Kotlin 类 – Kotlin 构造函数
在本教程中，我们将讨论Kotlin的面向对象编程概念。我们将详细介绍Kotlin类。我们还将研究Kotlin的构造函数、访问修饰符和抽象类。
Kotlin课堂
class FirstClass {
}
以下是如何实例化该类的示例：
val firstClass = FirstClass()
var new = FirstClass() //here new is the name of the var.
与Java不同，Kotlin中的new不是一个关键字。在Kotlin中，默认情况下，类是final的。因此，在Java中，上述定义的等效方式可能类似于以下代码：
public final class FirstClass {
}
因此，默认情况下，Kotlin中的类是不可继承的。为了使一个类非final，我们需要在其后面加上关键字open。
open class Me{
}
开放的注解允许其他类继承。
Kotlin 类示例
让我们创建一个带有几个函数和属性的类。我们将看到如何访问该类的函数和属性。此外，我们还将看到如何设置成员属性。
class User {
var loggedIn: Boolean = false
val cantChangeValue = "Hi"
fun logOn() {
loggedIn = true
}
fun logOff() {
loggedIn = false
}
}
fun main(args: Array<String>) {
val user = User()
println(user.loggedIn) //false
user.logOn()
println(user.loggedIn) //true
user.logOff()
println(user.loggedIn) //false
user.cantChangeValue = "Hey" //won't compile. Can't modify a final variable.
}
主函数属于Test.kt类。要访问成员和函数，我们需要使用点运算符。不能再使用点运算符来设置val属性。
Kotlin的初始化
Kotlin的init块定义如下所示。
class User {
init{
print("Class instance is initialised.")
}
var loggedIn: Boolean = false
val cantChangeValue = "Hi"
fun logOn() {
loggedIn = true
}
fun logOff() {
loggedIn = false
}
}
当类实例化时，init块中的代码将首先执行。每次实例化类时，init块都会运行，无论使用什么样的构造函数，下面我们将看到。一个类中可以编写多个初始化块。它们将按照下面的顺序依次执行。
class MultiInit(name: String) {
init {
println("First initializer block that prints ${name}")
}
init {
println("Second initializer block that prints ${name.length}")
}
}
fun main(args: Array<String>) {
var multiInit = MultiInit("Kotlin")
}
//Following is printed in the log console.
//First initializer block that prints Kotlin
//Second initializer block that prints 6
使用下面所示的 also 函数，Kotlin 类可以在声明本身中打印属性。
class MultiInit(name: String) {
val firstProperty = "First property: $name".also(::println)
init {
println("First initializer block that prints ${name}")
}
val secondProperty = "Second property: ${name.length}".also(::println)
init {
println("Second initializer block that prints ${name.length}")
}
}
fun main(args: Array<String>) {
var multiInit = MultiInit("Kotlin")
}
//Following gets printed.
//First property: Kotlin
//First initializer block that prints Kotlin
//Second property: 6
//Second initializer block that prints 6
Kotlin 构造函数
Kotlin的构造函数是用于初始化属性的特殊成员函数。与Java相比，Kotlin的构造函数的编写和结构有所不同。默认情况下，一个类具有以下空构造函数：
class Student {
var name: String
val age : Int
init {
name = "Anupam"
age = 24
}
init {
name = "Anupam Chugh"
//age = 26
}
}
fun main(args: Array<String>) {
val student = Student()
println("${student.name} age is ${student.age}")
student.name = "Your"
//student.age = 26 //won't compile. age is val
println("${student.name} age is ${student.age}")
}
//Following is printed on the console:
//Anupam Chugh age is 24
//Your age is 24
主构造函数
Kotlin中的主构造函数是在类头中定义的，如下所示。
class User(var name: String, var isAdmin: Boolean) {
init {
name = name + " @ JournalDev.com"
println("Author Name is $name. Is Admin? $isAdmin")
}
}
主要的构造函数定义在类头部。我们已经在构造函数本身中定义了属性类型（val/var）。注意：除非声明为var，否则默认情况下构造函数参数是val。
class User(name: String, isAdmin: Boolean)
在上面的代码中，无法重新分配name和isAdmin。或者，我们也可以将构造函数的参数分配给类中的成员属性，如下所示。
class User(name: String, val isAdmin: Boolean) {
var username = name
val _isAdmin = isAdmin
init {
username= username + " @ JournalDev.com"
println("Author Name is $name. Is Admin? $_isAdmin")
}
}
fun main(args: Array<String>) {
var user = User("Anupam",false)
user.isAdmin = true //won't compile since isAdmin is val
user._isAdmin = true //won't compile. Same reason.
user = User("Pankaj",true)
}
//Following is printed in the log console.
