[Kotlin] 學習筆記(2) - Basic Concept

在開始之前，附上一篇文章 17位Google Android 工程師對 Kotlin 的看法

來了解一下各方對 Kotlin 的見解

然後也提供一下學習資源 : Kotlin官方文檔

本篇大部分內容都是參考 Kotlin 的官方文檔來的，可以去該網站多挖挖寶

以下開始一些基礎介紹

• Variables

有兩個關鍵字，val 和 var。val 就是 java 裡的 final，var 是可以重新賦值的變數

	val number: Int = 1 //>> same as "final int number = 1;" in Java
	val number = 1 //>> Kotlin will infer type of number is Int。similar to "auto number = 1;" in C++
	val number: Int //>> if no initial value, type must be given
	number = 3

view raw basicKotlin_1.kt hosted with ❤ by GitHub

var 也有上面的宣告方式，差別在於 var 可以重新賦值

在 Kotlin，變數預設是 non-null type，也就是不能是 null

Kotlin 的一大特色就是能盡量減少 NullPointerException 發生的機率

以下是關於變數 null safety 的介紹

	var a: String = null //>> a is non-null type, compilation error
	var b: String? = null //>> b is nullable, okay
	val lengthA = a.length //>> okay because a is non-null type
	val lengthB = b.length //>> error because b might be null. see following usage
	//>> Usage 1:
	//>> use safe call operator, written as ?.
	//>> b?.length return b.length if b is not null and null otherwise
	//>> the type of lengthB is Int?
	val lengthB = b?.length
	//>> Usage 2:
	//>> let lengthB has default value when b is null and type of lengthB is Int instead of Int?
	val lengthB: Int = if (b != null) b.length else -1 //>> method 1
	val lengthB = b?.length ?: -1 //>> method 2, use Elvis operator, written as ?:
	//>> Usage 3:
	//>> use !! operator if you want NullPointerException
	//>> there will be a NullPointerException when b is null
	val lengthB = b!!.length

view raw basicKotlin_5.kt hosted with ❤ by GitHub

型態轉換也有 null safety 可以使用

	//>> the as? will return null if the cast was not successful
	val newInt: Int? = someVar as? Int

view raw basicKotlin_5-2.kt hosted with ❤ by GitHub

如果今天有一個 class 的 property，我們不希望為 null，但也不想在 construtor 去初始化，該怎麼做呢？

	class MyKotlinClass {
	private var mMsg: String //>> compilation error. non-null type need initial value
	fun initMsg() {
	mMsg = "init"
	}
	}
	//>> use lateinit keyword
	class MyKotlinClass {
	private lateinit var mMsg: String
	fun initMsg() {
	mMsg = "init"
	}
	}

view raw basicKotlin_5-3.kt hosted with ❤ by GitHub

lateinit 的用法有一些限制

(1) 只能用在 var 變數

(2) 不能用在 primitive type (Int, Float, ...) 

(3) 該變數不能有客製化的 setter 和 getter

• Functions

有兩個 Int 的 parameters，回傳 type 是 Int

	fun getSum(a: Int, b: Int): Int {
	return a+b
	}
	//>> can be simplified further
	//>> Kotlin will infer return type
	fun getSum(a: Int, b: Int) = a + b

view raw basicKotlin_3.kt hosted with ❤ by GitHub

如果沒有 return value，用 Unit 表示，也可以把 Unit 省略

	//>> no return value
	fun showMsg(msg: String): Unit {
	println(msg)
	}
	//>> omit Unit
	fun showMsg(msg: String) {
	println(msg)
	}

view raw basicKotlin_4.kt hosted with ❤ by GitHub

如果一個函式的參數比較多，這時候可以使用 named arguments 來增加可讀性

	fun doAction(msg: String, isTestMode: Boolean = false, ver: Int = 1) {
	//>> function body
	}
	//>> use named arguments
	doAction("Hello Kotlin", ver = 3, isTestMode = true)

view raw basicKotlin_4-1.kt hosted with ❤ by GitHub

function 也可以使用 variable number of arguments

使用 vararg 有幾點注意事項

1. function 只能有一個 parameter 被標示為 vararg
2. vararg parameter 通常要放在 parameter list 的最後一個。如果不是最後一個，則在使用的時候，必須搭配 named argument 來使用

