kotlin汇总8-其他2

1.Null安全

1.1通过代码来理解

var a: String = "abc"val l = a.length // oka = null // compilation errorvar b: String? = "abc"val l = b.length // error: variable 'b' can be nullb = null // ok

1.2安全调用

b?.lengthbob?.department?.head?.name//如果是java需要下面着这样写if(bob == null) return null;if(bob.department == null) return null;if(bob.department.head == null) return null;return bob.department.head.name;

1.3 ?:操作符

val l: Int = if (b != null) b.length else -1//和下面一样val l = b?.length ?: -1

也可以在后面接语句

fun foo(node: Node): String? {    val parent = node.getParent() ?: return null    val name = node.getName() ?: throw IllegalArgumentException("name expected")    // ...}

1.4 !!操作符

val l = b!!.length //如果b是null，会抛出空指针错误

2.Exceptions

在kotlin，所有的exception类都是继承自Throwable, 和Java基本一致。

try {    // some code}catch (e: SomeException) {    // handler}finally {    // optional finally block}

3.Annotations 注释

使用annotation修饰

annotation class Fancy

@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION,        AnnotationTarget.VALUE_PARAMETER, AnnotationTarget.EXPRESSION)@Retention(AnnotationRetention.SOURCE)@MustBeDocumentedannotation class Fancy

annotations可以有带参的构造函数

annotation class Special(val why: String)@Special("example") class Foo {}

参数类型可以是：
- types that correspond to Java primitive types (Int, Long etc.);
- strings;
- classes (Foo::class);
- enums;
- other annotations;
- arrays of the types listed above.
看下几个例子

annotation class ReplaceWith(val expression: String)annotation class Deprecated(        val message: String,        val replaceWith: ReplaceWith = ReplaceWith(""))@Deprecated("This function is deprecated, use === instead", ReplaceWith("this === other"))

import kotlin.reflect.KClassannotation class Ann(val arg1: KClass<*>, val arg2: KClass<out Any?>)@Ann(String::class, Int::class) class MyClass

lambdas也可以用annotation修饰

annotation class Suspendableval f = @Suspendable { Fiber.sleep(10) }

给使用指定位置加上注释,可以给属性或者基础构造函数的参数加上。

class Example(@field:Ann val foo,    // annotate Java field              @get:Ann val bar,      // annotate Java getter              @param:Ann val quux)   // annotate Java constructor parameter@file:JvmName("Foo")package org.jetbrains.democlass Example {     @set:[Inject VisibleForTesting]     var collaborator: Collaborator}

有下面是支持的
- file
- property (annotations with this target are not visible to Java)
- field
- get (property getter)
- set (property setter)
- receiver (receiver parameter of an extension function or property)
- param (constructor parameter)
- setparam (property setter parameter)
- delegate (the field storing the delegate instance for a delegated property)

另外Java的annotation也可以用在kotlin中。

4.反射

在java平台，需要引入kotlin-reflect.jar来使用反射。

4.1类反射

val c = MyClass::class //是KClass

4.2函数反射

fun isOdd(x: Int) = x % 2 != 0val numbers = listOf(1, 2, 3)println(numbers.filter(::isOdd)) // prints [1, 3] //::isOdd等于 (Int) -> Boolean

4.3属性反射

var x = 1fun main(args: Array<String>) {    println(::x.get()) // prints "1"    ::x.set(2)    println(x)         // prints "2"}

4.4和Java反射互用

import kotlin.reflect.jvm.*class A(val p: Int)fun main(args: Array<String>) {    println(A::p.javaGetter) // prints "public final int A.getP()"    println(A::p.javaField)  // prints "private final int A.p"}

4.5构造函数反射

class Foofun function(factory: () -> Foo) {    val x: Foo = factory()}function(::Foo) //::Foo是无参构造函数

5.类型安全的Builders

比较像groovy，可以阅读下面代码

import com.example.html.* // see declarations belowfun result(args: Array<String>) =    html {        head {            title {+"XML encoding with Kotlin"}        }        body {            h1 {+"XML encoding with Kotlin"}            p  {+"this format can be used as an alternative markup to XML"}            // an element with attributes and text content            a(href = "http://kotlinlang.org") {+"Kotlin"}            // mixed content            p {                +"This is some"                b {+"mixed"}                +"text. For more see the"                a(href = "http://kotlinlang.org") {+"Kotlin"}                +"project"            }            p {+"some text"}            // content generated by            p {                for (arg in args)                    +arg            }        }    }

6.类型别名

使用typealias关键字修饰

typealias NodeSet = Set<Network.Node>typealias FileTable<K> = MutableMap<K, MutableList<File>>typealias MyHandler = (Int, String, Any) -> Unittypealias Predicate<T> = (T) -> Booleanclass A {    inner class Inner}class B {    inner class Inner}typealias AInner = A.Innertypealias BInner = B.Inner