【Kotlin】基础语法学习

Kotlin简介

Kotlin 是一个基于 JVM 的新的编程语言，由 JetBrains 开发。Kotlin可以编译成Java字节码，也可以编译成JavaScript，方便在没有JVM的设备上运行。JetBrains，作为目前广受欢迎的Java IDE IntelliJ 的提供商，在 Apache 许可下已经开源其Kotlin 编程语言。Kotlin已正式成为Android官方开发语言。

设计目标

1.创建一种兼容Java的语言

2.让它比Java更安全，能够静态检测常见的陷阱。如：引用空指针

3.让它比Java更简洁，通过支持variable type inference，higher-order functions (closures)，extension functions，mixins and first-class delegation等实现。

4.让它比最成熟的竞争对手Scala语言更加简单。

基础语法

安装jdk，下载JetBrains开发的IDE：IntelliJ IDEA，就可以新建Kotlin工程，运行Kotlin代码。这里我使用目前最新版IntelliJ IDEA 2017.1.3 x64和JDK 1.8.0作为基础环境。

1. 定义函数

(1)定义一个带有两个Int类型参数，并且返回Int值的函数

fun sum(a: Int, b: Int): Int {    return a + b}fun main(args: Array<String>) {    print("3 and 5 is ")    println(sum(3, 5))}

运行结果：3 and 5 is 8

(2)使用表达式主体和推断返回类型的函数

fun sum(a: Int, b: Int) = a + bfun main(args: Array<String>) {    print("3 and 5 is ")    println(sum(3, 5))}

运行结果：3 and 5 is 8

(3)函数返回没有有意义的值

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：sum of -1 and 8 is 7

在IDE中查看Unit类型源码，可以看到注释说这种类型对应Java中的void类型，也就是空

package kotlin/** * The type with only one value: the Unit object. This type corresponds to the `void` type in Java. */public object Unit {    override fun toString() = "kotlin.Unit"}

(4)Unit 在函数返回类型中可以省略

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：sum of -1 and 8 is 7

2. 定义局部变量

(1)局部变量只赋值一次的(只读)

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：a = 1, b = 2, c = 3

如果强行再次给a、b、c其中之一赋值，例如我给a再次赋值5，IDE中就会直接报错a = 5这行，尝试运行就会得到报错信息：Error:(9, 5) Kotlin: Val cannot be reassigned

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    a = 5    println("a = $a, b = $b, c = $c")}

(2)可变变量

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

运行结果：x = 6

3.注释

就像Java和JavaScript，Kotlin支持//和/**/注释。不像Java，Kotlin可以在/**/注释中和嵌套注释块。

// This is an end-of-line comment/* This is a block comment   on multiple lines. */

Java中嵌套注释块，报错

public class Main {    public static void main(String[] args) {        /*/*vhfgh*/        System.out.println("Hello World!");*/    }}

Error:(5, 44) java: 非法的表达式开始

Error:(5, 45) java: 非法的表达式开始

Error:(6, 5) java: 非法的表达式开始

Error:(7, 2) java: 解析时已到达文件结尾

Kotlin中嵌套注释块正常

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

4.使用字符串模板

fun main(args: Array<String>) {    var a = 5    // simple name in template(模板中使用简单名字):    val s1 = "a is $a"    println(s1)    a = 100    // arbitrary expression in template(模板中使用任意表达式):    val s2 = "${s1.replace("is", "was")}, but now is $a"    println(s2)}

运行结果：

a is 5

a was 5, but now is 100

5.使用条件表达式

以下的普通maxOf函数和使用if表达式的maxVal函数是等价的

fun maxOf(a: Int, b: Int): Int {    if (a > b) {        return a    } else {        return b    }}/** * 使用if表达式 */fun maxVal(a: Int, b : Int) = if (a > b) a else bfun main(args: Array<String>) {    println("max of 0 and 42 is ${maxOf(0, 42)}")    println("max of 0 and 42 is ${maxVal(0, 42)}")}

运行结果：

max of 0 and 42 is 42

max of 0 and 42 is 42

6.使用可空值和检查空值

当null值是可能的，引用时必须显式地标记为可为空。

返回null，如果str不持有一个整数

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

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：

42

either ‘a’ or ‘7’ is not a number

either ‘a’ or ‘b’ is not a number

或者可以这样写

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：

42

Wrong number format in arg1: ‘a’

Wrong number format in arg2: ‘b’

7.使用类型检查和自动转换

这是操作符检查，是否表达式是类型的实例。如果一个不可变的局部变量或属性为特定类型检查，没有必要把它显式转换。

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：

‘Incomprehensibilities’ string length is 21

‘1000’ string length is … err, not a string

‘[java.lang.Object@2626b418]’ string length is … err, not a string

或者这样写

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()))}

还可以这样写

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)}

8.使用for循环

使用for循环，这和python、groovy语法非常相似

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

运行结果：

apple

banana

kiwi

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：

item at 0 is apple

item at 1 is banana

item at 2 is kiwi

9.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：

item at 0 is apple

item at 1 is banana

item at 2 is kiwi

10.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：

One

Greeting

Long

Not a string

Unknown

11.使用范围

使用in操作符检查数字是否在范围内

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：fits in range

注意边界，是包含最大值的，这里的范围1..y+1，代表[1,10]包含左右边界，使用in操作符测试1~10的数字都是在范围内的

检查数字不在范围内

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：

-1 is out of range

list size is out of valid list indices range too

迭代一个范围

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

运行结果：12345

使用步长迭代一个范围，还可以使用downTo从大到小

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

运行结果：13579 9630

12.使用集合

迭代集合，和迭代范围一样

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

运行结果：

apple

banana

kiwi

使用in操作符检查集合是否包含对象

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")    }}

运行结果：apple is fine too

使用lambda表达式过滤和映射集合，而且有点流式编程的意思

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：

APPLE

AVOCADO

13.定义包

包定义应该在源文件的最顶部。它不需要匹配目录和包：源文件可以放在任意的文件系统。

package my.demoimport java.util.*// ...