Listlist = new ArrayList ();
ListlistAll = new ArrayList ();
list.add("jack");
list.add("tom");
listAll.addAll(list);
listAll.add("zhuzhu");
System.out.println(listAll.size());
listAll.removeAll(list);
System.out.println(listAll.size());
清空,不需要内容就清空。或者new一个新的