Kotlin入门之基本语法（Basic Syntax）

包定义

包应位于源文件的顶部（最开始的地方）

package my.demo
import java.util.*
// ...

不要求包定义内容和实际的文件目录的一致性：源文件可以放置在文件系统中的任意位置。

函数定义

具有两个Int类型的参数和一个Int类型的返回值的函数的定义如下所示：

fun sum(a: Int, b: Int): Int {
    return a + b
}

fun main(args: Array<String>) {
    print("sum of 3 and 5 is ")
    println(sum(3, 5))
}

运行结果为：sum of 3 and 5 is 8

具有一个表达式方法体和一个推断类型返回值的函数的定义如下所示：

fun sum(a: Int, b: Int) = a + b

fun main(args: Array<String>) {
    println("sum of 19 and 23 is ${sum(19, 23)}")
}

运行结果为：sum of 19 and 23 is 42

返回值没有意义（传统意义上的返回值为void）的函数的定义如下所示：

fun printSum(a: Int, b: Int): Unit {
    println("sum of $a and $b is ${a + b}")
}

fun main(args: Array<String>) {
    printSum(-1, 8)
}

运行结果为：sum of -1 and 8 is 7

函数的返回值类型为Unit的话，该类型可省略不写，如下：

fun printSum(a: Int, b: Int) {
    println("sum of $a and $b is ${a + b}")
}

fun main(args: Array<String>) {
    printSum(-1, 8)
}

运行结果为：sum of -1 and 8 is 7

局部变量的定义

分配一次（只读）局部变量的定义如下：

fun main(args: Array<String>) {
    val a: Int = 1  // immediate assignment
    val b = 2   // `Int` type is inferred
    val c: Int  // Type required when no initializer is provided
    c = 3       // deferred assignment
    println("a = $a, b = $b, c = $c")
}

运行结果为：a = 1, b = 2, c = 3

可变变量的定义如下：

fun main(args: Array<String>) {
    var x = 5 // `Int` type is inferred
    x += 1
    println("x = $x")
}

运行结果为：x = 6

注释

类似Java和javaScript，Kotlin也提供了行尾注释和块注释

// This is an end-of-line comment

/* This is a block comment
   on multiple lines. */

与Java不同，Kotlin中的块注释可以嵌套。

使用字符串模板（templates，也译为属性单元）

fun main(args: Array<String>) {
    var a = 1
    // simple name in template:
    val s1 = "a is $a" 

    a = 2
    // arbitrary expression in template:
    val s2 = "${s1.replace("is", "was")}, but now is $a"
    println(s2)
}

运行结果为：a was 1, but now is 2

使用条件表达式

fun maxOf(a: Int, b: Int): Int {
    if (a > b) {
        return a
    } else {
        return b
    }
}

fun main(args: Array<String>) {
    println("max of 0 and 42 is ${maxOf(0, 42)}")
}

运行结果为：max of 0 and 42 is 42

使用if作为一个表达式，如下：

fun maxOf(a: Int, b: Int) = if (a > b) a else b

fun main(args: Array<String>) {
    println("max of 0 and 42 is ${maxOf(0, 42)}")
}

运行结果为：max of 0 and 42 is 42

使用可null的值和判null

一个引用变量的值可以为null的时候，必须明确标记该引用变量可null。

一个函数预期实现：如果形参str不包含一个整数就返回null。该函数可定义如下：

fun parseInt(str: String): Int? {
    // ...
}

调用一个返回值可null的函数的示例如下：

fun parseInt(str: String): Int? {
    return str.toIntOrNull()
}

fun printProduct(arg1: String, arg2: String) {
    val x = parseInt(arg1)
    val y = parseInt(arg2)

    // Using `x * y` yields error because they may hold nulls.
    if (x != null && y != null) {
        // x and y are automatically cast to non-nullable after null check
        println(x * y)
    }
    else {
        println("either '$arg1' or '$arg2' is not a number")
    }    
}

fun main(args: Array<String>) {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("a", "b")
}

运行结果为：42
          either 'a' or '7' is not a number
          either 'a' or 'b' is not a number

或者如下：

fun parseInt(str: String): Int? {
    return str.toIntOrNull()
}

fun printProduct(arg1: String, arg2: String) {
    val x = parseInt(arg1)
    val y = parseInt(arg2)
    
    // ...
    if (x == null) {
        println("Wrong number format in arg1: '${arg1}'")
        return
    }
    if (y == null) {
        println("Wrong number format in arg2: '${arg2}'")
        return
    }

    // x and y are automatically cast to non-nullable after null check
    println(x * y)
}

fun main(args: Array<String>) {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("99", "b")
}

运行结果为：42
          Wrong number format in arg1: 'a'
          Wrong number format in arg2: 'b'

