java (polymorphism)

Polymorphism

"Capture the instance vaguely" Instances can be assigned to variables of parent class type if the is-a relationship is established by inheritance.

SuperHero h = new SuperHero();

Regular SuperHero instance

Character c = new SuperHero();

Premise that the parent class of the SuperHero class has Character

Assigning an instance to a box of ambiguous type

public class Main {
  public static void main(String[] args) {
    Wizard w = new Wizard();
    Character c = w;             //Put in a Character type box
    Matango m = new Matango();
    c.name = "Wizard";
    c.attack(m);                 //Can be called
    c.fireball(m);               //Get an error
  }
}

・ Forget that the contents of the box (Character c) is Wizard ・ Only attack () can be called because other characters have attack. (Assuming that every character has attack ()) -The reason why fireball () could not be called is that another character does not have fireball () (because it does not necessarily have it), so an error occurs. ・ I can say for sure that the box contains only one type of character

How to forcibly call Wizard (downcast)

Character c = new Wizard();
Wizard w = c;                   //Get an error

Because the program is interpreted and translated line by line, c on the second line cannot be determined as Wizard.

Character c = new Wizard();
Wizard w = (Wizard)c;           //Instruct forced type conversion

Processing that works when a method can be called

** The Slime (child) class inherits the Monster (parent) class ** Monster System.out.println ("Monster escaped."); Slime System.out.println ("Slime has escaped.");

public class Main {
  public static void main(String[] args) { 
    Slime s = new Slime();
    Monster m = new Slime();
    s.run();
    m.run();
  }
}

Execution result The slime escaped as a result of s.run () The slime escaped as a result of m.run () The actual result depends on the type of contents (Slime ()), the type of box (Monster m) is not related to the result

Downcast failure (ClassCastException)

Character c = new Wizard();
Hero h = (Hero)c;                        

Compiles successfully, but fails when running Because it is a "lie composition"

Determine if you can cast safely (instanceof operator)

if(c instanceof Wizard) {  //If the contents of c can be regarded as Wizard
Wizard w = (Wizard)c;      //Think of it as a Wizard
w.fly();
}                       

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