//Author Name is Anupam. Is Admin? false
//Author Name is Pankaj. Is Admin? true
Kotlin 构造函数的默认值
在Kotlin中，我们可以在构造函数本身中指定默认值，如下所示。
class User(name: String, var website: String = "JournalDev") {
init {
println("Author $name writes at $website")
}
init {
website = website + ".com"
println("Author $name writes at $website")
}
}
fun main(args: Array<String>) {
var user = User("Anupam","JournalDev")
user = User("Pankaj","JournalDev")
}
//Following is printed on the console:
//Author Anupam writes at JournalDev
//Author Anupam writes at JournalDev.com
//Author Pankaj writes at JournalDev
//Author Pankaj writes at JournalDev.com
次要的构造函数
在类的主体内，通过添加关键字constructor在其前面来编写次要构造函数。下面的示例演示了相同的情况。
class Student {
var name: String
val age : Int
constructor(name: String, age: Int)
{
this.name = name
this.age = age
}
fun printDetails()
{
println("Name is $name and Age is $age")
}
}
fun main(args: Array<String>) {
var student = Student("Anupam", 24)
student.printDetails()
}
//Following is printed in the console.
//Name is Anupam and Age is 24
在子类中，次要构造函数最常见的用途是在需要以不同方式初始化类时使用。如果该类包含一个主构造函数，次要构造函数必须在声明中引用它。使用关键字“this”来进行声明。
class Student(var name: String, val age: Int) {
var skill: String
init {
skill = "NA"
}
constructor(name: String, age: Int, skill: String) : this(name, age) {
this.skill = skill
}
fun printDetails() {
if (skill.equals("NA"))
println("Name is $name and Age is $age")
else
println("Name is $name and Age is $age Skill is $skill")
}
}
//Following is printed in the log console:
//Name is Anupam and Age is 24
//Name is Anupam and Age is 24 Skill is Kotlin
初始化块用于初始化成员属性技能。次要构造函数使用”this”关键字委托给主构造函数。
自定义的获取器和设置器
直到现在，我们通过对类的实例使用点运算符来访问和修改属性。让我们使用set和get语法来看看如何自定义访问。
class Name{
var post: String = "default"
set(value) {if(!post.isNotEmpty()) {
throw IllegalArgumentException(" Enter a valid name")
}
field = value
print(value)
}
}
fun main(args: Array<String>) {
var name = Name()
name.post = "Kotlin Classes"
name.post = ""
name.post = "Kotlin Data Classes Our Next Tutorial"
}
以下内容被打印在log控制台中：
Kotlin Classes
Exception in thread "main" java.lang.IllegalArgumentException: Enter a valid name
at Name.setPost(Test.kt:16)
at TestKt.main(Test.kt:78)
在setter中，字段变量保存了旧值。让我们添加一个getter。
class Name{
var post: String = "default"
set(value) {if(!post.isNotEmpty()) {
throw IllegalArgumentException(" Enter a valid name")
}
field = value
}
get() {
return field.capitalize()
}
}
fun main(args: Array<String>) {
var name = Name()
name.post = "kotlin classes"
println(name.post)
name.post = "kotlin data Classes our next Tutorial"
println(name.post)
}
//Following is printed:
//Kotlin classes
//Kotlin data Classes our next Tutorial
capitalize()方法将字符串的第一个字母大写。注意：如果属性是val，则设置方法将无法编译通过。
Kotlin 可见性修饰符
- Public : Any class, function, property, interface, or object that has this modifier is visible and can be accessed from anywhere.
- Private: A class/function defined with this modifier can be only accessed within the same file. A member/property in a class/function with this modifier can be only accessed within that block.
- Protected : This modifier is same as private, except that it allows visibility and access within subclasses.
- Internal: A class/interface/function with this modifier is accessible only within the same module.
可见性修饰符同样适用于构造函数。将修饰符分配给主构造函数需要在类头中的构造函数旁指定关键字constructor。
class Student private constructor (var name: String, val age: Int) {
var skill: String
init {
skill = "NA"
}
constructor(name: String, age: Int, skill: String) : this(name, age) {
this.skill = skill
}
fun printDetails() {
if (skill.equals("NA"))
println("Name is $name and Age is $age")
else
println("Name is $name and Age is $age Skill is $skill")
}
}
fun main(args: Array<String>) {
var student = Student("Anupam",24,"Kotlin")
student.printDetails()
}
//prints
//Name is Anupam and Age is 24 Skill is Kotlin
私有构造函数不能在类外部调用。在上述代码中，我们只能使用辅助构造函数在不同的函数中实例化该类。
Kotlin的抽象类
与Java类似，Kotlin中使用abstract关键字来声明抽象类。抽象类不能被实例化，但可以被子类继承。默认情况下，抽象类的成员都是非抽象的，除非另有声明。
abstract class Person(name: String) {
init {
println("Abstract Class. init block. Person name is $name")
}
abstract fun displayAge()
}
class Teacher(name: String): Person(name) {
var age : Int
init {
age = 24
}
override fun displayAge() {
println("Non-abstract class displayAge function overridden. Age is $age")
}
}
fun main(args: Array<String>) {
val person = Teacher("Anupam")
person.displayAge()
}
//Following is printed in the console.
//Abstract Class. init block. Person name is Anupam
//Non-abstract class. Age is 24
注意：抽象类默认是开放的。所以不需要添加open修饰符来允许子类化。override关键字用于在子类中重写方法。在本教程中，我们已经介绍了Kotlin类的基础知识。还有很多其他内容，如数据类、密封类、继承等等。我们将在接下来的教程中涵盖它们。参考资料：Kotlin文档。