	//>> function with variable number arguments
	fun <T> toList(vararg input: T) : List<T> {
	val result = ArrayList<T>()
	input.forEach { n -> result.add(n) }
	return result
	}
	//>> example 1
	val list = toList(1, 2, 3, -1, -2, -3)
	//>> example2
	//>> use * to spread array a
	val a = arrayOf(1, 2, 3)
	val b = toList(7, 5, *a, -5)
	//==========================================================
	//>> vararg is not the last one in the list
	fun <T> toList(vararg input: T, ver: Int ) : List<T> {
	val result = ArrayList<T>()
	input.forEach { n -> result.add(n) }
	return result
	}
	//>> need to use named argument syntax to pass following parameter
	val list = myKotlinClass.toList(1, 2, 3, ver = 111)

view raw basicKotlin_4-2.kt hosted with ❤ by GitHub

• Class

在 Kotlin，如果 class 沒有指明父類別的話，都會是從 Any 這個例別衍生出來的

class 的基本宣告方式如下

	//>> this class cannot be inherited
	class MyClass {
	//>> class body
	}
	//>> this class can be inherited
	open class MyClass {
	//>> class body
	}

view raw basicKotlin_6-1.kt hosted with ❤ by GitHub

Kotlin 有個和 Java 不一樣的地方是，Kotlin 沒有 new 這個 keyword

	//>> create instance of class
	val myClass = MyClass() //>> there is no 'new' keyword in Kotlin

view raw basicKotlin_6-1-1.kt hosted with ❤ by GitHub

每個 class 都會有一個 primary constructor

	//>> this class will have auto-generated constructor with no arguments
	//>> the auto-generated constructor's visibility will be public
	class MyClass {
	//>> class body
	}
	//>> this class has a public primary constructor with one argument
	class MyClass constructor(msg: String) {
	//>> class body
	}
	//>> you can omit the constructor keyword when primary constructor has no
	//>> annotations or visiblity modifiers
	class MyClass(msg: String) {
	//>> class body
	}
	//>> want a private primary constructor
	class MyClass private constructor(msg: String) {
	//>> class body
	}
	//>> constructor example 1
	class MyKotlinClass(msg: String, ver: Int) {
	private var privateMsg = "[private]" + msg
	private var privateVer = ver + 20
	fun setup() {
	Log.d("TAG", "private message is $privateMsg, private version is $privateVer")
	}
	}
	//>> constructor example 2
	class MyClass(val msg: String, var ver: Int) {
	private var privateMsg = "[private]" + msg
	fun setup() {
	Log.d("TAG", "user's message is $msg, current version is $ver")
	ver = -1 //>> change the value
	Log.d("TAG", "private message is $privateMsg, modified version is $ver")
	}
	}
	usage:
	val myKotlinClass = MyKotlinClass("Hi Kotlin", 33)
	myKotlinClass.setup()
	=> outputs are:
	user's message is Hi Kotlin, current version is 33
	private message is [private]Hi Kotlin, modified version is -1

view raw basicKotlin_6-2.kt hosted with ❤ by GitHub

primary constructor 是不能寫 code 的，必須使用 initializer block 來達到這個目的

	//>> use initializer block to contain code for primary constructor
	class MyClass(msg: String) {
	init {
	Log.d("TAG", "user's message is $msg")
	}
	}

view raw basicKotlin_6-2-1.kt hosted with ❤ by GitHub

Kotlin 還有提供 secondary constructor，一個 class 可以有多個 secondary constructor

	//>> this class has no customized primary constructor
	//>> and has one secondary constructor
	class MyClass {
	constructor(secondMsg: String) {
	println("second message is ${secondMsg}")
	}
	}
	//>> if the class has a primary constructor
	//>> then each secondary constructor need to delegate to the primary constructor
	class MyClass(msg: String) {
	constructor(msg:String, number: Int) : this(msg){
	println("[$number] message is $msg")
	}
	constructor(msg: String, number: Int, ver: Double) : this(msg) {
	println("[$number] message is $msg and version is $ver")
	}
	}

