Kotlin基本语法（译）

Posted 2022-12-11 子不语归来

# 基本语法 ##定义包 软件包规范应位于源文件的顶部：

package my.demo

import java.util.*

// ...

不需要匹配目录和包：源文件可以任意放在文件系统中。 ## 定义功能 全Kotlin参考 编辑页面 基本语法 定义包 软件包规范应位于源文件的顶部： package my.demo import java.util.* // … 不需要匹配目录和包：源文件可以任意放在文件系统中。 参见包。 定义功能 具有返回类型的两个Int参数的Int函数：

//sampleStart
fun sum(a: Int, b: Int): Int 
    return a + b

//sampleEnd

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

函数与表达体和推断返回类型：

//sampleStart
fun sum(a: Int, b: Int) = a + b
//sampleEnd

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

函数返回无意义值：

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

//sampleEnd

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

Unit 返回类型可以省略：

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

//sampleEnd

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

## 定义局部变量 分配一次（只读）局部变量：

fun main(args: Array<String>) 
//sampleStart
    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
//sampleEnd
    println("a = $a, b = $b, c = $c")

可变变量：

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

## 注释 就像Java和javascript一样，Kotlin支持行尾和//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

fun main(args: Array<String>) 
//sampleStart
    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"
//sampleEnd
    println(s2)

## 使用条件表达式

//sampleStart
fun maxOf(a: Int, b: Int): Int 
    if (a > b) 
        return a
     else 
        return b
    

//sampleEnd

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

使用if表达式：

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

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

## 使用可空值并检查null 当空值可能时，引用必须被明确地标记为可空。 如果不持有整数则返回nullstr：

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

使用返回可空值的函数：

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


//sampleStart
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")
        

//sampleEnd


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

或者

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


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

//sampleStart
    // ...
    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)
//sampleEnd


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

## 使用类型检查和自动转换 这是操作者检查是否一个表达式是一个类型的一个实例。如果为特定类型检查不可变的局部变量或属性，则不需要显式转换：

//sampleStart
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

//sampleEnd


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()))

或者

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

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

//sampleEnd


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()))

甚至

//sampleStart
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

//sampleEnd


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)

## 使用for循环

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

或者

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

## 使用while循环

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

##使用when表达式

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

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

##使用范围 检查一个数字是使用范围内操作

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

检查一个数字是否超出范围：

fun main(args: Array<String>) 
//sampleStart
    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")
    
//sampleEnd

迭代一个循环：

fun main(args: Array<String>) 
//sampleStart
    for (x in 1..5) 
        print(x)
    
//sampleEnd

或过程：

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

## 使用集合 迭代集合：

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

检查如果集合包含对象使用in关键词 操作:

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

使用lambda表达式过滤和映射集合：

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

原文Kotlin基本语法