Interesting kotlin 0x08:what am I

Recently http://portal.kotlin-academy.com/#/ I see a lot about Kotlin An interesting topic . I think it's very suitable for Kotlin lovers , If you are interested, you can check it by yourself .
【 amusing Kotlin 】 Record your understanding of each question in the series .

0x08：What am I ?

fun main(args: Array<String>) {
    
    val whatAmI = {
    }()
    println(whatAmI)
}

Above code , What is the result of the operation ？ optional ：

1. "null"
2. "kotlin.Unit"
3. Doesn’t print anything
4. Doesn’t compile

Think about it , Record the answers in your mind .

analysis

Unknowingly, it has reached question 8 , Every time you look at a question , I feel I have something to gain , And the knowledge behind each question can be found in the official documents , While doing the questions, I also force myself to read the official documents again and again .

If you have carefully read the first seven questions, you should be able to get the answer to this question soon ：

Options 2 ："kotlin.Unit"

Friends in doubt go on , If you're smart and you've got the right answer , Please move freely .

As long as the variables are clear whatAmI The type of , The problem is solved .

Let's look at parentheses and parentheses separately , Leaving only large brackets whatAmI What type is it ？

val whatAmI = {
    }

To the right of the equal sign is a Lambda expression , The type is ()-> Unit , Namely variable wahtAmI The type of , and Lambda The expression is followed by () In fact, it's called invoke() function , Represents a call to a function type .

that , One type is ()-> Unit What is returned after the function type is called ？ Naturally Unit , Did I say a word of nonsense . therefore , In question

val whatAmI = {
    }()

wahtAmI The type of kotlin.Unit, So the answer is kotlin.Unit ? Not rigorous enough . We need to see again println Function in JVM Implementation logic on the platform , Although the content may be simple

/** Prints the given [message] and the line separator to the standard output stream. */
@kotlin.internal.InlineOnly
public actual inline fun println(message: Any?) {
    
    System.out.println(message)
}

Keep going

public void println(Object x) {
    
    String s = String.valueOf(x);
    synchronized (this) {
    
        print(s);
        newLine();
    }
}

Until then valueOf() Method

public static String valueOf(Object obj) {
    
    return (obj == null) ? "null" : obj.toString();
}

therefore , The final output of the title to the console depends on kotlin.Unit Of toString() Method .

public object Unit {
    
    override fun toString() = "kotlin.Unit"
}

here , We can say that the correct answer to this question is ：

Options 2 ："kotlin.Unit"

extend

We see the println() The function is implemented with the following code

/** Prints the given [message] and the line separator to the standard output stream. */
@kotlin.internal.InlineOnly
public actual inline fun println(message: Any?) {
    
    System.out.println(message)
}

Appear one actual keyword , Maybe some readers don't know , A little explanation .

Kotlin Language starts from the beginning of design , Positioning is cross platform language , Designed for developers to use Kotlin Language to develop applications on any platform . But in some cases, developers may need to write different code for different platforms . Now we need to go through expect Keywords define classes or methods that need to be implemented on different platforms in the common code part , Then, in the directory corresponding to each platform, through actual Keyword to implement the corresponding class or method , This is Kotlin Of expect/actual Mechanism . Let's look for println() Corresponding expect function .

In a similar way , We can also find other platforms println() Realization

Quote official pictures to illustrate the problem ：https://kotlinlang.org/docs/mobile/connect-to-platform-specific-apis.html