java - Convert sub-type to super-type -

i have abstract super class a, , subclass b extends class.

public abstract class {  }  public class b extends {   } 

i have method returns type a, object b

public fetchtype() {     a = new b();     return a;  } 

when call fetchtype method, want object a. in case, might have cast b a. can please advise how do that?

you cannot create object if type abstract.

here class a abstract class, means never able create object a.

this not allowed: a = new a();