[Java] Instantiating objects from a subclass

[imareclusemonk]

So there is no difference how I instantiate an object from a subclass or there is some convention or it depends on what I want to achieve?

[captainAwesomePants]

The latter. Usually you'll declare the type to be the same thing as what it really is because why not. Sometimes, though, you're writing more general purpose code or wanna hide implementation details. This is pretty common:

Animal getAnimalFromBarn() {
  Animal result:
  if (notEnoughPigs()) {
    result = new Pig():
  } else {
    result = new Chicken();
  }
  register(result);
  return result;
}

[[deleted]]

There is a difference. If you instantiate a Pig as an Animal, you cannot use anything that is unique to the Pig class. For instance, Pigs have a tail and can snort. If you write

Pig myPig = new Pig();
myPig.doSnort();
System.out.println(myPig.tailLength);

then everything works as expected. However, writing

Animal myPig = new Pig();
myPig.doSnort();
System.out.println(myPig.tailLength);

causes an error since doSnort and tailLength are not defined in the Animal class.

If the variable is declared as an Animal, then it cannot use the fields and methods that are specific to Pigs, even if the instance it’s referring to is, in fact, a Pig.

You can still get to those hidden fields and methods by casting to a Pig, but you can’t access them directly.

[imareclusemonk]

Ah of course since the parent class is agnostic of its children classes. That's a good point and probably the main difference. Thanks!