Kotlin入門基礎1--一句話教程

1，定義函數

fun?函數名(參數名:類型,參數名：類型,...):返回類型{
????......
}

比如

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

如果不需要返回值，則可以

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

2，定義變量

如果是只讀變量，用val聲明，如果是可修改的變量，用var聲明

val?a:?Int?=?1??
val?b?=?2???//?自動推斷類型`Int`?
val?c:?Int??//?如果沒有初始值，則需要提供類型
c?=?3???????//?稍后賦值
var?x?=?5?//?自動推斷類型`Int`
x?+=?1

3，字符串模板

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"

4，if表達式

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

5，對于可能為null的值，必須判斷

fun?parseInt(str:?String):?Int??{
????//?如果不是int，就返回null
}

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")
????}????
}

6, 用is 關鍵字判斷對象類型，相當于java的instanceOf

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

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

7, list遍歷

val?items?=?listOf("apple",?"banana",?"kiwifruit")
for?(item?in?items)?{
????println(item)
}

val?items?=?listOf("apple",?"banana",?"kiwifruit")
var?index?=?0
while?(index?<?items.size)?{
????println("item?at?$index?is?${items[index]}")
????index++
}

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

8, 范圍

val?x?=?10
val?y?=?9
if?(x?in?1..y+1)?{
????println("fits?in?range")
}

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")
}
//遍歷
for?(x?in?1..5)?{
????print(x)
}
//步長
for?(x?in?1..10?step?2)?{
????print(x)
}
println()
for?(x?in?9?downTo?0?step?3)?{
????print(x)
}

9，集合

for?(item?in?items)?{
????println(item)
}

when?{
????"orange"?in?items?->?println("juicy")
????"apple"?in?items?->?println("apple?is?fine?too")
}
//lambda表達式
val?fruits?=?listOf("banana",?"avocado",?"apple",?"kiwifruit")
fruits
??.filter?{?it.startsWith("a")?}
??.sortedBy?{?it?}
??.map?{?it.toUpperCase()?}
??.forEach?{?println(it)?}

10,創建對象

val?rectangle?=?Rectangle(5.0,?2.0)?//不需要'new'
val?triangle?=?Triangle(3.0,?4.0,?5.0)

參考文獻：?https://kotlinlang.org/docs/reference/coding-conventions.html