Difference between "group" and "component" in QuickFIX/J

Components aren't really... things. They're like macros in the FIX DataDictionary (DD). Many messages need the same set of fields, so instead of specifying the same fields in every message, the DD defines a component that other messages can include.

A Group, on the other hand, is a very real thing. It's a repeating sequence of fields that will appear 0 or more times in a message.

QuickFIX's (QF) programming interface largely ignores components as a concept. You can't extract a component from a message because a component isn't a concept in QF; you just extract the fields like any other field.

A hypothetical example: The following two message definitions are exactly the same.

1. With a component

   <message name="Automobile" msgtype="X" msgcat="app">
     <field name="Wheel" required="Y"/>
     <field name="Bumper" required="Y"/>
     <component name="Dashboard" required="Y"/>
   </message>
   
   <component name="Dashboard">
     <field name="Radio" required="Y"/>
     <field name="AirConditioner" required="Y"/>
     <field name="Heater" required="Y"/>
   </component>
   

2. Without a component

   <message name="Automobile" msgtype="X" msgcat="app">
     <field name="Wheel" required="Y"/>
     <field name="Bumper" required="Y"/>
     <field name="Radio" required="Y"/>
     <field name="AirConditioner" required="Y"/>
     <field name="Heater" required="Y"/>
   </message>
   

See? A component is pretty much just a macro.

Either way it's defined, you just end up calling msg.GetHeater() (or whatever).