一. Kotlin语言基础(语法篇)

Kotlin 基本语法官方文档:
https://kotlinlang.org/docs/basic-syntax.html
以下是罗列的详细点

1. source files can be placed arbitrarily in the file

package my.demo

import kotlin.text.*

2. an entry point of a Kotlin application is the main function

fun main(args: Array<String>) {
    println(args.contentToString())
}

3. Print and Println

fun main() {
    print("Hello ")
    print("world!")
}

print:
Hello world!

fun main() {
    println("Hello world!")
    println(42)
}

print:
Hello world!
42

4.Function --- using fun keyword

Unit return Type can be omitted(可以不写返回值)

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

or

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

using default arguments can reduces a number of overloads compared to other languages

fun read(
    b: ByteArray,
    off: Int = 0,
    len: Int = b.size,
) { /*...*/ }

5. val & var

val : read-only local variables are defined using the keyword
var: can be reassigned

6. class

Inheritance between classes is declared by a colon (: ). Classes are final by default; to make a class inheritable, mark it as open.

open class Shape

class Rectangle(var height: Double, var length: Double): Shape() {
    var perimeter = (height + length) * 2
}

7. comments

/**
 * A group of *members*.
 *
 * This class has no useful logic; it's just a documentation example.
 *
 * @param T the type of a member in this group.
 * @property name the name of this group.
 * @constructor Creates an empty group.
 */
class Group<T>(val name: String) {
    /**
     * Adds a [member] to this group.
     * @return the new size of the group.
     */
    fun add(member: T): Int { ... }
}

往后需要编写api文档的可以参考:
https://kotlinlang.org/docs/kotlin-doc.html#block-tags

8.String templates

using $[变量] or ${ [变量.子变量] }

val s2 = "${s1.replace("is", "was")}, but now is $a"

9. Conditional expresssion

if 官方文档 : https://kotlinlang.org/docs/control-flow.html#if-expression

if 在简单函数中的运用

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

or 简写成

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

没有三目运算,所以 if 是很重要的角色
可以替代三目运算写成

val max = if (a > b) a else b

也可以使用if 在进行复杂运算之后为变量赋值 ( else branch is mandatory )

val max = if (a > b) {
   /** 稍微复杂运算 */
    a /** 返回值  没有 return关键字*/
} else {
    print("Choose b")
    c
}

10 for loop

使用value

  for ( `[value]` in`[数组]`) {
        println(value)
    }

or 使用index

for (index in items.indices) {
    println("item at $index is ${items[index]}")
}

or 使用 index&value

for ((index, value) in array.withIndex()) {
    println("the element at $index is $value")
}

11. while and do-while

 var a = 10
    do {
        a --
        print(a)
    }while (a > 5)

print:
9876

12 nothing type

val s = person.name ?: return

13. 打破并继续标签

Kotlin 中的任何表达式都可以用label 标记。标签具有标识符后跟@符号的形式，例如：abc@, fooBar@。要标记一个表达式，只需在它前面添加一个标签。

为for循环贴上标签

loop@ for (i in 1..10) {
    // ……
}

用标签限制break

loop@ for (i in 1..10) {
    for (j in 1..10) {
        if (……) break@loop
    }
}

被标签限制的break 跳转到指定标签的执行点,也就是一次break 跳出两重循环

用标签限制continue 如果将上述代码中的break 换成continue 那结果就是直接进行第一重循环的下一次迭代

14. return

当前函数中 后续代码不执行

15.When 类似于 switch

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

fun main() {
    println(describe(1))
    println(describe("Hello"))
    println(describe(1000L))
    println(describe(2))
    println(describe("other"))
}

print:
One
Greeting
Long
Not a string
Unknown

16.? 类似于swift 用法