类继承2

重写实例方法

在子类中，你可以重写继承来的实例方法或类方法，提供一个定制或替代的方法实现。

class Person{    var name:String    var age:Int    func description() -> String{           //定义实例方法description        return "\(name) 年龄是: \(age)"    }    class func printClass() -> (){        print("Person 打印...")             //静态方法不能访问实例属性    }    init(name:String, age:Int){        self.name = name        self.age = age    }}class Student:Person{    var school:String    convenience init(){        self.init(name:"Tony", age:18, school:"四川师范大学")    }    init(name:String, age:Int, school:String){        self.school = school        super.init(name:name, age:age)    }    override func description() -> String {                //在子类中重写方法        print("父类打印 \(super.description())")            //调用父类方法，super代指父类        return "\(name) 年龄是: \(age), 所在学校: \(school)。"    }    override class func printClass() ->(){            //重写静态方法        print("Student 打印...")    }}let student1 = Student()print("学生1: \(student1.description())")Person.printClass()Student.printClass()//输出结果：//父类打印 Tony 年龄是：18//学生1：Tony 年龄是：18，所在学校：清华大学。//Person 打印...//Student 打印...

重写静态方法

静态方法使用class或static关键字，但是使用哪一个要看子类中是否重写该方法。class修饰的静态方法可以被重写，static关键字的就不能。

class Account{    var owner:String = "Tony"       //账户名    //class不能换成static    class func interestBy(amount:Double) -> Double{    //定义静态方法        return 0.08886 * amount    }}class TermAccount:Account{     //定期账户    //class可以换成static    override class func interestBy(amount:Double) -> Double{    //重写静态方法 这里的class可以换成static 除非在TermAccount的子类重写interestBy        return 0.09 * amount    }}//调用静态方法print(Account.interestBy(10_000.00))print(TermAccount.interestBy(10_000.00))

下标重写

对下标的重写同子类属性重写一样也是重写下标的getter和setter访问器

class DoubleDimensionalArray{    let rows:Int, columns:Int    var grid:[Int]    init(rows:Int, columns:Int){        self.rows = rows        self.columns = columns        grid = Array(repeating:0, count:rows * columns)    }    subscript(row:Int , col:Int)->Int{         //定义下标        get{            return grid[(row * columns) + col]        }        set{            grid[(row * columns) + col] = newValue        }    }}class SquareMatrix:DoubleDimensionalArray{  //类的继承    override subscript(row:Int, col:Int)->Int{  //重写父类下标        get{            return super.grid[(row * columns) + col]        }        set{            super.grid[(row * columns) + col] = newValue * newValue//给父类grid属性附值        }    }}var ary2 = SquareMatrix(rows: 5, columns: 5)for var i in 0..<5{    for var j in 0..<5{        ary2[i, j] = i + j    }}for var i in 0..<5{    for var j in 0..<5{        print("\t \(ary2[i, j])", terminator:" ")    }    print("\n")}

使用final关键字

我们可以在类的定义中使用final关键字声明类、属性、方法和下标。final声明的类不能被继承，final声明的属性、方法、和下标不能被重写。

final class Person{ //声明final ，说明Person类不能被继承    var name:String    final var age:Int    final func description() -> String{        return "\(name) 年龄是: \(age)"    }    final class func printClass() -> (){        print("Person 打印...")    }    init(name:String, age:Int){        self.name = name        self.age = age    }}class Student:Person{            //报错    var school:String    convenience init(){        self.init(name: "Tony", age:18, school:"清华大学")    }    init(name:String, age:Int, school:String){        self.school = school        super.init(name:name, age:age)    }    override func description() -> String {    //报错        print("父类打印 \(super.description())")        return "\(name) 年龄是: \(age), 所在学校: \(school)。"    }    override class func printClass() -> (){   //报错        print("Student 打印...")    }    override var age:Int{  //报错        get{            return super.age        }        set{            super.age = newValue < 8 ? 8: newValue        }    }}

类型检查与转换

定义一个类的继承

class Person{    var name:String    var age:Int    func description() -> String{        return "\(name) 年龄是: \(age)"    }    convenience init(){//便利构造函数        self.init(name: "Tony")//调用本类的第二个（其他）便利构造函数        self.age = 19    }    convenience init(name:String){//便利构造函数        self.init(name: name, age:8)//调用同一类（下面）的指定构造函数    }    init(name:String, age:Int){//指定构造函数        self.name = name        self.age = age    }}class Student:Person{    var school:String    init (name:String, age:Int, school:String){//指定构造函数        self.school = school        super.init(name:name, age:age)//调用Person中的指定构造函数    }}class Worker:Person{    var factory:String    init(name:String, age:Int, factory:String){        self.factory = factory        super.init(name:name, age:age)    }}

使用is进行类型检查，它可以判断一个实例是否是某个类的类型。如果实例是目标类型，结果返回true，否则返回false

let student1 = Student(name:"Tom", age:18, school:"清华大学")let student2 = Student(name: "Ben", age: 28, school: "北京大学")let student3 = Student(name: "Tony", age: 38, school: "香港大学")let worker1 = Worker(name: "Tom", age: 18, factory: "钢厂")let worker2 = Worker(name: "Ben", age: 20, factory: "电厂")let people = [student1, student2, student3, worker1, worker2]//把实例放进数组集合中var studentCount = 0var workerCount = 0for item in people{      //用循环判断其属于哪个子类    if item is Worker{        workerCount = workerCount + 1    }else if item is Student{        studentCount = studentCount + 1    }}print("工人人数: \(workerCount), 学生人数: \(studentCount)。")

