SparseBooleanArray.get returns a boolean, but I believe you need to check it for each position in your list, e.g.
int len = listView.getCount();
SparseBooleanArray checked = listView.getCheckedItemPositions();
for (int i = 0; i < len; i++)
if (checked.get(i)) {
String item = cont_list.get(i);
/* do whatever you want with the checked item */
}