view raw basicKotlin_6-3.kt hosted with ❤ by GitHub

在 Kotlin，function 和 constructor 的 arguments 都可以有 default value，這可以簡化原本 Java 的寫法

	//>> Java Style
	public class MyJavaClass {
	public MyJavaClass(String msg, int number) {
	this(msg, number, 1.8);
	}
	public MyJavaClass(String msg, int number, double ver) {
	Log.d("MyJavaClass", "[" + number + "] message is " + msg + ", and version is " + ver);
	}
	}
	=> val myJavaClass = MyJavaClass("Hi Java", 23)
	output: [23] message is Hi Java, and version is 1.8
	=> val myJavaClass = MyJavaClass("Hi Java", 23, 6.6)
	output: [23] message is Hi Java, and version is 6.6
	//>> Kotlin Style
	class MyKotlinClass {
	constructor(msg: String, number:Int, ver: Double = 1.8) {
	Log.d("MyKotlinClass", "[$number] message is $msg, and version is $ver")
	}
	}
	=> val myKotlinClass = MyKotlinClass("Hi Kotlin", 33)
	output: [33] message is Hi Kotlin, and version is 1.8
	=> val myJavaClass = MyJavaClass("Hi Kotlin", 33, 10.2)
	output: [33] message is Hi Kotlin, and version is 10.2

view raw basicKotlin_6-4.kt hosted with ❤ by GitHub

在繼承 class 的時候，constructor 的寫法有下列兩種情況

	//>> if the derived class has primary constructor
	//>> the base class must be initialized right here
	open class Base(msg: String) {
	//>> class body
	}
	class Derived(msg: String) : Base(msg) {
	//>> class body
	}
	//>> if the derived class has no primary constructor
	//>> each secondary constructor must use super to initialize base class
	open class Base(msg: String) {
	//>> class body
	}
	class Derived : Base {
	constructor(msg: String, ver: Int) : super(msg)
	}

view raw basicKotlin_6-5.kt hosted with ❤ by GitHub

以下示範 derived class 如何複寫 function

	open class Base {
	fun doAction() {
	//>> function body
	}
	open fun showMsg() {
	//>> function body
	}
	}
	class Derived : Base() {
	//>> this is a compilation error
	//>> derived class can only override open function
	override fun doAction() {
	//>> function body
	}
	//>> okay
	override fun showMsg() {
	//>> function body
	}
	}

view raw basicKotlin_6-6.kt hosted with ❤ by GitHub

當 derived class 繼承或實現多個 base class or interface 的情況呢

	open class BaseClass {
	open fun doAction() {
	println("don't do it")
	}
	open fun showMsg() {
	println("Hi Kotlin")
	}
	}
	interface BaseInterface {
	//>> interface members are 'open' by default
	fun doAction() {
	println("just do it")
	}
	fun power() {
	println("interface only")
	}
	}
	class Derived : BaseClass(), BaseInterface {
	//>> doAction() must be override because both BaseClass and BaseInterface have doAction()
	override fun doAction() {
	super<MyKotlinClass>.doAction() //>> call to BaseClass.doAction()
	super<BaseInterface>.doAction() //>> call to BaseInterface.doAction()
	}
	}

view raw basicKotlin_6-7.kt hosted with ❤ by GitHub

可以看到因為有 doAction() 的 supertype 不只一個，所以在 Derived class 就一定得用 override 去複寫這個函式

而在這種情況下，要指定使用哪個 supertype 的 doAction()，就可以使用 super<BaseName> 來指定

至於 showMsg() 和 power()，因為來源都只有一個，所以不會被強迫要求用 override 複寫

• What's Next

本篇介紹一些 Kotlin 基礎的語法，可以看出來 Kotlin 有簡化一些 Java 語法，並且引入一些新的概念

不過也可以看出來，Kotlin 在某些部分也有一些限制，像是為了 null safety，programmer 必須在寫 code 的時候遵從一些語法才行

下一篇會繼續介紹 Kotlin 的基礎語法，讓各位更熟悉 Kotlin

[LeetCode] 523. Continuous Subarray Sum

轉自LeetCode

Given a list of non-negative numbers and a target integer k, write a function to check if the array has a continuous subarray of size at least 2 that sums up to the multiple of k, that is, sums up to n*k where n is also an integer.

