Stack Overflow Asked by Stefan on December 29, 2020
As this is not so famous concept, I will make a little intro.
Covariant return type of a method is one that can be replaced by a
"narrower" type when the method is overridden in a subclass.
So I can compile just fine this little program (as String is child of Object):
public class House {
Object someMethod(){
return null;
}
}
class DogHouse extends House{
@Override
String someMethod() {
return null;
}
}
The rule is easy enough to remember, but I don’t understand it. My question is this:
Why can the return type in someMethod in DogHouse only be the same or child of return type in someMethod in class House? I hope the question is quite clear.
Or..(for example) why this code wouldn’t compile if I had put return type in someMethod in class House Integer
for example? (String is not a child of Integer)
What is happening ‘behind the scenes’ so I can understand it?
The way to understand this is to think of the subclass as a specific type of the parent class. This means it still needs to adhere to the behavior defined by the parent. The parent class defines a someMethod
method that returns an Object
. Subclasses can't break this behavior, but they can further specify it - a DogHouse
's someMethod
still returns an Object
, it just happens to be a String
.
Correct answer by Mureinik on December 29, 2020
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