I have seen other answers to similar questions but all of them rely on the fact that the language is defined to be like this. Following is what I am seeking an explanation to:
In an inheritance hierarchy, the parent types can hold child objects implicitly (why?), however for the child references to hold a parent object, explicit downcast is necessary (why?).
Please cite some example that explains why not doing this will fail, I mean using Animal, Dog type etc. If this question is already answered and I have missed it, citing that also will be helpful.
For example:
class Animal{
public void eat(){};
}
class Dog extends Animal{
public void bark(){};
}
class App{
Animal an = new Dog(); //1. why can animal store a dog but not a dog store an animal
Dog dog = (Dog) new Animal(); //2. Dog has the behavior of an Animal so why explicit downcast
}
I just want to know how this lines make sense, other than just knowing they are language semantics. Better if your answer looks like that as a granny explaining this to her grandchild.
Edit:
I just was wondering that Dog inherits Animal and has all the behavior. Hence number 2 above should have been allowed without explicit downcasting.
Or, that (I think I got it now) when I ask a Dog to store an Animal there are possibilities that I actually get a Cow or a Horse, because Animal being parent can hold any of its subtypes. If that's the case then why has Java allowed Animal to hold subtypes since there might be behavior typical to subtypes like a Dog will bark(), for that again compiler has to check and report. I know the rules just trying to reason out in the simplest of sense.
The gain of strict type binding in Java is that you get compile time errors, instead of runtime errors, when possible.
Example:
class Animal {
void eat() {}
}
class Dog extends Animal {
void bark() {}
}
class Pigeon extends Animal {
void pick() {}
}
class MyFunction {
void run() {
Animal first = new Pigeon();
// the following line will compile, but not run
((Dog)first).bark();
}
}
If you have such code in a simple example like this, you will spot the problem at once. But consider having such a problem in a project, in a seldom called function at the depth of thousands of lines of code, in hundreds of classes. One day in production, the code fails and your customer is upset. And its up to you to find out why it failed, what happened and how to fix it. Its a horrible task.
So, with this somewhat complicated notation Java nudges you into thinking again about your code, the next example would be how its done better:
class MyFunction {
void run() {
Pigeon first = new Pigeon();
// the following line will NOT compile
first.bark();
// and neither will this. Because a Pigeon is not a Dog.
((Dog)first).bark();
}
}
Now you see your problem at once. This code, will not run. And you can avoid the problems ahead by using it correctly.
If you make your Animal class abstract (which you should), you will see that you can only instantiate specific animals, but not general ones. After that you will start using the specific ones when required and be relieved that you can reuse some code when using the general class.
Background
Conceptually, runtime errors are harder to find and debug, then compile time errors. Like, seriously hard. (Search for NullPointerException here on Stack Overflow and you will see hundreds of people who struggle to fix runtime exceptions)
In a Hierarchy of things (in general, not programming related) you can have something general "Thats an animal". You can also have something specific "Thats a dog". When someone talks about the general thing, you can't expect to know the specific thing. An animal can't bark up a tree, because birds could not, neither do cats.
So, in Java in particular the original programmers found it wise to decide that you need to know an object specific enough to call the functions of that object. This ensures that if you didn't pay attention, the compiler will warn you, instead of your runtime.
Your particular case
You assume that:
Dog dog = (Dog) new Animal();
should work, because a Dog is an Animal. But it won't, because not all Animals are Dogs.
BUT:
Animal an = new Dog();
works, because all Dogs are Animals. And in this specific case
Animal an = new Dog();
Dog dog = (Dog)an;
will work too, because the specific runtime state of that animal happens to be Dog. Now if you change that to
Animal an = new Pigeon();
Dog dog = (Dog)an;
it still WILL compile, but it WILL NOT run, because the second line Dog dog = (Dog)an; fails. You can't cast a Pigeon to a Dog.
So, in this case you WILL get a ClassCastException. Same if you try to cast new Animal() to Dog. An Animal is NOT a Dog. Now, that will happen at runtime and that is bad. In the Java way of thinking, compile time errors are better then runtime errors.
Yes. One simple real time example to understand the theory is
Every Truck driver is Driver but not you cannot say every Driver is a Truck Driver.
