Generics Wildcards: Assigning Super class object to a Subclass object list

Question

Question_1

class A {}
class B extends A {}

A aObj = new A();
B bObj = new B();

We know that below will result in class cast exception at runtime .

List<B> list_B = new ArrayList<B>();
list_B.add((B)aObj); //ClassCast exception

But

List<? extends A> list_4__A_AND_SubClass_A = new ArrayList<B>();
List<A> list_A = (ArrayList<A>)list_4__A_AND_SubClass_A;
list_A.add(aObj); // No exception here, Why?

Question_2

In a method like below

void method(List<? extends A> list_4__A_AND_SubClass_A){
   //How do i find the passed list is of Type A or B ??
   // To decide List<A> list_A = (ArrayList<A>)list_4__A_AND_SubClass_A;
   //        OR List<B> list_B = (ArrayList<B>)list_4__A_AND_SubClass_A;
}

UPDATE

Q_1

Better I should avoid referencing the collection like below for modification; Unless We are very sure about the passed list is type of A/B; to maintain the integrity on that list.

 List<A> list_A = (ArrayList<A>)list_4__A_AND_SubClass_A;

Q_2

I think we can pass one more argument from the caller method to the calling method to maintain the integrity over the list while modification at calling method.

void method(List<? extends A> list_4__A_AND_SubClass_A, Class<? extends A> obj){
     if (A.class.isAssignableFrom(obj)) {
         List<A> list_A = (List<A>)list_4__A_AND_SubClass_A;
            list_A.add(new A());
            list_A.add(new B());
     } else if (B.class.isAssignableFrom(obj)){
         List<B> list_B = (List<B>)list_4__A_AND_SubClass_A;
            //list_B.add(new A()); //only we can add B
            list_B.add(new B());
     }
}

score 2 · Answer 1 · answered Mar 06 '13 at 19:14

2

Ideally, the cast from List<B> to List<A> should throw exception at runtime; unfortunately Java can't decide that at runtime. Fortunately, at compile time you get a warning for that cast.

answered Mar 06 '13 at 19:14

irreputable

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I got the warning like "Type safety: Unchecked cast from List to ArrayList" for line "List list_A = (ArrayList )list_4__A_AND_SubClass_A;" – Kanagavelu Sugumar Mar 06 '13 at 19:27
Could you please answer the Q2 also?? – Kanagavelu Sugumar Mar 06 '13 at 19:30

score 1 · Accepted Answer · answered Mar 06 '13 at 19:42

1

This line:

List<A> list_A = (ArrayList<A>)list_4__A_AND_SubClass_A;

Produces the 'unchecked' warning, because you may ( and indeed are ) casting a list that contains only Bs to a list that can contain Bs or As (Remember, because B is a subclass of A, it IS A), so the next line

list_A.add(aObj);

is perfectly OK, because you did not pay attention to a warning.

In general, with generics, List< ? extends A> is not always the same as List<A>.

To answer question 2.
Unfortunately, all generics are checked at compile time and not much information is available at runtime, so at runtime, you only have a list of objects, and cannot effectively test what kind of list that is. This "feature" was a trade off between backward compatibility and generics when they were introduced in Java 1.5.

answered Mar 06 '13 at 19:42

Alexander Pogrebnyak

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+1 Thanks! Please let me know if you have any comments over my updates in the question. – Kanagavelu Sugumar Mar 06 '13 at 20:43
@Kanagavelu Sugumar. You are breaking data integrity in 'Q_1', as before your cast, the list could only have elements of type `B` and after the cast it can have elements of both `A` and `B`, it may not be a big deal in the beginning, when everything is contained in 1 method, but what if the list is a parameter to your method, you really cannot do this cast then. As for Q_2, it may work, but it sure does not look right. Ask yourself, do you really need inheritance? Read this -> http://stackoverflow.com/questions/49002/prefer-composition-over-inheritance – Alexander Pogrebnyak Mar 06 '13 at 23:09

score 0 · Answer 3 · answered Mar 06 '13 at 19:19

You are here List<? extends A> defining wildcard type which means that list accepts anything that is A or a derived from A. Therefore it will not throw an exception
You can't detect here the concrete inner type of the list You have to pass that directly, i.e. your method signature must be as follows

void method(List list)

Generics Wildcards: Assigning Super class object to a Subclass object list

3 Answers3

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