    // MyStringField.java

    import quickfix.StringField;

    public class MyStringField extends StringField
    {
      public MyStringField() { super(6123); }
      public MyStringField(String data) { super(6123, data); }
    }

    // MyPriceField.java

    import quickfix.DoubleField;

    public class MyPriceField extends DoubleField
    {
      public MyPriceField() { super(8756); }
      public MyPriceField(double data) { super(8756, data); }
    }