使用类型检查和自动转换

is操作符用于判断一个表达式是否是一个类型的实例。如果一个不可变（只读）局部变量或者属性被is操作符判断为是某类型的，则不必再对其强转，如下所示：

fun getStringLength(obj: Any): Int? {
    if (obj is String) {
        // `obj` is automatically cast to `String` in this branch
        return obj.length
    }

    // `obj` is still of type `Any` outside of the type-checked branch
    return null
}

fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
    }
    printLength("Incomprehensibilities")
    printLength(1000)
    printLength(listOf(Any()))
}

运行结果为：'Incomprehensibilities' string length is 21 
          '1000' string length is ... err, not a string 
          '[java.lang.Object@6b884d57]' string length is ... err, not a string 

或者：

fun getStringLength(obj: Any): Int? {
    if (obj !is String) return null

    // `obj` is automatically cast to `String` in this branch
    return obj.length
}


fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
    }
    printLength("Incomprehensibilities")
    printLength(1000)
    printLength(listOf(Any()))
}

运行结果为：'Incomprehensibilities' string length is 21 
          '1000' string length is ... err, not a string 
          '[java.lang.Object@6b884d57]' string length is ... err, not a string 

甚至：

fun getStringLength(obj: Any): Int? {
    // `obj` is automatically cast to `String` on the right-hand side of `&&`
    if (obj is String && obj.length > 0) {
        return obj.length
    }

    return null
}


fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, is empty or not a string at all"} ")
    }
    printLength("Incomprehensibilities")
    printLength("")
    printLength(1000)
}

运行结果为'Incomprehensibilities' string length is 21 
         '' string length is ... err, is empty or not a string at all 
         '1000' string length is ... err, is empty or not a string at all ：

使用for循环

fun main(args: Array<String>) {
    val items = listOf("apple", "banana", "kiwi")
    for (item in items) {
        println(item)
    }
}

运行结果为：apple
          banana
          kiwi

或者：

fun main(args: Array<String>) {
    val items = listOf("apple", "banana", "kiwi")
    for (index in items.indices) {
        println("item at $index is ${items[index]}")
    }
}

运行结果为：item at 0 is apple
          item at 1 is banana
          item at 2 is kiwi

使用while循环

fun main(args: Array<String>) {
    val items = listOf("apple", "banana", "kiwi")
    var index = 0
    while (index < items.size) {
        println("item at $index is ${items[index]}")
        index++
    }
}

运行结果为：item at 0 is apple
          item at 1 is banana
          item at 2 is kiwi

使用when表达式

fun describe(obj: Any): String =
when (obj) {
    1          -> "One"
    "Hello"    -> "Greeting"
    is Long    -> "Long"
    !is String -> "Not a string"
    else       -> "Unknown"
}

fun main(args: Array<String>) {
    println(describe(1))
    println(describe("Hello"))
    println(describe(1000L))
    println(describe(2))
    println(describe("other"))
}

运行结果为：One
          Greeting
          Long
          Not a string
          Unknown

使用范围

使用in操作符来判定一个数字是否在指定范围内：

fun main(args: Array<String>) {
    val x = 10
    val y = 9
    if (x in 1..y+1) {
        println("fits in range")
    }
}

运行结果为：fits in range

判定一个数字不在某个范围，示例如下：

fun main(args: Array<String>) {
    val list = listOf("a", "b", "c")

    if (-1 !in 0..list.lastIndex) {
        println("-1 is out of range")
    }
    if (list.size !in list.indices) {
        println("list size is out of valid list indices range too")
    }
}

运行结果为：-1 is out of range
          list size is out of valid list indices range too

范围迭代：

fun main(args: Array<String>) {
    for (x in 1..5) {
        print(x)
    }
}

运行结果为：12345

步进：

fun main(args: Array<String>) {
    for (x in 1..10 step 2) {
        print(x)
    }
    for (x in 9 downTo 0 step 3) {
        print(x)
    }
}

运行结果为：135799630

使用集合

迭代（遍历）一个集合示例如下：

fun main(args: Array<String>) {
    val items = listOf("apple", "banana", "kiwi")
    for (item in items) {
        println(item)
    }
}

运行结果为：apple
          banana
          kiwi

使用in操作符来判定一个集合是否包含某个对象，示例如下：

fun main(args: Array<String>) {
    val items = setOf("apple", "banana", "kiwi")
    when {
        "orange" in items -> println("juicy")
        "apple" in items -> println("apple is fine too")
    }
}

实现结果为：apple is fine too

使用Lambda表达式来过滤和映射一个集合：

fun main(args: Array<String>) {
    val fruits = listOf("banana", "avocado", "apple", "kiwi")
    fruits
    .filter { it.startsWith("a") }
    .sortedBy { it }
    .map { it.toUpperCase() }
    .forEach { println(it) }
}

运行结果为：APPLE
          AVOCADO