Why an ArrayList clear automatically?

Hi everybody. I have two ArrayList "corso_list" and "corsoitems_list"
that I use like temporary array. When I filled up them, I save them in
two ArrayList that is a property of the object "a" and, at the
beginning of the while I clear the temp array. When I do this
operation, even the array in the object a were cleared. Why? Below
there is the code used. Thanks to all.

ArrayList corso_list = new ArrayList();
ArrayList corsoitems_list = new ArrayList();
while(iter.hasNext())
{
    corso_list.clear();
    corsoitems_list.clear();
    a=new Assioma();
    i=(Individual)iter.next();
    l=(Literal) (i.getPropertyValue(model.getProperty(modello_conoscenza
+"nome_elemento_conoscenza")));
    a.setNome(l.getString());
    l=(Literal) (i.getPropertyValue(model.getProperty(modello_conoscenza
+"testo_assioma")));
    a.setTesto(l.getString());
    a.setTipo(((String)(lista_sottoclassi.get(j))));
    iter1 = i.listPropertyValues(model.getProperty(modello_conoscenza
+"sono_riferiti"));
    while (iter1.hasNext())
    {
        nome_corso = (String)(((Resource)(iter1.next())).getLocalName());
        corso_temp=model.getIndividual(modello_conoscenza+nome_corso);

l=(Literal)corso_temp.getPropertyValue(model.getProperty(modello_conoscenza
+"nome_corso"));
        corso_list.add(l.getString());
        corsoitems_list.add(new SelectItem(nome_corso,l.getString()));
    }
    if ( !corsoitems_list.isEmpty() )
        Collections.sort(corsoitems_list, new
ObjectComparator("getLabel",true));
    a.setSono_riferiti(corso_list);
    a.setSono_riferitiitems(corsoitems_list);
    temp_list.add(a);
}