Since
2008
Since
2008
Since
2008
Object oriented programming (OOP) has a principle named substitutability. In this tutorial, let us learn about substitutability and support for covariant return type in Java. Covariant return type uses the substitutability principle.
Substitutability was introduced by eminent Barbara Liskov and Jeannette Wing. It is also called as Liskov substitution principle.
Let T be a super type and S be its subtype (parent and child class). Then, instances (objects) of T can be substituted with instances of S. Parent’s instances can be replaced with the child’s instances without change in behavior of the program.
Let WildAnimal be a supertype and Lion be a subtype, then an instance obj1 of WildAnimal can be replaced by an insance obj2 of Lion
.
The subtyping principle which we discussed above as Liskov principle is called covariant. The reverse of it (instead of child replacing the parent, the reverse of it as parent replacing the child) is called contravariant. If no subtyping is allowed then, it is called invariant.
From the release of JDK 1.5, covariant types were introduced in Java. Following example source code illustrates the covariant types in java. In the below example, method overriding is used to demonstrate the covariant type.
In class Zoo, the method getWildAnimal returns ‘WildAnimal’ which is a super type. AfricaZoo extends Zoo and overrides the method getWildAnimal. While overriding, the return type of this method is changed from WildAnimal to Lion. This demonstrates covariant type / Liskov substitution principle. We are replacing the supertype’s (WildAnimal) instance with subtype’s (Lion) instance. This was not possible before JDK 1.5 IndiaZoo is just another example which demonstrates the same covariant type.
class WildAnimal {
public String willYouBite(){
return "Yes";
}
}
class Lion extends WildAnimal {
public String whoAreYou() {
return "Lion";
}
}
class BengalTiger extends WildAnimal {
public String whoAreYou() {
return "Bengal Tiger";
}
}
class Zoo {
WildAnimal getWildAnimal() {
return new WildAnimal();
}
}
class AfricaZoo extends Zoo {
@Override
Lion getWildAnimal() {
return new Lion();
}
}
class IndiaZoo extends Zoo {
@Override
BengalTiger getWildAnimal() {
return new BengalTiger();
}
}
public class Covariant {
public static void main(String args[]){
AfricaZoo afZoo = new AfricaZoo();
System.out.println(afZoo.getWildAnimal().whoAreYou());
IndiaZoo inZoo = new IndiaZoo();
System.out.println(inZoo.getWildAnimal().whoAreYou());
}
}
Comments are closed for "Covariant Return Type in Java".
Sorry, with Java 7 above example desn’t compile without cast
AfricaZoo afZoo = new AfricaZoo();
System.out.println((afZoo.getWildAnimal()).getClass());
System.out.println(((Lion) afZoo.getWildAnimal()).whoAreYou());
AfricaZoo afZoo = new AfricaZoo();
System.out.println((afZoo.getWildAnimal()).getClass());
// System.out.println(((Lion) afZoo.getWildAnimal()).whoAreYou());
Lion lion = (Lion) afZoo.getWildAnimal();
System.out.println(lion.whoAreYou());
Janusz,
No issues, this compiles without any error in JDK 1.7.0_25
Why do we need a cast here?
How annotation works in Java? @Override is an interface how its checks method is overriden propery or not by subclasses?
How to de-serilize array of objects? Say I have stored array of employees into a flat file.
Good example for Covariant.
Kindly give any short example for Contravariant & invariant.
thanks
Is it possible to you to use contravariant subtyping in java and if yes can you please give some example on it.
Thanks,
Sameer.
Siddu,
Please refer my tutorial on Java annotations https://javapapers.com/core-java/java-annotations/
Siddu,
De-serializing an array is no different.
To know about serialization please refer https://javapapers.com/core-java/java-serialization/
Invariant means no support for subtyping. Java does not support contravariant.
There is no support for contravariant in Java as of now.
Nice and simple as always.. Thank you Joe
Nice Example. I never thought about this before.
Everything OK
The same code copied and pasted on my personal laptop with true and real Java 7 works fine. Some problems with my remote account in lab temporarily disabled/changed/maintained/whatever the day before beginning of the new academic year
What about clear and simple invarian exaple?
Good to see you back, hope you are doing well, missed your articles a lot.
Thanks for posting this article but I don’t get the idea of this article , it’s very simple
Great Thanks a lot now i have clear picture.
Really nice to see you after long time.
Janusz, declaring as final also the example of invariant.
Hey Joe,
I have one request once you done with OOP concept.
We like to read your experience with Threads/multithreading.
Thanks, I got it.
I understood about annotations. How @Override checks whether method is being overidden properly or not. I mean will it supply any information at runtime.
Welcome Pushkar.
I have been working on few things that kept me busy. Those few things are going to be good for javapapers.
Going forward, you will see lot more articles and interaction than usual.
Welcome Vinuraj.
Thanks Pratik, I am doing well. I was focusing on some changes in my personal and career front. Going forward, I can dedicate more time on javapapers :-)
Yes, this is a simple article. This gives an introduction for a feature in Java 5. Since the audience is in large numbers, I wish to write on mixed depth stuff. Some I want to be in-depth and high complex, some to be of very fundamental and introductory. This is to serve all type of readers.
So the message is, you have to be selective, but do not go away since there might be some articles coming which will be of interest to you.
Thanks Pratik.
Yes Pushkar.
Threads are pending for a long time and I am really eager to write on it. Sure I will start immediately on Threads.
Thanks , will be looking forward for nice articles like all previous ones.
Hi Joe,
I have read your every post. Its awsome..!
And today this Gandhiji’s shadow also looking great on javapaper logo. Keep it up and thanks for contribution for java community.
Thanks Dhaval.
It is our tribute to the Mahatma.
Very nice explanation :)
Hey Joe, That’s an excellent guide on covariant return type. Thanks a lot for sharing.
[…] polymorphism. This is covariant type and the reverse of it is contravariant. We have discussed the substitution principle and covariant types, contravariant and invariant earlier in the linked tutorial. This is demonstrated with a code […]
Hi Joe,
looking forward to some articles on multithreading.