----《第一季Kotlin崛起：次世代Android开发 》学习笔记
总目录：每天学一点 Kotlin ---- 目录
上一篇：每天学一点 Kotlin -- 类的属性
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1. 继承
1.1 对于继承，父类名称前面需要加上关键字 open，这个类叫作父类(或超类)。需要在继承父类的类(子类)后面加上冒号，把想要继承的类放在后面即可。示例如下：
open class Person5 {
var name = ""
var age = 0
}
class Student1 : Person5() {
var schoolName = ""
var studentId = 0
}
在继承的时候调用的是这些超类的默认构造器，所以我们在子类后面跟着的父类的括号中不需要加上任何构造参数，空的就行。
1.2 对子类的实例化并且对属性赋值也是非常简单的，子类也能够访问和设置其超类的属性。举个栗子：
open class Person6(name: String, age: Int, height: Int, likeFood: String, constByMonth: Int) {
val name: String = name
val age: Int = age
val height: Int = height
val likeFood: String = likeFood
val constByMonth: Int = constByMonth
fun printInfo() =
println("name: " + name + " age: " + age + " height: " + height + " likeFood: " + likeFood + " constByMonth: " + constByMonth)
}
class Student2(
name: String, age: Int, height: Int, likeFood: String, constByMonth: Int, schoolName: String
) : Person6(name, age, height, likeFood, constByMonth) {
val schoolName: String = schoolName;
}
class Worker(
name: String, age: Int, height: Int,
likeFood: String, constByMonth: Int, nameOfFactory: String, salary: Int
) : Person6(name, age, height, likeFood, constByMonth) {
val nameOfFactory: String = nameOfFactory
val salary: Int = salary
}
可以看到，如果父类中有主构造函数，那么子类必须对父类的主构造函数参数就地初始化。也就是说，子类中的主构造函数传入的参数要父类的属性进行初始化，也对子类本身新增加的属性进行初始化。在 main 函数中调用的写法为：
fun main() {
testStudent2()
}
fun testStudent2() {
val student2 = Student2("xiaoming", 20, 185, "pingguo", 30, "xiaoming-school")
student2.printInfo()
val worker = Worker("worker", 21, 190, "shupian", 40, "worker-factory", 8000)
worker.printInfo()
}
打印结果：
name: xiaoming age: 20 height: 185 likeFood: pingguo constByMonth: 30
name: worker age: 21 height: 190 likeFood: shupian constByMonth: 40
(1) 可以看到子类新加的成员变量没有输出，这时要打印子类中新加的变量，只要重写父类的 printInfo() 方法即可，但前提是父类的 printInfo() 方法是 open 的。因为如果不加任何修饰，Kotlin 会默认在前面添加 final，这样就是不可继承，不可覆盖重写的。
(2) 用 open 修饰父类中 printInfo() 方法，然后在子类中重写父类的方法：
fun main() {
testStudent3()
}
fun testStudent3() {
val student3 = Student3("xiaoming", 20, 185, "pingguo", 30, "xiaoming-school")
student3.printInfo()
}
class Student3(name: String, age: Int, height: Int, likeFood: String, constByMonth: Int, schoolName: String) :
Person7(name, age, height, likeFood, constByMonth) {
val schoolName: String = schoolName;
override fun printInfo() =
println("name: " + name + " age: " + age + " height: " + height + " likeFood: " + likeFood + " constByMonth: " + constByMonth + " schoolName: " + schoolName)
}
open class Person7(name: String, age: Int, height: Int, likeFood: String, constByMonth: Int) {
val name: String = name
val age: Int = age
val height: Int = height
val likeFood: String = likeFood
val constByMonth: Int = constByMonth
open fun printInfo() =
println("name: " + name + " age: " + age + " height: " + height + " likeFood: " + likeFood + " constByMonth: " + constByMonth)
}
打印结果为：
name: xiaoming age: 20 height: 185 likeFood: pingguo constByMonth: 30 schoolName: xiaoming-school
1.3 如果超类没有主构造函数，只有次构造函数，那么每个次构造函数必须用 super 关键词调用超类的构造函数对超类的属性进行初始化，具体的语法为 "constructor 子类(次构造函数)：super(次构造函数)"。举个栗子：
fun main() {
testStudent4()
}
fun testStudent4() {
val student4 = Student4("st4", 20, "st4_schoolName");
student4.printInfo()
}
open class Person8 {
constructor(name: String, age: Int) {
this.name = name
this.age = age
}
var name: String = ""
var age: Int = 0
open fun printInfo() =
println("name: " + name + " age: " + age)
}
class Student4 : Person8 {
constructor(name: String, age: Int, schoolName: String)
: super(name, age) {
this.schoolName = schoolName
}
var schoolName: String = ""
override fun printInfo() {
println("name: ${name}, schoolName: ${schoolName}")
}
}
打印结果;
name: st4, schoolName: st4_schoolName
1.4 上面的代码中，子类重写了父类的 printInfo() 方法。子类也可以重写父类中的属性，但是有一条原则：不能把父类中 var 变量重写为 val 常量，也就是：不能把父类中可读可写的属性改为可读不可写的。即覆盖属性只能扩充外部功能，不能减少。举个栗子：
// 第一种情况
open class Parent1 {
open var str = "str"
}
class Child1 : Parent1() {
override var str = "str_child"
}
// 第二种情况
open class Parent2 {
open val str = "str"
}
class Child2_1 : Parent2() {
override val str = "str_child"
}
class Child2_2 : Parent2() {
override var str = "str_child"
}
// 第三种情况
class Child3 : Parent1() {
// override val str = "str_child" // 编辑器直接报红了
}
1.5 在主构造函数中覆盖属性：
open class Parent4{
open val str: String = "Parent4"
}
class Child4(override val str: String = "Child4"):Parent4(){}
上面的情况也是一种属性的重写，因为在写构造器参数的时候如果在参数名称前加一个 var 或 val 修饰符等于就是把它自动变为一个当前类的属性了。这里在参数名前加了 override val，也就是自动将它变为超类中相应属性的继承了。
1.6 Kotlin 中，如果一个类没有声明继承任何类，那么这个默认继承了 Any 类。Any 这个类中有 equals()、hashCode()、toString() 这些方法。
