Object-oriented (polymorphic downward transformation) of the first season of Java entry

Last blog (http://blog.csdn.net/qq_32059827/article/details/51328638) finally on the shortcomings of the polymorphic show, here to solve this drawback. As follows:

/* polymorphic Disadvantages: You cannot use the unique features of subclasses. I just want to use the special features of subclasses, okay? How to use it? A: You can call A method to create A subclass object. (yes, but a lot of the time is unreasonable.) And, too much memory) B: casts a reference to the parent class into a reference to the subclass. Transformation between objects (downward transition): upward transformation: Fu f = new Zi (); downward transformation: (prone to problems) Zi Z = (Zi) F; Requires that the F must be capable of being converted to Zi. Type to match the non-row case: Ren r = (cat) cat (); it's obviously impossible for a cat to turn into an adult. */class Fu {public void Show () {System.out.println ("show Fu");}} Class Zi extends Fu {public void Show () {System.out.println ("show Zi");} public void Method () {System.out.println ("method Zi");}} Class DuoTaiDemo4 {public static void main (string[] args) {//Test fu F = new Zi (); F.show ();//f.method ();//no symbol found, no method in parent class ( //Create subclass object, yes, but many times unreasonable. And, too much memory//zi z = new Zi ();//z.show ();//z.method (); Zi z = (Zi) f;//Note Format Oh! Z.show (); Z.method ();}}

Now that there is a problem with the downward transition, the next step is to demonstrate a problem with the downward transition. (This problem is due to grammatical nonstandard)

/*classcastexception: Type conversion exceptions are generally easy to appear in a polymorphic downward transition */class Animal {public void Eat () {}}class Dog extends Animal {public void Eat ( {}public void Lookdoor () {}}class Cat extends Animal {public void Eat () {}public void PlayGame () {}}class DuoTaiDemo5 {p Ublic static void Main (string[] args) {//In memory is the dog animal a = new dog (); System.out.println (a);D og d = (Dog) a;//downward transformation System.out.println (d);//Memory is cat//animal a = new Cat (); Animal A remember that you cannot define this. ERROR: Variable A has been defined in method main (string[]). Personal understanding is the occurrence of variable assignment "conflict"//(the reader wants to see the specific reasons for the above error) A = new Cat ();//Downward Transition System.out.println (a); Cat C = (cat) a;//downward Transformation System.out.println (c);//memory is cat//dog dd = (Dog) A; Classcastexception,a is the cat's content and cannot be transformed downward to the dog. A cat becomes a dog.}

The memory plots for polymorphic problems are as follows:

Object-oriented (polymorphic downward transformation) of the first season of Java entry