Using "super" in Generics

[Malatesh Karabisti]

class GenAnimal { }
class Horse extends GenAnimal { }

public class TestGeneric {

    public static void main(String[] args) {
        List<GenAnimal> arr=new ArrayList<GenAnimal>();
        arr.add(new Horse());
        arr.add(new Horse());
        arr.add(new GenAnimal());
        add(arr);
    }
    private static void add(List<? super Horse> arr) {
        arr.add(new Horse()); // this ok no error
        arr.add(new GenAnimal()); // error  here 
    }
}

compilation error
The method add(capture-of ? super Horse) in the type List<capture-of ? super Horse> is not applicable for the arguments (GenAnimal)

I have learned that method add(List<? super Horse> arr) can add any thing that is super type of Horse, ok. I am adding "GenAnimal" which is super type of Horse, what made compiler unhappy

can any body please expain ?

[Harpreet Singh janda]

This means that you can pass a generic list of type Horse or super class of Horse. This does not mean that you can add super class object to this list.

[Minhaj Mehmood]

by saying this

List<? super Horse>

you mean the list can accept horse type of object or a object of which super type is horse, take a look on following code.

import java.util.List;
import java.util.ArrayList;

class GenAnimal { }
class Horse extends GenAnimal { }
class WhiteColorHorse extends Horse { }

public class TestGeneric {

	public static void main(String[] args) {
		List<GenAnimal> arr = new ArrayList<GenAnimal>();
		arr.add(new Horse());
		arr.add(new Horse());
		arr.add(new GenAnimal());
		add(arr);

	}

	private static void add(List<? super Horse> arr) {
		arr.add(new Horse()); //the horse type of object are acceptable
		arr.add(new WhiteColorHorse()); // the object which super type is Horse are also acceptable.
		arr.add(new GenAnimal()); 
		// compile-time-error. Only the type of horse or the objects of which super type is Horse are only acceptable.
	}

}

Hope this helps.

Minhaj

[Edit: spelling mistakes correct and added following]
Furthermore, before posting any question here please take a look on HowToAskQuestionsOnJavaRanch guide link.

[Shanky Sohar]

import java.util.*;
class GenAnimal { } 
class Horse extends GenAnimal { } 
class Dog extends Horse{}
public class TestGeneric { 

public static void main(String[] args) { 
List<GenAnimal> arr=new ArrayList<GenAnimal>(); 
arr.add(new Horse()); 
arr.add(new Horse()); 
arr.add(new GenAnimal()); 
add(arr); 

} 

private static void add(List<? super Horse> arr) { 
arr.add(new Horse()); // this ok no error 
arr.add(new Dog()); // see now compiler is ok
} 

point out the difference between the two you will understand

[Malatesh Karabisti]

From K & B Book

public void addAnimal(List<? super Dog> animals)
is essentially, "Hey compiler, please accept any List with a generic type that is of type Dog, or a supertype of Dog.
Nothing lower in the inheritance tree can come in, but anything higher than Dog is OK."

I think my source of confusion is these words still these words are not very clear to me ?

[Shanky Sohar]

Malatesh Karabisti wrote:From K & B Book

public void addAnimal(List<? super Dog> animals)
is essentially, "Hey compiler, please accept any List with a generic type that is of type Dog, or a supertype of Dog.
Nothing lower in the inheritance tree can come in, but anything higher than Dog is OK."

I think my source of confusion is these words still these words not very clear to me ?

its confusing.......because it doesnot work in that way.......can anybody comment

[Shanky Sohar]

public void addAnimal(List<? super Dog> animals)

this means that any subtype of dog can be accepted

like
Class whitecolor exends Dog{}
class Blackcolor extends Dog{}
class Brown extends Dog....

all three are accepted.......
anything which is having supertype as Dog is accepted......


can anybody comment if i am wrong

[Minhaj Mehmood]

Malatesh Karabisti wrote:From K & B Book
"Hey compiler, please accept any List with a generic type that is of type Dog, or a super type of Dog.
Nothing lower in the inheritance tree can come in, but anything higher than Dog is OK."

