Kotlin Reference (二) Idioms

most from reference

一些常用操作

创建单例类 object 数据类data class
List、Map、Array的简单操作
Lazy延迟加载属性
空类型?
空类型表达式?.、?:、?.let{}
try catch finally语句块 
无参函数表示一个值
条件判断语句 if else 及 when else 
with语句块
IO流操作并使用lambda表达式
函数声明时直接内联一个其它方法，相当于其方法实现 inline 
类Java的void类型：Unit

package com.jackie.basic

import java.io.File
import java.nio.file.Files
import java.nio.file.Paths

/**
 * Created by Jackie on 2017/7/28.
 * Idioms
 * A collection of random and frequently used idioms in Kotlin.
 * If you have a favorite idiom, contribute it by sending a pull request.
 */
class Idioms {
}

/**
 *Create DTOs(POJOs/POCOs)
 * Provides a Customer class with the following functionality:
 *   getters (and setters in case of vars) for all properties
 *   equals()
 *   hashCode()
 *   toString()
 *   copy()
 *   component1(), component2(), …, for all properties (see Data classes)
 */
data class Customer(val name: String, val email: String) //数据类的属性必须用val(不可变)或者var(可变)来修饰 属性无法修改
data class User(var name: String = "", val email: String = "") //为属性添加默认值  之后创建对象时，就可以不输或少输构造方法参数

//Creating a singleton
object Resource {
    val name = "Name"
}

fun main(args: Array<String>) {
    var customer = Customer("Jackie", "717702148@qq.com")
//    customer.name = "Jack"  //var couldn't be modified

    var list1 = listOf(-5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5)
    //Filtering a list
    var positives = list1.filter { x -> x > 0 }
    //Or alternatively, even shorter:
//    val positives = list1.filter { it > 0 }

    println(positives)

    //Traversing a map/list of pairs
    var map1 = HashMap<Int, String>()
    map1[0] = "Good"; map1[1] = "Good"; map1[2] = "Study"
    map1[3] = "Day"; map1[4] = "Day"; map1[5] = "Up"

    for ((key, value) in map1) {
        println("$key -> $value")
    }

    //key, value can be called anything.

    //Using ranges
    for (i in 1..100) {
    }  // closed range: includes 100
    for (i in 1 until 100) {
    } // half-open range: does not include 100
    for (x in 2..10 step 2) {
    }

//    for (x in 10 downTo 1) {
//        if (x in 5..8) {
////            print(x + " ")  //x是Int 编译报错 同 var result = 5 + " "
////            print(" " + x)  //编译通过
//            print(x.toString() + " ")
//        }
//    }

    //上面的简写
    (10 downTo 1)
            .filter { it in 5..8 }  //it是固定命名，表示Int
            .forEach { print(it.toString() + " ") }

    println()

    //Read-only list
    val list2 = listOf("a", "b", "c")
    for (list in list2) {
        println(list)
    }

    //按index打印
    for ((i, e) in list2.withIndex()) {
        println("$i $e")
    }

    //Read-only map
    val map2 = mapOf(0 to "Good", 1 to "Good", 2 to "Study", 3 to "Day", 4 to "Day", 5 to "Up")
    //Accessing a map
    println(map2[0])

    for (key in map2.values) {
        print(key + " ")
    }

    println()

    //Lazy property
    fun lazy() {
        val p: String by lazy {
            println("in this")
            var a = "aa"  //do something
            "aaa" //最后必须给lazy属性赋值  之前可做些其它事
        }

        println("lazy value p = $p")
    }

    lazy()

    //Extension Functions
    fun String.convertToUpperCase(): String {  //为String 添加扩展函数，把字符串转为大写字母
        return this.toUpperCase()  //任何字符串都可以调用这个扩展函数，this指接收者对象(Receiver Object)(也就是调用扩展函数时, 在"."号之前指定的对象实例).
    }

    print("Convert this to uppercase".convertToUpperCase())

//    fun Activity.toast(message: CharSequence, duration: Int = Toast.LENGTH_SHORT) {
  //为Activity 添加扩展函数
//        Toast.makeText(this, message, duration)
.show()
//    }

    println()

    //If not null shorthand
    val files1 = File("Test").listFiles()
    println(files1?.size)  //可以为null

    //If not null and else shorthand
    //使用Elvis操作符来给定一个在是null的情况下的替代值
    val files2 = File("Test").listFiles()
    println(files2?.size ?: "empty")

    //Executing a statement if null
    val data = mapOf("email" to null)
//    val email = data["email"] ?: throw IllegalStateException("Email is missing!")

    //Execute if not null
    data?.let {
        // execute this block if not null
        println("data is not null")
    }

    //Map nullable value if not null
//    val mapped = data?.let { transformData(it) } ?: defaultValueIfDataIsNull

    fun transform1(color: String): Int {
        return when (color) {
            "Red" -> 0
            "Green" -> 1
            "Blue" -> 2
            else -> throw IllegalArgumentException("Invalid color param value")
        }
    }

    //'try/catch' expression
    fun test() {
        val result = try {
//            count()
        } catch (e: ArithmeticException) {
            throw IllegalStateException(e)
        }

        // Working with result
    }

    //'if' expression
    fun foo(param: Int) {
        val result = if (param == 1) {
            "one"
        } else if (param == 2) {
            "two"
        } else {
            "three"
        }
    }

    //Builder-style usage of methods that return Unit
    fun arrayOfMinusOnes(size: Int): IntArray {
        return IntArray(size).apply { fill(10) } //生成一个size长度的int数组，每个索引都填充成10
    }

    print(arrayOfMinusOnes(5).filter { it > 5 }.forEach { println(it) })

    //Single-expression functions
    fun theAnswer1() = 42

    //This is equivalent to
    fun theAnswer2(): Int {
        return 42
    }

    //This can be effectively combined with other idioms, leading to shorter code. E.g. with the when-expression:
    fun transform2(color: String): Int = when (color) {
        "Red" -> 0
        "Green" -> 1
        "Blue" -> 2
        else -> throw IllegalArgumentException("Invalid color param value")
    }

    //Calling multiple methods on an object instance ('with')
    fun withObject(obj: Idioms) {
        //使用with(object) ，在block中 可直接调用 对象实例的 fun, 不需要obj.前缀
        with(obj) {
            foo(2)
        }
    }

    //Java 7's try with resources
//    val stream = Files.newInputStream(Paths.get("/some/file.txt"))
//    stream.buffered().reader().use { reader ->
//        println(reader.readText())
//    }

    //Convenient form for a generic function that requires the generic type information

//  public final class Gson {
//     ...
//     public <T> T fromJson(JsonElement json, Class<T> classOfT) throws JsonSyntaxException {
//     ...

    //    private fun <T: Any> fromJson(str: String, clazz: Class<T>) {
    /*private*/ fun <T> fromJson(str: String, clazz: Class<T>): T? {
        return null
    }

    /*
    inline 内联一个方法， 前面为fun定义，且使用了泛型，后面直接 = 内联的方法
     */
//    inline fun <reified T: Any> Gson.fromJson(json: JsonElement): T = this.fromJson(json, T::class.java)

    //Consuming a nullable Boolean
//    val b: Boolean = false
    var b: Boolean? = false
    if (b == true) { //Boolean?的比较，不能省略 ==
        // 'b' is true
    } else {
        // 'b' is false or null
    }
}