java - Polymorphism vs Inheritance -

suppose have 2 classes: animal , dog. dog subclass of animal. following code:

animal = new dog(); 

now can call methods of dog class through variable.

but question this: if can call of animal's methods through dog objects (inheritance) why should use polymorphism principle? can declare:

dog d = new dog(); 

with declaration can use of animal's methods , dog methods. why use polymorphism? thank answer.

in java, concepts of polymorphism , inheritance "welded together"; in general, not have way:

• polymorphism lets call methods of class without knowing exact type of class
• inheritance lets derived classes share interfaces , code of base classes

there languages inheritance decoupled polymorphism:

• in c++ can inherit class without producing polymorphic behavior (i.e. not mark functions in base class virtual)
• in objective c can implement method on unrelated class, , call place knows signature of method.

going java, reason use polymorphism decoupling code details of implementation of counter-parties: example, if can write method feed(animal animal) works sorts of animals, method remain applicable when add more subclasses or implementations of animal. in contrast feed(dog dog) method, tightly coupled dogs.

as far the

dog d = new dog(); 

declaration goes, there no general reason avoid if know rest of method deals dogs. however, in many cases later not case: example, class or methods insensitive exact implementation, example

list<integer> numbers = new arraylist<integer>(); 

in cases that, can replace new arraylist<integer>() new linkedlist<integer>(), , know code going compile. in contrast, had numbers list been declared arraylist<integer> numbers, such switchover may not have been certainty.

this called "programming interface". there answer on stack overflow explaining it.