使用as、as！和as？进行类型转换

某类型的一个常量或变量可能在幕后实际上属于一个子类。当确定是这种情况时，你可以尝试向下转到它的子类型，用类型转换操作符（as? 或 as!）。
因为向下转型可能会失败，类型转型操作符带有两种不同形式。条件形式（conditional form）as? 返回一个你试图向下转成的类型的可选值（optional value）。强制形式 as! 把试图向下转型和强制解包（force-unwraps）转换结果结合为一个操作。
//当你不确定向下转型可以成功时，用类型转换的条件形式（as?）。条件形式的类型转换总是返回一个可选值（optional value），并且若下转是不可能的，可选值将是 nil。这使你能够检查向下转型是否成功。只有你可以确定向下转型一定会成功时，才使用强制形式（as!）。当你试图向下转型为一个不正确的类型时，强制形式的类型转换会触发一个运行时错误。

let p1:Person = Student(name: "Tom", age: 20, school: "清华大学")let p2:Person = Worker(name: "Tom", age: 18, factory: "钢厂")let p3:Person = Person(name: "Tom", age: 28)let p4:Student = Student(name: "Ben", age: 40, school: "北京大学")let p5:Worker = Worker(name: "Tony", age: 28, factory: "电厂")

创建了实例，p1、p4是Student实例，p2、p5是Work实例，p3是Person实例
类型转换有两个方向：向下转型——将父类类型转换为子类类型，向上转型——将子类类型转换为父类类型。通常情况下的类型转换都是向下转型
as操作符在类中仅仅只用于向上转型,很少用.还可以进行模式匹配

let p41:Person = p4 as Person//向上转型let p41:Person = p4

将Student类型的p4转化为Person类型是向上转型，向上转型通常可以省略as Person部分。

as!操作符可以应用于三种情况：将非可选类型为非可选类型／将可选类型为非可选类型／将可选类型为可选类型

//1.将非可选类型为非可选类型let p11 = p1 as! Student//let p111 = p2 as! Student  //报错  在转换过程中不能转换为目标类型//2.将可选类型为非可选类型let p6 :Person? = Student(name: "Tom", age: 20, school: "清华大学")let p12 = p6 as! Student//报错 p6为nil  as！在转换过程对可选址进行拆包 //3.将可选类型为可选类型let p13 = p6 as! Student?//可选类型转换为可选类型

as?操作符可以应用于两种情况：将非可选类型为可选类型， 将可选为可选

//向上转型，使用as?//将非可选类型为可选类型let p21 = p1 as? Studentlet p211 = p2 as? Studentprint(p21)print(p211)//将可选为可选let p7 :Person? = Student(name: "Tom", age: 20, school: "清华大学")let p22 = p7 as? Student?print(p7)print(p22)var arr = [p1, p2, p3, p4, p5]var i:Int = 1for item in arr{    if let m = item as? Worker{        print("Worker factory: \(m.factory)")    }else if let n = item as? Student{        print("Student school: \(n.school)")    }    i += 1}let stud1 = arr[0] as? Student //它是Student可选类型，具体使用时往往还需要拆包print(stud1)print(stud1!.school)let woker1 = arr[1] as! Workerprint(woker1)print(woker1.name)

所以我们在使用as?操作符进行类型转换时你最好采用可选绑定方式，也就是将转换语句放到if或while语句中

两种不确定的类型：Any类型和AnyObject类型

AnyObject:任何类的类型, Any:任何类型

let people1:[Person] = [student1, student2, student3, worker1, worker2]let people2:[AnyObject] = [student1, student2, student3, worker1, worker2]let people3:[Any] = [student1,student2,student3, worker1, worker2]for item in people2{    if let Student = item as? Student{        print("Student school: \(Student.school)")    }else if let Worker = item as? Worker{        print("Worker factory: \(Worker.factory)")    }}let w1:AnyObject = arr[1] as! Worker

使用Any类型来和混合的不同类型一起工作，包括函数类型和非类类型

var things = [Any]()//创建了一个可以存储Any类型的数组things.append(0)things.append(0.0)things.append(42)things.append(3.14159)things.append("hello")things.append((3.0, 5.0))things.append(Student(name: "Tina", age: 10, school: "四川师范大学"))things.append({ (name: String) -> String in "Hello, \(name)" })print("\n")for thing in things {    switch thing {    case 0 as Int:        print("zero as an Int")    case 0 as Double:        print("zero as a Double")    case let someInt as Int:        print("an integer value of \(someInt)")    case let someDouble as Double where someDouble > 0:        print("a positive double value of \(someDouble)")    case is Double:        print("some other double value that I don't want to print")    case let someString as String:        print("a string value of \"\(someString)\"")    case let (x, y) as (Double, Double):        print("an (x, y) point at \(x), \(y)")    case let stu as Student:        print("a student name: '\(stu.name)', age: \(stu.age), school: \(stu.school)")    case let stringConverter as (String) -> String:        print(stringConverter("Michael"))    default:        print("something else")    }}

原则上若能够使用具体的数据类型，则尽量不要使用AnyObject类型，更要少考虑使用Any类型。从集合取出这些实例时，请尽可能地将AnyObject或Any类型转换为特定类型，然后在进行接下来的操作