Super in Generics PROBLEM

[Mohit G Gupta]

Super in Generics by Malatesh Karabisti

  import java.util.ArrayList;  
import java.util.Iterator;  
import java.util.List;  
  
public class TestGeneric1 {  
   
 public static void main(String[] args) {   
 List<? super Horse1> list=new ArrayList<Horse1>();  
  
//here list variable can be assigned with any thing ie of   
//type List and typed for either Horse1 or super type of   
//Horse1 any thing lower then the Horse1 is not allowed  
//but the point to note here is, to this list we can add either Horse1 or   
//sub type Horse1 never never we can add Animal1( super of Horse1)  
  
list.add(new Horse1()); // ok no errror   
list.add(new Horse2()); // ok no error  
/*list.add(new Animal1()); //No we can't add the super of Horse1 
list.add(new Dog1());  // No we can't add the Dog1 which is sister of Horse1 */   
  
for (Animal1 animal1 : list) { System.out.println(); } // compiler error  
for (Horse1 horse1 : list) {System.out.println(); }  // compiler error  
  
}  
  
class Animal1 { }  
class Horse1  extends Animal1 { }  
class Horse2 extends Horse1 { }  
class Dog1  extends Animal1 { }  
  

i know that this question has been answered but still i am not convinced

  list.add(new Animal1());  

why does this line give compiler error ?
Animal1 is superclass of Horse1,so why cannot Animal1 be added.

similarly,why line 16,17 give complier error ?

i saw the final code by Malatesh Karabisti

 
import java.util.ArrayList;  
import java.util.Iterator;  
import java.util.List;  
  
public class TestGeneric1 {  
  
    /** 
     * @param args 
     */  
    public static void main(String[] args) {  
        // TODO Auto-generated method stub  
     List<? super Horse1> list=new ArrayList<Horse1>();  
     //here list variable can be assigned with any thing ie of   
     //type List and typed for either Horse1 or super type of   
     //Horse1 any thing lower then the Horse1 is not allowed  
     //but the point to note here is, to this list we can add either Horse1 or   
     //sub type Horse1 never never we can add Animal1( super of Horse1)  
       
     list.add(new Horse1()); // ok no errror   
     list.add(new Horse2()); // ok no error  
     /*list.add(new Animal1()); //No we can't add the super of Horse1 
     list.add(new Dog1());  // No we can't add the Dog1 which is sister of Horse1 */   
      
     for (Object animal1 : list) { System.out.println(); } // ok works fine  
    // for (Horse1 horse1 : list) {System.out.println(); }  // compiler error  
      
     Object Objtye=list;   
     // type cast is not required  
     List<Animal1> animalList=(List<Animal1>)list;   
     // This type cast is required otherwise it throws Unresolved compilation problem:  
  
       
     animalList.add(new Horse1()); // No Compiler error no runtime error  
     animalList.add(new Horse2()); // No Compiler error no runtime error  
     animalList.add(new Animal1()); // No Compiler error no runtime error  
     animalList.add(new Dog1());  // No Compiler error no runtime error  
      
      for (Animal1 ani : animalList)// compiles and runs fine  
     { System.out.println(ani); }  
  
     List<Object> animalObj=(List<Object>) list;  
     // This type cast is required otherwise it throws Unresolved compilation problem:  
     
     for (Object obj : animalList)//  // compiles and runs fine  
     { System.out.println(obj); }  
       
     List<Horse1> animalHorse1111=(List<Horse1>) list;  
    //This type cast is required otherwise it throws Unresolved compilation problem:  
       
     for (Horse1 horse1 : animalHorse1111)  
     {  System.out.println(horse1); }  
     // Since animalHorse1111 has Animal1 and Dog1 it throws Class cast exception  
       
     List<Horse2> animalHorse2222=(List<Horse2>) list;  
     //This type cast is required otherwise it throws Unresolved compilation problem:  
     List<Dog1>  animalDog1111=(List<Dog1>) list;  
     //This type cast is required otherwise it throws Unresolved compilation problem:  
     List<String> animalString=(List<String>) list;  
     //This type cast is required otherwise it throws Unresolved compilation problem:  
    }  
  
}  
  
class Animal1 { }  
class Horse1  extends Animal1 { }  
class Horse2 extends Horse1 { }  
class Dog1  extends Animal1 { }      

why is casting required in the below code ?

