Groovy Converting List of objects to Comma Separated Strings

If you don't want to create a new list (which you say you don't want to do), you can use inject

currenciesList.inject( '' ) { s, v ->
    s + ( s ? ', ' : '' ) + v
}

Try currenciesList.code.join(", "). It will create list at background, but it's minimal code solution.

Also do you know, that your code may be even Groovier? Look at Canonical or TupleConstructor transformations.

//This transform adds you positional constructor.
@groovy.transform.Canonical 
class CurrencyType {
  int id
  String code
  String currency
}

def currenciesList = [
     new CurrencyType(1,"INR", "Indian Rupee"),
     new CurrencyType(1,"USD", "US Dollar"),
     new CurrencyType(1,"CAD", "Canadian Dollar")
]

//Spread operation is implicit, below statement is same as
//currenciesList*.code.join(/, /)
currenciesList.code.join(", ")