Yes, in other words its written: please accept any list with a generic type that is of type Dog or a list which super type is dog (the child of dog).

[Malatesh Karabisti]

From K & B Book
"Hey compiler, please accept any List with a generic type that is of type Dog, or a super type of Dog.
Nothing lower in the inheritance tree can come in, but anything higher than Dog is OK."


ok logically these words are correct with respect to method signature but not with add() method of the list. In fact this becomes inverse with add method since it can take only objects sub type or class which has been mentioned in right hand side of the super keyword(Incase any thing wrong please correct me).

class GenAnimal { }
class Horse extends GenAnimal { }
class WhiteColorHorse extends Horse { }

public class TestGeneric {
public static void main(String[] args) {
List<WhiteColorHorse> arr=new ArrayList<WhiteColorHorse>();
add(arr); // #1 error
}

private static void add(List<? super Horse> arr) {
arr.add(new Horse()); // this ok no error
arr.add(new WhiteColorHorse()); // this ok no error
arr.add(new GenAnimal()); //#2 error
}

}

#1 error The method add(List<? super Horse>) in the type TestGeneric is not applicable for the arguments (List<WhiteColorHorse>)

#2 error The method add(capture-of ? super Horse) in the type List<capture-of ? super Horse> is not applicable for the arguments (GenAnimal)

[Venu Chakravorty]

Malatesh Karabisti wrote:From K & B Book

public void addAnimal(List<? super Dog> animals)
is essentially, "Hey compiler, please accept any List with a generic type that is of type Dog, or a supertype of Dog.
Nothing lower in the inheritance tree can come in, but anything higher than Dog is OK."

I think my source of confusion is these words still these words are not very clear to me ?

Of course the method can accept any supertype of Dog or Dog itself. We get the error when we are adding elements to the list (List).

Regards,
venu.

[Shanky Sohar]

Venu Chakravorty wrote:

Of course the method can accept any supertype of Dog or Dog itself. We get the error when we are adding elements to the list

can you please tell this? how.....

[Venu Chakravorty]

consider a case of using <? extends Animal>
here the method can accept anything that is an Animal and you can't use the add() method in this case.

consider a case of using <? super Animal>
here the method can accept anything that is a supertype of Animal. That might include Vertibrates, LivingThings or even Object. So the method can accept all these and you can add elements which are (IS-A) Animal(s).

Please correct me if I am wrong.

regards,
venu.

[Ankit Garg]

Malatesh please Use Code Tags when you post a source code. That way your code looks formatted. Unformatted code is hard to read. You can add code tags by wrapping your code in [code] [/code] tags. You can edit your message using button and then add code tags to it...

[Dieter Quickfend]

there is a special rule that allows such code under very specific circumstances in
which the code can be proven to be safe. This rule, known as wildcard capture,
allows the compiler to infer the unknown type of a wildcard as a type argument
to a generic method.

http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf Chapter 9, Page 20


If you make the list a List<GenAnimal> and you pass it with <? super GenAnimal>, it will work.

But you can only add subtypes of the lowest type, so the compiler is sure that everything you're adding is safe.

 
   class GenAnimal { }
   class Horse extends GenAnimal { }
   class WhiteColorHorse extends Horse { }
   public class TestGeneric {
       public static void main(String[] args) {
           List<GenAnimal> arr = new ArrayList<GenAnimal>();
           arr.add(new Horse());
           arr.add(new Horse());
           arr.add(new GenAnimal());
           add(arr);
       }
       private static void add(List<? super GenAnimal> arr) {
           //The Lowest list you can add is GenAnimal, so GenAnimal
           //and all its subtypes will be safe
           arr.add(new Horse());
           arr.add(new WhiteColorHorse());
           arr.add(new GenAnimal());   
       }
   }