Where
Driver -Parent
Truck Driver - Child.
Animal an = new Dog(); //1. why can animal store a dog but not a dog store an animal
You ordered for an Animal and the shop keeper gave a Dog for you and you are happy since Dog is an Animal.
Dog do = (Dog) new Animal(); //2. Dog has the behavior of an Animal so why explicit downcast
You are asking for a Dog and the shop keeper gave an Animal to you. So it is so obvious that you check that the Animal is a Dog or not. Isn't you ?? Or you just assume that you got a Dog ?
Think.
In java references are used. Suppose we have below classes
class A{
Integer a1;
public A(){
// a1 is initialized
}
public void baseFeature(){
super.baseFeature();
// some extra code
}
}
and
class B extends A{
Integer b1;
Integer b2;
public B(){
super();
// b1 , b2 are initialized
}
#Override
public void baseFeature(){
super.baseFeature();
// some extra code
}
public void extraFeature(){
// some new features not in base class
}
}
Below all 3 statements are valid
A a = new A();
B b = new B();
A b1 = new B();
In java references are used to refer to the objects kept in Heap.
A reference of type A should not be considered as if it can not hold the objects which have more memory required than an object of class A. Its the references and not the objects holders.
In case of sub type object creation, Constructor call follows : Parent-constructor call followed by constructor call of the actual class whose object is being created.
Sub class object can be said to be having features at-least as much as the Parent type has.
A b = new B() has no confusion, as object of B has all the features of its parent.
Sub class object has all the features as defined in its parent, so any parent class method can be called on the object of sub class
Sub classes can have much more features, which parent class does not have, so calling a method of sub class on object of parent will lead to problems.
Suppose In Java B a = new A() is valid then if a.extraFeature() is invoked then it is obvious of an error.
This is prevented by compile time error. If in case downcasting is needed, then the programmer must do it with extra care. This is the intention of compile time error. Below cases down-casting would not lead to issues but the onus is on the programmer to see if situation is of this kind or not.
public void acceptChild(Child c){
c.extraMethodNotWithParent();
}
Parent p = new Child();
acceptChild((Child)p);
Here programmer is given compile time warning if down-casting is not done. Programmer can have a look and can see if the actual object is really of sub class type then he/she can do explicit down-casting. Thus issues will only come if the programmer has not taken care.
All the replies here help. I was confused.
Removing all the chains in my brain, if I simply think, it now looks like:
Parent can hold subtypes.
You mean thats the benefit of polymorphism and how its achieved.
Child types can hold parent but need explicit downcast.
Since polymorphism is allowed, this downcast is a safeguard. The compiler is asked to trust that the Animal in code is actually going to be a Dog instance that the Dog type wants to hold.
This is because inheritance is specialization, when you inherit a class T, the child class S is a specialization of T, for that Java don't need a cast, but T can have many childs, S' inherits T, S' too, so here if you have an object of S and its referenced as T and you want to get your origin type again, you should make a cast to S and Java check the type at runtime.
an example :
A man and a woman are humans, but a human is a man or woman.
Man man = new Man();
Human human = man //Legal.
Man man2 = (Man) humain; //Legal
Woman human = (Woman) humain; //Error, Java can't cast the humain to Woman, because its declared originaly as Man.
If Java is sure made the cast implecitly, if not, it reports it and don't decide in your place, This check is made at runtime.
Related
I have a basic doubt in polymorphism in Java. I have written the code below in one file named AnimalTestDrive.java. According to me the code below should work specially the line in bold but unfortunately its not. Can you please explain why, I have given the error below:
class Dog extends Animal {
public void dogMethod() {
System.out.println("In Dog method");
}
}
public class AnimalTestDrive {
public static void main(String args[]) {
Dog d = new Dog();
d.dogMethod();
d.animalMethod();
Animal animal = new Animal();
animal.animalMethod();
animal = d;
**animal.dogMethod(); // THIS IS NOT WORKING**
}
}
Let's try to look at this line the same way that the compiler would:
animal.dogMethod();
First, it needs to work out what animal means. That's nice and easy - it's a local variable in the current method, so it doesn't need to look far.
