The following script: def a = [ 'a' , 'b' , 'c' , 'd' ] as Set def c = a.collect { it + it } println c.getClass ( ).getName ( ) + ": " + c results in: Caught: java.lang.ClassCastException: java.util.ArrayList at kolesa.run(kolesa.groovy:8) at kolesa.main(kolesa.groovy)