inheritance - Is there a good trick for instantiating a subclass with an instance of its superclass? -

preferably in java, there way instantiate subclass instance of superclass?

say have classb subclasses classa, here constructor classa:

 public classa(){     } 

i want like:

public class classb extends classa{       public classb(classa aclassa){         super(aclassa);      } } 

is there way instance of subclass instance of superclass, when subclass might have 1 field superclass, , therefore there isn't difference between them?

for instance,

classa = new classa();

classb b = a;

(the compiler going want me this: classb b = (classb)a; ... not going work.)

how do this?

is there way instance of subclass instance of superclass

no , superclass instance doesn't contain subclass instance . trying cast object referenced superclass type reference variable object of subclass.

classb b = (classb)a; 

it dangerous , may fail if a didn't refer instance of classb @ run time.

remember instances of classb instanceof classa , reverse not true.

i not sure trying achieve , may @ abstract factory pattern , factory method pattern , see if helps.