Java Type Casting Problem

Question

Possible Duplicates:
How to Cast a Vector<ObjectA> to Vector<ObjectB> in Java?
Is List<Dog> a subclass of List<Animal>? Why aren't Java's generics implicitly polymorphic?

I've a logical problem with the java type casting. I have a class A which extends class B and therefore I can have a statement like A a = new B(); but why I get a compiler error for Vector<A> va = new Vector<B>(); or Vector<A> va = (Vector<A>)new Vector<B>();

You want a wildcard, `Vector extends A> va`. Lots of SO questions will point to info, e.g. http://stackoverflow.com/questions/252055/java-generics-wildcards — Michael Brewer-Davis, May 26 '11 at 21:24
This is a common misconception with generics. Vector is NOT a subtype of Vector. Have a read through the [Generics Book] (http://www.amazon.com/Java-Generics-Collections-Maurice-Naftalin/dp/0596527756) — wolfcastle, May 26 '11 at 21:26

score 2 · Answer 1 · answered May 26 '11 at 21:20

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java generics don't support co-variance, so Vector<B> does NOT extend Vector<A>

answered May 26 '11 at 21:20

amit

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score 1 · Answer 2 · edited May 23 '17 at 12:26

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Think of it like this;

If C also extends B then you could put C into Vector<B>.

Now if you could cast it to a Vector<A> and you did a get you would get an instance of C!

That wouldn't make much sense so the compiler doesn't allow it. As @amit says Co-variance is the name for this.

See this posting

edited May 23 '17 at 12:26

Community

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answered May 26 '11 at 21:30

Jim

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score 0 · Answer 3 · answered May 26 '11 at 21:45

The type of the variable must match the type you pass to the actual object type.

When it comes to polymorphism with regards to generic type, java does not allow you to as follows:

class A { } 
class B extends A { } 
List<A> myList = new ArrayList<B>();

This is because on compilation, the compiler gets rid of generic typing (through something called type erasure) and does was it used to do in pre java 5 code. This is confusing because it is not the same behavior with arrays. With arrays you can do:

A[] myArr = new B[10];

But such is not the case with generics. Again, when it comes to generics the type of the variable must match the type you pass to the actual object type.

Hope that helps!

Java Type Casting Problem

3 Answers3