java - usage of generics as return type -

i've structure this:

abstract class mydomain{...} abstract class foodomain extends mydomain{...} abstract class bardomain extends mydomain{...} class firstconcretebardomain extends bardomain{...} class secondconcretebardomain extends bardomain{...} 

i need factory creates mydomain objects. first attempt this:

public interface ispecializedobjectsfactory {     public <t extends mydomain> t create(class<?> clazz); } 

implementend as:

public class firstspecializedobjectsfactory implements ispecializedobjectsfactory {      @override     public <t extends mydomain> t create(class<?> clazz) {         if(clazz.equals(bardomain.class))             return new firstbardomain();         throw new invalidparameterexception();     } 

same secondbardomain.

first question: why generating error says cannot cast firstbardomain t?

after error i've introduced cast: return (t) new firstbardomain();.

the problem cast unsafe , want confident result, i've introduced constraint (assuming each mydomain object have 2 levels of derivation):

public <t extends anagrafedomain, s extends t> s create(class<t> clazz) 

second question: assuming factory entry point mydomain objects created, , calls factory never use concrete classes (but like: bardomain subj = specializedobjectsfactory.getfactory().create(bardomain.class);), question is: new version safe?

the reason cast unsafe because of particular line:

public <t extends mydomain> t create(class<?> clazz) { 

this infers return type call site; in other words, consider following class:

public abstract class myfakedomain extends mydomain { } 

the following code compile, fail @ runtime:

ispecializedobjectsfactory factory = new firstspecializedobjectsfactory(); myfakedomain broken = factory.create(bardomain.class); 

this throw classcastexception because of type inference; inferred type myfakedomain, resulting in attempt cast firstbardomain myfakedomain, illegal cast - hence unsafe warning.

the type inference reason why cast must present; whilst firstbardomain subclass of mydomain, not know if of type t, t any mydomain subclass, not firstbardomain.

however, if caller careful, code work fine - whether consider acceptable or not you.

this gives answer second question: using bardomain type inferred not safe, subclass of mydomain. type safe here mydomain - however, if planning on using mydomain type, might remove generic type bound , make return type mydomain.