A a = aa.getObject();
if (a instanceof B) {
   B b = (B) a;
   b.do();
}
// ...

public abstract class A {
   public abstract void do();
}

public class B extends A {
   public void do() {
       System.out.println("In B");
   }
}

public class Test {
    public static void main(String[] args) {
        A obj = returnA();

        obj.do();  // Will invoke class B's do() method
    }

    /** Method returning BaseClass A's reference pointing to subclass instance **/
    public static A returnA() {
         A obj = new B();
         return obj;
    }
}

if (obj instanceof B) {
    B obj1 = (B) obj;

}

public class E {

public static void doMethod(A a) {
    Class<?> class1 = a.getClass();
    Method method;
    try {
        method = class1.getDeclaredMethod("doMethod", null);// B, C, D has doMethod
        method.invoke(a, null);
        // I know to many exceptions
    } catch (SecurityException e) {
        e.printStackTrace();
    } catch (NoSuchMethodException e) {
        e.printStackTrace();
    } catch (IllegalArgumentException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (InvocationTargetException e) {
        e.printStackTrace();
    }

}
}

A a = new B();

public interface Doer {
    public void do();
}

public class B extends A implements Doer {
    //implement do method
}

//.. same for other subclass