Level 2: polymorphism

Task description

     Our mission ： The topic of this section is to take the mobile phone brand as an example , understand Scala For method rewriting and polymorphism in object-oriented programming, please read the following carefully “ Related knowledge ” The content in , Understand what needs to be done for each mobile phone class , completion huawei Functions in class , Make the program run as expected .

Related knowledge

     In order to complete this mission , You need to master ：1. How to override methods ,2. How to achieve polymorphism .
##### Method rewriting
     Method rewriting refers to when a subclass inherits from its parent class , Methods inherited from the parent class cannot meet their own needs , Subclasses want to have their own implementation , At this time, you need to override the methods of the parent class （override）, Method rewriting is the key to realize polymorphism and dynamic binding .Scala Method rewriting and Java Method rewriting in language is the same , through override Keyword to override the method in the parent class , So as to realize the processing logic of the subclass itself .

class Phone(var phoneBrand:String,var price:Int){  
   // For the parent class Any Medium toString Method   
   override def toString=s"Phone($phoneBrand,$price)"  
}
class Apple(phoneBrand:String,price:Int,var place:String)extends Phone(phoneBrand,price){  
  // For the parent class Phone Medium toString Method   
   override def toString=s"Apple($phoneBrand,$price,$place)"  
}
object TestPhone{  
   def main(args:Array[String]){  
    // call Apple Class's own toString Method returns the result   
      println(new Apple("iphone",5400,"Shenzhen"))  
   }  
}  

Running results ：
Apple(iphone,5400,shenzhen)

     If you don't override the parent class toString Method , The returned result is the class name plus hashcode value , for example ：

scala> class Phone(var phoneBrand:String,var price:Int)  
defined class Phone
scala> new Phone("HuaWei",4500)  
res1: Phone = Phone@55881f40  

Polymorphism and inheritance

     After implementing inheritance , A child class can have the properties and methods of the parent class , You can also add new class members to the subclass or override the methods in the parent class . Polymorphism is a language feature implemented on the basis of inheritance , It refers to allowing different objects to respond to the same message , That is to say, the same message can adopt different behavior modes according to different sending objects .
     Polymorphism is also called dynamic binding or delayed binding , It refers to determining the actual type of the referenced object during execution rather than compilation , Call its corresponding method according to its actual type , In other words, the reference of the subclass can be assigned to the parent class , When the program runs, it calls the corresponding method according to the actual type . The main function of polymorphism is to eliminate the coupling relationship between types . There are three necessary conditions for polymorphism ： There must be inheritance ; There should be rewriting ; The parent class reference points to the subclass object .

  // Define parent class Phone  
class Phone(var phoneBrand:String,var price:Int){  
    def buy():Unit=println("buy() method in Phone") //buy Method , No parameter   
  //compare Method , Parameter is Phone type   
    def compare(p:Phone):Unit=println("compare() method in Phone")  
}
   // Defining subclasses Apple  
class Apple( phoneBrand:String,price:Int) extends Phone(phoneBrand,price){  
   private var AphoneNo:Int=0  
  // Override parent class compare Method   
   override def compare(p:Phone):Unit={  
       println("compare() method in Apple")  
       println(this.phoneBrand+" is compared with "+p.phoneBrand)  
  }  
} 

     If you create Apple Class object , Parent class Phone The reference points to the object ：

1. val p1:Phone=new Apple("iphone",6400)

     Use parent class reference p1 call compare Method , Then what is really called is the subclass Apple Rewritten in compar Method ; If the parent class references p1 call buy Method , Because of subclasses Apple The... In the parent class is not overridden in buy Method , Will undoubtedly call... In the parent class buy Method .
#### Programming requirements
     The programming task of this pass is to complete... According to the expected results HuaWei Class from Phone Functions inherited by class , Consider whether you need to rewrite each method .

Test instructions

The following is a description and sample test of how the platform evaluates the functions you have implemented .
Test input ： No input
Expected results ：
buy() method in HuaWei
compare() method in HuaWei
huawei is compared with iphone
-----------------------------
buy() method in Phone
compare() method in Apple
iphone is compared with huawei

Start your mission , wish you success ！

object TestPhone{

    // Define parent class Phone
    class Phone(var phoneBrand:String,var price:Int){
        def buy():Unit=println("buy() method in Huawei") //buy Method , No parameter 
        //compare Method , Parameter is Phone type 
        def compare(p:Phone):Unit=println("compare() method in Huawei")
    }

    // Defining subclasses Apple
    class Apple( phoneBrand:String,price:Int) extends Phone(phoneBrand,price){
        private var AphoneNo:Int=0
        // Override parent class compare Method 
        override def buy():Unit=println("buy() method in Phone")
        override def compare(p:Phone):Unit={
            println("compare() method in Apple")
            println(this.phoneBrand+" is compared with "+p.phoneBrand)
        }
    }

    // Defining subclasses HuaWei
    class HuaWei(phoneBrand:String,price:Int) extends Phone(phoneBrand,price){

        private var HphoneNo:Int=0
        //**************Begin*************************
        override def buy():Unit=println("buy() method in HuaWei")
        override def compare(p:Phone):Unit={
            println("compare() method in HuaWei")
            println(this.phoneBrand+" is compared with "+p.phoneBrand)
        }

        //**************Begin*************************

    }

    // Operation entrance 
    def main(args: Array[String]){

        val p1:Phone=new HuaWei("huawei",4500)
        val p2:Phone=new Apple("iphone",6400)

        p1.buy()

        p1.compare(p2)
        println("-----------------------------")

        /*p2 The reference is Apple Object of type ,Apple Class is not on the buy Method , Therefore, what it calls is... Inherited from the parent class buy Method */
        p2.buy()

        //p2.compare(p1) The actual type passed in is HuaWei, It's called Apple After class rewriting compare Method 
        p2.compare(p1)
    }
}