Object as an array

Question

I need to create class Dog and PurebredDog extending Dog. Problem is that Dog can be at once single object and array of objects (Dogs and PurebreedDogs :

 Dog pack[]={new Dog(76589,"As","black",18,
                    "Ann","Kowalsky"),
           new PurebreedDog(45321,"Labrador","Elf","black",25, 
                          "Angus","Mati","Barbara","Smith"),
           new Dog(102467,"Gamma","brown",89,
                    "Josh","Coke"),
            new PurebreedDog(9678,"York","Theta","brown",8,
                    "Emka","Figaro","Alice","Cat")};    

for(int i=0; i < pack.length; i++)
  System.out.println(pack[i]+"\n\n");

How to write proper constructor for Dog ? You could do :

public Dog(String name, etc){
}

but how to write constructor for array of dogs ?

public Dog(Dog[]tab) ?

And then how to recall it's elements ? Is pack[] a 2d array ?

Answer 1

To simplify things, an instance of Dog really should refer to a single Dog. So your constructor should look similar to (the data types are just examples):

Dog(int ID, String color, String name, ...)

PurebreedDog would subclass Dog and provide any additional constructor parameters (and members) such as breed, etc.

To deal with multiple dogs, I recommend you store instances of the class in a List, HashTable, or other type of data structure that is designed to hold multiple elements. The actual structure you use will depend upon your requirements.

Answer 2

In your specific case, a Dog should represent one dog, and I agree with Justin's answer.

Just for the sake of completeness, I think it's worth mentioning the composite pattern though.

There are indeed some circumstances where an aggregation of objects can acts the same way as its individual parts. A traditional example is the folder/file hierarchy. Both responds to the same methods such as size, delete, etc.

That could even be used with you Dog example if you plan to model a genealogy tree. A dog is a dog, but can have N children.

Answer 3

The way to make uniform the representation of one dog and many dogs is for each entity to be a list. The one-dog entity is simply a list with one dog in it; the multiple-dog entity, of course, has more than one dog in it. But all you need to code is the (single) Dog, and then, as Justin and ewernli said, use a Collection - probably an ArrayList of Dog.

Trying to make the un-enlisted single Dog the "same" as a list of dogs is just crazy.

Answer 4

Okay, so did it the following way :

PurebreedDog class:

public class PurebreedDog extends Dog {

private String breed;
private String mother;
private String father;

public PurebreedDog(int id, String breed, String name, String color, int age,
        String owner_name, String owner_surname, String mother, String father){
    super(id, name, color, age, owner_name, owner_surname);
    this.breed = breed;
    this.mother = mother;
    this.father = father;
}

@Override
public String toString(){
    return super.toString()+"\n Pure breed dog \n"+"-------------------------------\n"+
            "BREED: "+this.breed+"\n"+"MOTHER: "+this.mother+"\n"+
            "FATHER: "+this.father+"\n-------------------------------";
}

}

Dog class:

public class Dog {

private int id;
private String name;
private String owner_name;
private String owner_surname;
private String color;
private int age;
private Dog[]pack;

public Dog(int i, String n, String c, int w, String on, String os){
    id = i;
    name = n;
    color = c;
    age = w;
    owner_name = on;
    owner_surname = os;
}

public Dog(Dog[]p){

    Dog[]group = new Dog[p.length];
    for(int i=0; i < p.length; i++){
        group[i] = p[i];
    }

    this.pack = group;

}

@Override
public String toString(){

    return "-------------------------------\n"+
            "idr: "+this.id+"\n"+"OWNER: "+this.owner_name+
            this.owner_surname+"\n"+"NAME: "+this.name+"\n"+
            "color: "+this.color+"\n-------------------------------";
}