The compile-time type of that variable is Animal. The compiler doesn't care what the value of the variable will be at execution time - it only uses the information about the declared type.
So, that's what it uses to try to look up what dogMethod() means within the context of animal, i.e. with type Animal. First it looks in Animal, then in java.lang.Object (the implicit superclass of Animal) - but neither of those classes contains a declaration of dogMethod. At that point, the compiler has to give up with an error - it can't find the method. It doesn't matter that the method is available on the execution-time type of the object that the value that animal refers to. It has to bind it at compile-time, using only the information available at compile time.
The only decision made at execution time is which implementation of a method is used - for example, if you called animal.toString() and the Dog class had an override, e.g.
#Override public String toString() {
return "I'm a dog";
}
then the compiler would find the toString() method from java.lang.Object, so it would know that the method call was valid - but the implementation in Dog would be used because of the execution-time type of the object.
As far as Java can tell, animal is just an animal, so it can only do the things defined in your Animal class. If you want to be able to use the Dog method, and you know your animal is a Dog, you have to cast it to a Dog for the method to be visible.
In other words, the only methods and fields available for a variable are the ones defined by it's left hand side type. You can either say ((Dog)animal).dogMethod(); to refer to animal as a Dog, or create a new variable Dog animalAsDog = animal; and call your method on animalAsDog.
Imagine there's two Java classes, Dog & Poodle. The Poodle class extends the Dog class.
When creating an instance of these classes, this is the common syntax:
Dog myDog = new Dog();
Poodle myPoodle = new Poodle();
I don't understand what's happening though when you create an object of type parent class and then call the child class constructor, like so:
Dog myDog = new Poodle();
If myDog is a Dog class, why would we call the constructor of one of its child classes, and what does that even mean? Why not just create an object of type child class (Poodle)?
To put another way, I don't know what this sentence means: "I'm going to create an object of type Dog but I'm going to call the constructor of one of its child classes, even though this object is not of the child class type".
I can't make any logical sense of what this means. Please help me understand what is going on here.
EDIT - the linked duplicate question indeed appears to be asking the same question. Frustratingly, the two most upvoted answers on that page don't provide an explicit example of a meaningful use of the code segment that the questioner specifically asked about:
Parent parent = new Child();
I understood and followed all code segments provided by the two most upvoted answers on the linked duplicate question. I even understood where the polymorphism took place. But neither of the answers used the line of code specifically called out in the question. I would really, really appreciate if someone could please show me an example of where
Parent parent = new Child();
is used usefully and meaningfully in a broader segment of code. Please. I would really, really appreciate it.
The concept is called Dynamic method dispatch, by using this run time polymorphism(Overriding) is achieved in java.
Let's extend the example you wrote as below
class Dog{
public void bark(){
// parent class implementation
System.out.println("I bark");
}
}
Here we have defined a Dog class which has a method called bark and provides it's implementation as well.
Let's say Dog is having two subclasses as Poodle and Chihuahua as below.
class Poodle extends Dog{
#Override
public void bark(){
//Poodle implementation
System.out.println("I bark like Poodle");
}
}
class Chihuahua extends Dog{
#Override
public void bark(){
//Chihuahua implementation
System.out.println("I bark like Chihuahua");
}
}
Poodle and Chihuahua classes have their implementation of the bark method.
And you created a child object and assigned a parent reference to it and call bark method as below.
Dog dog1 = new Poodle();
dog1.bark(); //prints **I bark like Poodle**
At the run time JVM understands that you have a Poodle Object and checks if Poodle overrides the bark method and then calls it.
This pattern can be observed in Collections when you do something like below.
List<String> list = new ArrayList<>();
list.add("test string"); //this gets added according to ArrayList implementation.
This question already has answers here:
Calling overloaded inherited methods using super class reference
(10 answers)
Closed 6 years ago.
I'm currently working through a Java Book and i've started reading about inheritance and polymorphism.
I'm making a test program that stores animal information, and I want to overload the method that sets the animals size. The user knows the sizes of the dog, but no other animals, so sets animal size to 0 if it's not a dog, and for the dog, it uses its own setSize method with parameters.
