With quickfixj 1.6.0 when I do :

quickfix.fix44.NewOrderSingle o = new quickfix.fix44.NewOrderSingle();

I get the following error :

Exception in thread "main" java.lang.VerifyError: Bad type on operand stack
Exception Details:
Location:
quickfix/fix44/NewOrderSingle.get(Lquickfix/field/SettlType;)Lquickfix/field/SettlType; @2: invokevirtual
Reason:
Type 'quickfix/field/SettlType' (current frame, stack[1]) is not assignable to 'quickfix/CharField'
Current Frame:
bci: @2
flags: { }
locals:

Unknown macro: { 'quickfix/fix44/NewOrderSingle', 'quickfix/field/SettlType' }

stack:

Bytecode:
0000000: 2a2b b600 3057 2bb0

I foundd that there is two quickfix.field.SettlType class :
One in quickfixj-core which extends StringField
One in quickfixj-messages-fix44 which extends CharField