Kotlin 教程：从基础到进阶

1. OOP 版本的 HelloWorld

Kotlin 中的 HelloWorld 示例，展示了类和成员函数的基本使用方法。

class Greeter(val name: String) {    fun greet() {        println("Hello, $name")    }}fun main(args: Array
   
    ) {    Greeter(args[0]).greet()}

2. 构造函数传参

通过分解对应式声明，简洁地创建多个变量。

fun main(args: Array
   
    ) {    val pair = Pair(1, "one")    val (num, name) = pair    println("num = $num, name = $name")        val triple = Triple(10, "B", 10.0)    val (a, b, c) = triple    println("a=$a, b=$b, c=$c")}data class Pair
    
     (    val first: K,    val second: V) {    operator fun component1(): K = first    operator fun component2(): V = second}data class Triple
     
      (    val first: K,    val second: V,    val third: T) {    operator fun component1(): K = first    operator fun component2(): V = second    operator fun component3(): T = third}

3. 数据类

数据类自动推导标准功能，适合仅用于存储数据的类。

data class User(    val name: String,    val id: Int) {    override fun toString(): String {        return "User(name=$name, id=$id)"    }}fun getUser(): User {    return User("Alex", 1)}fun main(args: Array
   
    ) {    val user = getUser()    println("name = ${user.name}, id = ${user.id}")}

4. 接口与实现

定义接口并实现其方法，保证代码的可扩展性。

interface Clickable {    fun click()}class Button : Clickable {    override fun click() {        println("I was clicked")    }}fun main(args: Array
   
    ) {    Button().click()}

5. POJO Bean

创建实体类，包含必要的字段和 getter/setter 方法。

package jason.chen.mini_springboot.restful.entityimport java.util.*import javax.persistence.*@Entityclass Customer(    val firstName: String,    val lastName: String,    val gmtCreated: Date,    val gmtModified: Date,    val isDeleted: Int,    val deletedDate: Date,    @Id    @GeneratedValue(strategy = GenerationType.AUTO)    val id: Long = -1) {    override fun toString(): String {        return "Customer(firstName=$firstName, lastName=$lastName, gmtCreated=$gmtCreated, gmtModified=$gmtModified, isDeleted=$isDeleted, deletedDate=$deletedDate, id=$id)"    }}data class Shop(    val name: String,    val customers: List
   
    )data class Customer(    val name: String,    val city: City,    val orders: List
    
     ) {    override fun toString(): String {        return "$name from ${city.name}"    }}data class Order(    val products: List
     
      ,    val isDelivered: Boolean)data class Product(    val name: String,    val price: Double) {    override fun toString(): String {        return "'$name' for $price"    }}data class City(    val name: String) {    override fun toString(): String {        return name    }}

6. 定一个 Rectangle 对象

创建一个矩形对象，包含高度和宽度属性。

package geometry.shapesclass Rectangle(    val height: Int,    val width: Int) {    val isSquare: Boolean        get() = height == width    fun createRandomRectangle(): Rectangle {        val random = Random()        return Rectangle(            random.nextInt(),            random.nextInt()        )    }}

7. 封装日期工具类

创建一个日期工具类，提供丰富的日期操作功能。

package jason.chen.mini_springboot.restful.utilsimport java.text.SimpleDateFormatimport java.util.*enum class DateOptUnit {    YEAR, MONTH, DATE}data class DateOperator(    val unit: DateOptUnit,    val value: Int)fun Any.year(value: Int): DateOperator {    return DateOperator(DateOptUnit.YEAR, value)}fun Any.month(value: Int): DateOperator {    return DateOperator(DateOptUnit.MONTH, value)}fun Any.day(value: Int): DateOperator {    return DateOperator(DateOptUnit.DATE, value)}operator fun Date.plus(nextVal: Int): Date {    val calendar = GregorianCalendar()    calendar.time = this    calendar.add(Calendar.DATE, nextVal)    return calendar.time}operator fun Date.minus(nextVal: Int): Date {    val calendar = GregorianCalendar()    calendar.time = this    calendar.add(Calendar.DATE, nextVal * -1)    return calendar.time}operator fun Date.plus(nextVal: DateOperator): Date {    val calendar = GregorianCalendar()    calendar.time = this    calendar.add(        nextVal.unit.parseType(),        nextVal.value    )    return calendar.time}operator fun Date.minus(nextVal: DateOperator): Date {    val calendar = GregorianCalendar()    calendar.time = this    calendar.add(        nextVal.unit.parseType(),        nextVal.value * -1    )    return calendar.time}operator fun Date.inc(): Date {    val calendar = GregorianCalendar()    calendar.time = this    calendar.add(Calendar.MONTH, 1)    calendar.set(Calendar.DAY_OF_MONTH, 0)    return calendar.time}operator fun Date.dec(): Date {    val calendar = GregorianCalendar()    calendar.time = this    calendar.set(Calendar.DAY_OF_MONTH, 1)    return calendar.time}operator fun Date.get(position: Int): Int {    val calendar = GregorianCalendar()    calendar.time = this    var value = 0    when (position) {        0 -> value = calendar.get(Calendar.YEAR)        1 -> value = calendar.get(Calendar.MONTH) + 1        2 -> value = calendar.get(Calendar.DAY_OF_MONTH)        3 -> value = calendar.get(Calendar.HOUR)        4 -> value = calendar.get(Calendar.MINUTE)        5 -> value = calendar.get(Calendar.SECOND)    }    return value}operator fun Date.compareTo(compareDate: Date): Int {    return (time - compareDate.time).toInt()}fun Date.stringFormat(formatType: String): String {    return SimpleDateFormat(formatType).format(this)}

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