However i've tried two methods of creating a dog, by creating an animal object and also by creating a dog object. I assumed that even though the testDog1 variable is of type Animal, it would still be able to overload methods from the Dog class as it is a dog object.
Could anyone please explain why testDog1 does not work but testDog2 does work?
public class Loader {
public static void main(String[] args){
int dogSize = 100;
Animal testDog1 = new Dog();
testDog1.setSize(dogSize);
Dog testDog2 = new Dog();
testDog2.setSize(dogSize);
}
}
public class Animal {
public int size;
public void setSize(){
size = 0;
System.out.println(size);
}
}
public class Dog extends Animal {
public void setSize(int dogSize){
size = dogSize;
System.out.println(size);
}
}
If you are going to refer to testDog1 in your code, the compiler will consider it to be of class Animal. Meaning you can reassign it with an instance of Cat or Bird or anything else, where setSize(int) doesn't exist.
After testDog1 was declared of type Animal, you need to comply with the contract of that class.
The fact that it is currently a Dog doesn't make the specific methods of Dog visible. You will probably learn about casting in the next chapter of the book, though.
Animal testDog1 = new Dog();
An instance of Dog would be created and assigned to the testDog1 variable only when you run the program. ie.) Runtime.
Compiler doesn't run the program or make hard and fast assumptions of what the nature of each statements or variables would be while checking for correctness.Had that been the case , the following stmt would have been a compile time error too.
Animal testDog = null;
testDog.setSize(); // How can you call setSize() on null right? for the same reason mentioned above.
At compile time, all that the compiler knows is that in testDog1 you will have a reference to an Animal. Since an Animal does not have a setSize(int), the compiler cannot locate it and it fails.
It is important to know which operations are done at compile time (the compiler knows the type of the variable) and which one are done at runtime (the runtime knows the actual instance of the object).
Your testDog1 does not find setSize(int dogSize) method in Animal Class Hence it is a compilation error.
where as testDog2 find setSize(int dogSize) method in Dog class and it will compile.
testDog1 is an Animal object. Without casting it as a Dog, it doesn't know that setSize(int) exists.
(Dog)testDog1.setSize(dogSize) tells the compiler that it's actually a Dog.
I'm having some trouble understanding Java's inheritance. I understand that something like this is possible, but what is the purpose of it?
Animal pig0 = new Pig();
What is happening here? If someone wanted to create a new Pig object in this example, why not just do this?
Pig pig0 = new Pig();
Or if they wanted an Animal object, why not just do this?
Animal pig0 = new Pig();
What is the purpose of allowing this kind of thing, and what is really going on? Note that these examples assume that Pig is a subclass of Animal.
Thanks.
Because Pig extends Animal, any Animal reference is assignable to instances of Pig. This is known as polymorphism.
So why would you want to do this? If you write code that can deal with any Animal, then you can reuse that code for any extension of Animal, regardless of whether the Animals you are dealing with are Pigs or Goats or Elephants.
This also helps with the principle of information hiding - in many cases, you either don't need to know whether you're dealing with specific subclasses, or you don't want to know. Polymorphism lets you think about your code design at a more abstract level than if you had to deal with specific Animal extensions. Here's a quick example:
static abstract class Animal{
public abstract void speak();
}
static class Pig extends Animal{
#Override public void speak(){ System.out.println("oink"); }
}
static class Goat extends Animal{
#Override public void speak(){ System.out.println("baah"); }
}
static void converse(List<Animal> list){
for(Animal a : list) System.out.println(a.speak());
}
public static void main(String[] args){
List<Animal> l = new LinkedList<Animal>();
l.add(new Pig());
l.add(new Goat());
converse(l);
}
At the command line, you'll see the following printed:
oink
baah
Even though our List was defined as containing instances of type Animal, we get different behaviors each time we call a.speak() because the first item in the list was actually a Pig and the second was a Goat.
There's a second example of polymorphism in this code snippet - I'll leave that as an exercise to you to figure out where it is :)
A variable of type Animal will hold either null or else the identity of some object of type Animal or a derivative thereof. Unless the variable is declared final, it may be changed to identify a different Animal (or changed to null if it had previously identified an animal).
