How to access array of objects with specific indexes from a different class in Java?

Question

I know there are many questions similar to mine already but I still haven't found a solution that applies to my problem. I am required to use three different classes for this and to give you an idea, here is what some of my code looks like (included only those that I think are relevant):

public class Main {
   //this is where I instantiate 
   public static void main(String[] args){
      Habitat habitat1 = new Habitat(some arguments/parameters here);
      Habitat habitat2 = new Habitat(some arguments/parameters here);

      Animal type1 = new Animal(some arguments/parameters here);
      Animal type2 = new Animal(some arguments/parameters here);
      Animal type3 = new Animal(some arguments/parameters here);
      Animal type4 = new Animal(some arguments/parameters here);
   }
}

public class Habitat {
   //other attributes here
   Animal[] animals;

   public Habitat(some arguments/parameters here) {

  //here I want to be able to reference/assign **Animal type1 and type2 to habitat1** and **Animal type3 and type4 to habitat2** 
}


public class Animal {
   //other attributes here
   Habitat habitat;
   
   public Animal(some arguments/parameters here){
    
   }

I know that in order to have an array of object animals, I need to write something like this:

Animal[] animals = new Animal[4];

        animals[0] = type1;
        animals[1] = type2;
        animals[2] = type3;
        animals[3] = type4;
    

The problem is that I cannot put this block of code in class Habitat because it cannot access type1-type4. When I put this in class Main, I am unable to reference it in class Habitat.

Basically, the specific output I want in this particular problem is that when I print the contents of habitat1 for example, it will look something like this:

Habitat 1 - Savanna
Mixed woodland-grassland
List of Animals: 
     Type 1
     Name: Tiger
     Diet: Carnivore

     Type 2
     Name: Elephant
     Diet: Herbivore

Answer 1

Add a method addAnimal( Animal animal ) to Habitat and call this from the constructor of Animal like this

…
habitat.addAnimal( this );
…

Of course this assumes that the fourth argument of the Animal constructor is named habitat .

And the animals attribute should not be an array, but an instance of java.util.List (preferrably a java.util.ArrayList ). Then the new method would look like this:

public final void addAnimal( final Animal animal )
{
  animals.add( animal );
}

Perhaps you have to add some checks (not-null, for example), and when you want to make sure that an animal can be added just once, you should consider to use java.util.Set instead of a List (But this would mean some changes to the Animal class as well).

If you really insist on an array for the attribute, the addAnimal() method would get a bit more complex:

public final void addAnimal( final Animal animal )
{
  if( animals == null )
  {
     animals = new Animals [1];
     animals [0] = animal;
  }
  else
  {
    var curLen = animals.length;
    var newAnimals = new Animal [curLen + 1];
    System.arraycopy( animals, 0 newAnimals, 0, curLen )
    animals = newAnimals;
    animals [curLen] = animal;
  }
}

or

public final void addAnimal( final Animal animal )
{
  if( animals == null )
  {
     animals = new Animals [1];
     animals [0] = animal;
  }
  else
  {
    var curLen = animals.length;
    animals = Arrays.copyOf( animals, curLen + 1 );
    animals [curLen] = animal;
  }
}

Still error handling is missing.

Answer 2

Provide method(s) on Habitat for associating the Animal[] .

For instance, if you define a constructor that accepts the Animal[] then you can build that array first and then pass it into the constructor. You can also provide a setter that allows you to swap to a different Animal[] at any point.

public class Habitat {
  private Animal[] animals; // note that this should be private

  public Habitat(Animal[] animals) {
    this.animals = animals;
  }

  public void setAnimals(Animal[] animals) {
    this.animals = animals;
  }
}

This allows the main method to pre-generate the Animals or assign them later as needed.

public class Main {
  public static void main(String[] args) {
    Animal[] animals = ... build the array
    Habitat habitat1 = new Habitat(animals);

    Habitat habitat2 = new Habitat();
    Animal[] moreAnimals = ... build another array of Animals
    habitat2.setAnimals(moreAnimals);
  }
}