 List<Horse1> animalHorse1111=(List<Horse1>) list; 

since list is of <Horse1> generic type then why such casting
TAKE FOR eg:

List<Integer> a=new List<Integer>();

List<Integer> b=a;

here no casting is required,then WHY IN THE ABOVE CASE

[Mohit G Gupta]

CAN ANYONE ANSWER MY QUERY ??

PLEASE MODERATORS HELP ME

[Divyeshh Patel]

Regarding your first problem, ? super Horse1 allows you to add objects to the list but only the subtypes of Horse1(and of course, Horse1). You cannot add anything which is above Horse1 in the inheritance hierarchy. That is the reason for compilation error.
If you use List<? super Horse1> as a method argument, that would mean you can pass any List declared with Horse1(and any other class from the hierarchy between Object and Horse1) as the generic type. e.g. List<Object>, List<Animal>, List<Horse1>, etc
Hope this clears your first doubt.

Regarding the second doubt, you need to cast because though the actual list is of type List<Horse1>, it has been declared as List<? super Horse1> which is not same as the type of animalHorse1111 (which is List<Horse1>).

[Dieter Quickfend]

You assigned it to a List<? super Horse1>. The compiler has to be sure about generics at compile time. So when you assign something to a List<? super Horse1>, the compiler knows that your List could be any supertype of Horse1 (Meaning Horse1, Animal1 or Object). Now, you can add Horse1 or any of its subtypes (because they can all fit in a List<Horse1> or a List<Animal1> or a List<Object>, but you CANNOT add a supertype of Horse1, because the compiler isn't sure that the collection you passed can take a supertype of Horse1.

Imagine the following:

List <? super Horse1> horseList = new ArrayList<Horse1>(); //should compile fine
horseList.add(new Object()); //Do you REALLY want this to be possible?

and casting was required because he was using an Object reference before. You can cast that List referred to by an Object reference to a List of any type, which is dangerous at runtime, but part of the downsides of making sure non-generic code doesn't break.

[Jaikiran Pai]

mohitkumar gupta wrote:CAN ANYONE ANSWER MY QUERY ??

PLEASE MODERATORS HELP ME

Please read this http://faq.javaranch.com/java/KeepItDown

Also where did you get that question from? http://faq.javaranch.com/java/QuoteYourSources

[Mohit G Gupta]

KB book PG-619 GENERIC METHODS

Regarding your first problem, ? super Horse1 allows you to add objects to the list but only the subtypes of Horse1(and of course, Horse1). You cannot add anything which is above Horse1 in the inheritance hierarchy. That is the reason for compilation error.
If you use List<? super Horse1> as a method argument, that would mean you can pass any List declared with Horse1(and any other class from the hierarchy between Object and Horse1) as the generic type. e.g. List<Object>, List<Animal>, List<Horse1>, etc
Hope this clears your first doubt.

    public void addAnimal(List<? super Dog> animals) {
animals.add(new Dog()); // adding is sometimes OK with super
}
public static void main(String[] args) {
List<Animal> animals = new ArrayList<Animal>();
animals.add(new Dog());
animals.add(new Dog());
AnimalDoctorGeneric doc = new AnimalDoctorGeneric();
doc.addAnimal(animals); // passing an Animal List
}

in the above code,animals of type is passed to addAnimal method that is supertype of Dog.then why not in the First Problem i posted


----------------------------


2.why do line 22 ,21 give error ?

for (Animal1 animal1 : list) { System.out.println(); } // compiler error    
for (Horse1 horse1 : list) {System.out.println(); }  // compiler error   

list is of Horse Type,then why such error

-------------------------

3.why the casting is required in the line 42 but not in 28?

