Instance physics is: solid physics
Instance equation is: Hertz
Instance physics is: Fluid Dynamics
Instance formulae is: Giga Hertz
Subjects in chemistry contains 2 topics and maths contains 3 topics
向下转换
向下类型转换的子类型可以有两个操作符（如：as? 和 as!）。as? 当值是nil，返回一个可选值。它是用来检查成功向下转型。
“as!” 返回强制解包裹，如可选链，向下转换返回 nil 值。它用来触发运行时错误在向下转型出现故障时
复制代码class Subjects {
var physics: String
init(physics: String) {
self.physics = physics
}
}
class Chemistry: Subjects {
var equations: String
init(physics: String, equations: String) {
self.equations = equations
super.init(physics: physics)
}
}
class Maths: Subjects {
var formulae: String
init(physics: String, formulae: String) {
self.formulae = formulae
super.init(physics: physics)
}
}
let sa = [
Chemistry(physics: "solid physics", equations: "Hertz"),
Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz"),
Chemistry(physics: "Thermo physics", equations: "Decibels"),
Maths(physics: "Astro Physics", formulae: "MegaHertz"),
Maths(physics: "Differential Equations", formulae: "Cosine Series")]
let samplechem = Chemistry(physics: "solid physics", equations: "Hertz")
println("Instance physics is: (samplechem.physics)")
println("Instance equation is: (samplechem.equations)")
let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")
println("Instance physics is: (samplemaths.physics)")
println("Instance formulae is: (samplemaths.formulae)")
var chemCount = 0
var mathsCount = 0
for item in sa {
