public class dataHere {
        public List<Accounts> accounts;
    }

    public class Accounts {
        public String accountNumber;
    }

public class handleHere {
        public List<String> keyList = new List<String>();
        dataHere getData = dataHere.Class;
        keyList.add('accountNumber');
        system.debug(getData.accounts[0].(keyList[0]))
    }