🚀 go-pugleaf

Does the double dispatching technique of GOF's Visitor pattern not always
work in Java ?

I saw that if you have an overloaded method of which a version take as an
argument an object of class A,
and another takes an object of class B which extends A, if you invoke it
with a reference of type A which points to
a class B object, the JVM selects the first version (the one which declares
an instance of class A as its parameter).

Is this behavior correct ?

If you like, play with this sample of mine.
=======
package edotest;

public class Fruit
{
  public void print()     // (3)
  {
    System.out.println("Fruit");
  }
}
=======
package edotest;

public class Apple extends Fruit
{
  public void print()  // (4)
  {
    System.out.println("Apple");
  }
}
=======
package edotest;

public class Main
{
  Fruit fruit = new Fruit();
  Apple apple = new Apple();
  Fruit fruit2 = new Apple();

  public static void main(String[] args)
  {
    Main main = new Main();
    main.test();
  }


  void test()
  {
    print(fruit);   // invokes (1 + 3 )
    print(apple); // invokes (2+ 4 )
    print(fruit2); // invokes (1 + 4 )
  }


  void print(Fruit aFruit)        // (1)
  {
    System.out.print("Main.print(Fruit aFruit) : ");
    aFruit.print();
  }


  void print(Apple anApple) // (2)
  {
    System.out.print("Main.print(Apple anApple) : ");
    anApple.print();
  }

}
=======


bye,
Edo.

ing. Edoardo Comar
<comar@tecnest.it>
Tecnest S.r.l
via Tullio, 11/13 - 33100 Udine Italy
ph. +39 432 511550
fax +39 432 508725
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