  List<Object> animalObj=(List<Object>) list;  //line 42 of second program  
Object Objtye=list; //line 28 of second program  

[Divyeshh Patel]

mohitkumar gupta wrote:
in the above code,animals of type is passed to addAnimal method that is supertype of Dog.then why not in the First Problem i posted

Passing the list as an argument and adding something to the list are two different things, aren't they?

mohitkumar gupta wrote:
2.why do line 22 ,21 give error ?

for (Animal1 animal1 : list) { System.out.println(); } // compiler error    
for (Horse1 horse1 : list) {System.out.println(); }  // compiler error   

list is of Horse Type,then why such error

No doubt, list is of type Horse, but has been declared as <? super Horse> so the only way to access it is (Object o:list)

mohitkumar gupta wrote:
3.why the casting is required in the line 42 but not in 28?

  List<Object> animalObj=(List<Object>) list;  //line 42 of second program  
Object Objtye=list; //line 28 of second program  

Since Object is the supertype of all objects, you can assign anything to Object without a cast which is not the case with line 42
e.g.

 
Integer i=1;
Object o=null;
o=i; //is valid, but
i=o; //is invalid without a cast
   

[Mohit G Gupta]

i compiles and ran the TestGeneric1 program and got the folllowing errors:
-------------------------------------------------------------------------------------------

C:\Documents\Jcreator FIles\TestGeneric1.java:55: inconvertible types
found : java.util.List<capture#955 of ? super Horse1>
required: java.util.List<Horse2>
List<Horse2> animalHorse2222=(List<Horse2>) list;
^
C:\Documents\Jcreator FIles\TestGeneric1.java:57: inconvertible types
found : java.util.List<capture#442 of ? super Horse1>
required: java.util.List<Dog1>
List<Dog1> animalDog1111=(List<Dog1>) list;

^
C:\Documents\Jcreator FIles\TestGeneric1.java:59: inconvertible types
found : java.util.List<capture#822 of ? super Horse1>
required: java.util.List<java.lang.String>
List<String> animalString=(List<String>) list;


-------------------------------------------------------------------------------------------


i have just inserted import java.util.*;

[Malatesh Karabisti]

mohitkumar,

Thanks for digging the question more.

i compiles and ran the TestGeneric1 program and got the folllowing errors:

The code compiles in eclipse IDE but not with command line compiler. just comment the code after 55
and check, for me it worked. since there is Run time error in line 52 the code won't reaches to line number
55 and hence eclipse could not spot the error in Runtime. I don't know why it failed to notice while typing(compile time)
and if you have eclipse IDE just check how it behaves.


Let me come to your question

why the casting is required in the line 42 but not in 28?

Line 28 Object Objtye=list;
Line 42 List<Horse1> animalHorse1111=(List<Horse1>) list;

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

public class TestGeneric1 {

    public static void main(String[] args) {

	 List<? super Horse1> list=new ArrayList<Horse1>();
	 List<? super Horse1> list1=new ArrayList<Object>(); // ok No error
	 List<? super Horse1> list2=new ArrayList<Animal1>(); // ok No error

	 list.add(new Horse1());
	 list.add(new Horse2());

	 list1=list; // ok without type cast is no problem since both are same type

    // [b] for questions of line 28[/b]
      Object obj=list;
     /* list : As we know for 200% sure list can't have any thing higher then Object this 
     is the oldest father of all the  classes, I think  one rule holds here is if you have "?" 
    in the reference variable (like list, list1,list2) then this reference can be assigned to only
     two things without casting
     1)Same type like list1=list;
     2)To Object like Object obj=list; */

	 // [b] for question of line 42[/b]
	 List<Horse1> animalHorse1111 = (List<Horse1>)list1;
     // just think about this list1  ( <Object> ) to assign animalHorse1111 (<Horse1>) without type cast



    }

}

class Animal1 { }
class Horse1  extends Animal1 { }
class Horse2 extends Horse1 { }
class Dog1  extends Animal1 { }    

This time I compiled with command line compiler it should work

// All Java Gurus there please correct me If I am wrong any where