design patterns - Can a java method return value depending upon condition? -

i have icoreclient interface , aclient , bclient classes implements this.

icoreclient exposed users.

i need add new method in icoreclient interface. so, needs implemented in both clients. can not make method generic has different signature similar functionalities.

i have 2 interfaces xx , yy

clienta implements xx , clientb implements yy

so, decided add new testmethod in icoreclient provide me instance of xx or yy depending upon clients.

i want return instance of these interfaces single method depending upon condition.

in clienta:

public xx testmethod(){   return instanceof xx; } 

in clientb:

public yy testmethod(){   return instanceof yy; } 

what should write in icoreclient interface?

public zz testmethod() 

i tried putting dummy interface zz (acting common supertype) both xx , yy implementing this. still not able expose methods of xx , yy in respective clients got typecasted in zz.

is there known approach kind of scenario?

edit: if make return type object, method of these interfaces not exposed. although, object contains instance of xx or yy,

user still needs cast (xx or yy how user know?) using methods in interface.. want expose methods of clientx without having cast clienta or clientb...

after edit looks may looking generics. can make interface this

interface icoreclient<t>{// t set each class implementing interface     t testmethod(); } 

and each of classes can like

class clienta implements icoreclient<xx>{     xx testmethod(){         //return xx     } }  class clientb implements icoreclient<yy>{     yy testmethod(){         //return yy     } }