A statement Animal myPet = new Pig(); creates a variable called myPet which will initially identify a newly-constructed instance of Pig, but could be changed to identify any other object which derives from Animal (e.g. a Zebra). By contrast, had the statement been Pig myPet = new Pig();, then it would not be possible to change myPet to e.g. identify anything other than either a Pig or null.
It is most useful if the static type (the class name “on the left”) is an interface such as in
List<String> names = new ArrayList<String>();
If you later decide to use a LinkedList instead, you'll only need to go and edit one single line in your code. The two kinds of list can be used interchangeably except for performance where there are operations that are faster on one or the other.
If you take this a little further, you can move the new into a so-called factory method and hide the concrete class completely from the code that uses the interface.
Suppose I have two classes: Animal and Dog. Dog is a subclass of Animal. I do the following code:
Animal a = new Dog();
Now I can call methods of the Dog class through the a variable.
But my question is this: if I can call all of Animal's methods through the Dog objects (inheritance) than why should I use the polymorphism principle? I can just declare:
Dog d = new Dog();
With this declaration can use all of Animal's methods and Dog methods. So why use polymorphism? Thank you very much for your answer.
In Java, the concepts of polymorphism and inheritance are "welded together"; in general, it does not have to be that way:
Polymorphism lets you call methods of a class without knowing the exact type of the class
Inheritance lets derived classes share interfaces and code of their base classes
There are languages where inheritance is decoupled from polymorphism:
In C++ you can inherit a class without producing polymorphic behavior (i.e. do not mark functions in the base class with virtual)
In Objective C you can implement a method on an unrelated class, and call it from a place that knows only the signature of the method.
Going back to Java, the reason to use polymorphism is decoupling your code from the details of the implementation of its counter-parties: for example, if you can write a method Feed(Animal animal) that works for all sorts of animals, the method would remain applicable when you add more subclasses or implementations of the Animal. This is in contrast to a Feed(Dog dog) method, that would be tightly coupled to dogs.
As far as the
Dog d = new Dog();
declaration goes, there is no general reason to avoid this if you know that the rest of your method deals specifically with dogs. However, in many cases the later is not the case: for example, your class or your methods would often be insensitive to the exact implementation, for example
List<Integer> numbers = new ArrayList<Integer>();
In cases like that, you can replace new ArrayList<Integer>() with new LinkedList<Integer>(), and know that your code is going to compile. In contrast, had your numbers list been declared as ArrayList<Integer> numbers, such switchover may not have been a certainty.
This is called "programming to an interface". There is a very good answer on Stack Overflow explaining it.
You can have other implementations of the Animal class, such as Cat. Then you can say
Animal a = new Dog();
Animal b = new Cat();
You can call methods of the Animal class without caring which implementation it really is, and polymorphism will call the correct method. E.g.
a.speak(); // "Woof"
b.speak(); // "Meow"
Really, it's not "Polymorphism vs Inheritance" but "Polymorphism using Inheritance".
Polymorphism allows you to write a method that works for any Animal:
public void pet(Animal animal) {
...
}
This method would accept Dog, Cat, etc, including subclasses of Animal that are yet to be written.
If the method were to take Dog, it would not work for Cat etc.
If you are certain that it will always be a dog there is no reason for it. You might aswell use Dog d = new Dog(); as you described. But let's say you used a method instead of a constructor. The method returned an animal and you wouldn't know which implementation of animal you would get. You would still be able to use the same methods on the animal (even if it's a Dog, Elephant cat etc).
For extensibility purposes inheritance simplifies things. When you want to create an elephant or cat which also share some animal methods, You can easily get those by having animal as super class.
Normally the question you've asked is more similar to Inheritance vs Composition :) More "real life" example of why it's good to use polymorphism is for example usage of strategy design pattern. You can have many TaxPolicy implementation: UsaTaxPolicy, CanadaTaxPolicy, EuTaxPolicy, etc. If you have method calculateFinalPrice, which have to also calculate tax, then you inject the proper implementation and good calculation is executed, no matter you've passed Usa, Canada or Eu implementation.
inheritance is the dynamic polymorphism. I mean when you remove inheritance you can not override anymore.