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Swift

} else if item is Maths { ++mathsCount } }

println("Subjects in chemistry contains \(chemCount) topics and maths contains \(mathsCount) topics") When we run the above program using playground, we get the following result: 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

Downcasting Downcasting the subclass type can be done with two operators (as? and as!).'as?' returns an optional value when the value returns nil. It is used to check successful downcast. 'as!' returns force unwrapping as discussed in the optional chaining when the downcasting returns nil value. It is used to trigger runtime error in case of downcast failure 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) } }

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