: PersonalInfo?)
class PersonalInfo (val email: String?)
interface Mailer {
    fun sendMessage(email: String, message: String)
}