为什么Java中的多态性从超类调用方法

Question

///Example code here
Superclass{ 
    method1(){ 
      print(do1);
     }
    method2(){}
  }
Subclass extends Superclass{
   ///override method1
 method1(){ 
      print(do2);
    }
 method3(){}
 }

I have a question for the polymorphism in java, which is when Superclass s =new Subclass() . the "s" object always invoke the method in the Superclass, but when the override method happened, the "s" will point back to the override methods.

Update: So, the question is who will create a heap address for the reference "s", if it can be compiled and running in the end. If the Superclass created it, why new Subclass() rather than new Superclass(). If the Subclass created it, why cannot use s.method3().

Answer 1

s can call method3. You just have to cast it first. If they allowed s to call method3 as Superclass, it wouldn't make sense because not ALL Superclass can call method3.

Answer 2

Dynamic binding of method will decide at run times , so first when you call method with s as its reference of super class compiler will always first check if superclass has that method or not. there are 2 case

1. If super class has the method JVM will go ahead and check in child class, If child class has overriding method then that method will be called , if child class doesn't have that method than normally parent class method will call.

2. If super class does not have that method , then compilation fails as compiler could not identified any method in class of reference variable

So in your case if super class doesn't have method3 then it will defiantly gives compilation error