// 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); }
}