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Covariant return types

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Join Date: Aug 2006

Posts: 16

Reputation: VinC is an unknown quantity at this point

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VinC

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Newbie Poster

Covariant return types

Nov 12th, 2006

I'm been working through my textbook and just can't seem to understand the section on covariant return types. ie) what are covariant return types, what they are used for, etc. The definition in the book is as follows.

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An overridden method in a derived class can return a type derived from the type returned by the base-class method.

Any explanation, link to a simple tutorial or example would be greatly appreciated!

The example from the text. What is the author trying to get across?

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
//: polymorphism/CovariantReturn.java
 
class Grain {
  public String toString() { return "Grain"; }
}
 
class Wheat extends Grain {
  public String toString() { return "Wheat"; }
}
 
class Mill {
  Grain process() { return new Grain(); }
}
 
class WheatMill extends Mill {
  Wheat process() { return new Wheat(); }
}
 
public class CovariantReturn {
  public static void main(String[] args) {
    Mill m = new Mill();
    Grain g = m.process();
    System.out.println(g);
    m = new WheatMill();
    g = m.process();
    System.out.println(g);
  }
} /* Output:
Grain
Wheat
*///:~
//: polymorphism/CovariantReturn.java

class Grain {
  public String toString() { return "Grain"; }
}

class Wheat extends Grain {
  public String toString() { return "Wheat"; }
}

class Mill {
  Grain process() { return new Grain(); }
}

class WheatMill extends Mill {
  Wheat process() { return new Wheat(); }
}

public class CovariantReturn {
  public static void main(String[] args) {
    Mill m = new Mill();
    Grain g = m.process();
    System.out.println(g);
    m = new WheatMill();
    g = m.process();
    System.out.println(g);
  }
} /* Output:
Grain
Wheat
*///:~

Last edited by VinC; Nov 12th, 2006 at 4:22 pm.

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Join Date: Oct 2006

Posts: 89

Reputation: TylerSBreton is an unknown quantity at this point

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TylerSBreton TylerSBreton is offline

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Junior Poster in Training

Re: Covariant return types

Nov 12th, 2006

All it looks like that's showing you is that an object of type wheat can be stored in a grain variable, since wheat extends grain, but the proper method will be called even though the variable is of type grain.