construct model combobox contain given us retriev target manipul combobox clearabl user select combobox clear attribut model param properti chang event fire set select item programmat chang model param clearabl flag indic user select special clear valu valu combobox attribut shown combobox set make sure entri list object user opportun select clear attribut illeg argument except argument uml combo box model clearabl equal illeg argument except argument better didn need contain target constructor zero paramet soon improv target chang clearabl clearabl properti set 