How to select a base class at runtime depending upon an instance variable of parent class in Java?

Problem: With the following classes, I am trying to select a base class at runtime depending upon an instance variable of parent class in Java. I believe there should be some fundamental property in OOPS which can help me achieve it which I am just missing.

class Transaction{
   int id;
   enum transactionType;
}

class ATypeTransaction extends Transaction{
   /* bunch of instance variable specific to ATypeTransaction */
}

class BTypeTransaction extends Transaction{
   /* bunch of instance variable specific to BTypeTransaction */
}

My Approach: I am trying to use something like this (static polymorphism within switch-case). The problem with this approach is I am still not able to get hold of child class. Rather I am getting parent class which stops me from accessing child class attributes.

public Transaction selectTransaction(TransactionType transactionType){
        Transaction transaction;
        switch(transactionType) {
            case = "AType":
                transaction= new ATypeTransaction();
                break;
            case = "BType":
                transaction= new BTypeTransaction();
                break;
        }
   return transaction;
}

Please help me out with this problem and provide a good design to solve this. Thanks in advance.