Covariant return type

“In object-oriented programming, a covariant return type of a method is one that can be replaced by a "narrower" type when the method is overridden in a subclass.”

Since JDK 5.0 it is possible to provide covariant return types in methods of subclasses.^[2] Before this release, the Java programming language was invariant with regard to method return types.

Unfortunately, covariance is not possible with method parameters. If you wish to use that, reference [1] has a good explanation of how to do this using Generics.

Example

	public abstract class Animal
	{
	public abstract Food eats();
	}
	public abstract class Food{}
	public class Grass extends Food
	{
	public Grass() {}
	@Override
	public String toString()
	{
	return "grass";
	}
	}
	public class Meat extends Food
	{
	public Meat() {}
	@Override
	public String toString()
	{
	return "meat";
	}
	}
	public class Cow extends Animal
	{
	@Override
	public Grass eats()
	{
	return new Grass();
	}
	@Override
	public String toString()
	{
	return "cow";
	}
	}
	public class Lion extends Animal
	{
	@Override
	public Meat eats()
	{
	return new Meat();
	}
	@Override
	public String toString()
	{
	return "lion";
	}
	}

view raw covariant.java hosted with ❤ by GitHub

Scala

In Scala all three are possible, contravariant, covariant and invariant for both method parameters as well as method return types. It is used fairly frequently.

For more information, the blog in [3] has some excellent explanation.

References

[1] Covariant Parameter Types

https://www.java-tips.org/java-se-tips-100019/24-java-lang/482-covariant-parameter-types.html

[2] Wikipedia - Covariant return type

http://en.wikipedia.org/wiki/Covariant_return_type

[3] Atlassian Blogs - Covariance and Contravariance in Scala

http://blogs.atlassian.com/2013/01/covariance-and-contravariance-in-scala/