Example 1:

Input: [23, 2, 4, 6, 7],  k=6
Output: True
Explanation: Because [2, 4] is a continuous subarray of size 2 and sums up to 6.

Example 2:

Input: [23, 2, 6, 4, 7],  k=6
Output: True
Explanation: Because [23, 2, 6, 4, 7] is an continuous subarray of size 5 and sums up to 42.

Note:

1. The length of the array won't exceed 10,000.
2. You may assume the sum of all the numbers is in the range of a signed 32-bit integer.

<Solution>

[LeetCode] 560. Subarray Sum Equals K

轉自LeetCode

Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.

Example 1:

Input:nums = [1,1,1], k = 2
Output: 2

Note:

1. The length of the array is in range [1, 20,000].
2. The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

<Solution>

[Kotlin] 學習筆記 (1) - Hello Kotlin

在 Google I/O 2017 的大會上

Google 正式宣布 Android Studio 3.0 將會正式支援 Kotlin

這也讓 Kotlin 成為目前熱烈討論的程式語言之一

和 Java 一樣，Kotlin 也是一個 JVM 的語言，由 JetBrains 所開發

因此，Kotlin 和 Java 是可以同時並存一起使用的

至於 Kotlin 會不會取代 Java，以及 Kotlin 和 Java 誰比較好的問題

在這邊就不多作討論，因為這是滿主觀的問題

所以在這邊主要會介紹，怎麼運用 Kotlin 在 Android APP 的開發上

• 安裝 Kotlin plugin

因為目前 Android Studio 3.0 還沒有正式釋出，所以目前還是得透過安裝 plugin 的方式來使用 Kotlin

到 Preference 下的 plugin 頁面來安裝 Kotlin plugin

• Hello Kotlin App

接下來寫一個簡單的 App

先看原本 Java 的寫法

	public class MainActivity extends AppCompatActivity {
	@Override
	protected void onCreate(Bundle savedInstanceState) {
	super.onCreate(savedInstanceState);
	setContentView(R.layout.activity_main);
	TextView textView = (TextView) findViewById(R.id.textView);
	textView.setText("Java Style");
	}
	}

view raw helloKotlin_1.java hosted with ❤ by GitHub

接下來看怎麼用 Kotlin 改寫

首先，先設置好 Kotlin，按下 cmd + shift + a，搜尋 kotlin，選擇 configure kotlin in project

Kotlin 寫法

	class MainActivity : AppCompatActivity() {
	override fun onCreate(savedInstanceState: Bundle?) {
	super.onCreate(savedInstanceState)
	setContentView(R.layout.activity_main)
	val textView = findViewById(R.id.textView) as TextView
	textView.text = "Kotlin Style"
	}
	}

view raw helloKotlin_1.kt hosted with ❤ by GitHub

<ps> 在一開始不知道怎麼寫 kotlin 的時候，也可以用 cmd + shift + a -> Convert Java File to Kotlin File 來看怎麼寫

整體來看，好像沒有差別很大，也沒有變得比較簡潔

比較有差別的地方是，可以不用使用 setText()，而變成直接用 property 的方式來設定 text

其實 Kotlin 還有更簡潔的寫法(資料來源)

在 module 的 build.grandle 裡面，加入下面這段 code

apply plugin: 'kotlin-android-extensions'

這時候可以進一步改寫成

不用再使用 findViewById 來找到 TextView，可以直接使用 ID 來存取該 TextView

• 下一步

這次先簡單介紹一下 Kotlin，之後會再陸續介紹 Kotlin 的各種語法