String[] array = {"9", "1", "2", "a", "3", "8"};
        List<String> listArray = Arrays.asList(array);
//        for (int i = 0; i < listArray.size(); i++) {
//            System.out.println(listArray.get(i));
//        }

//      hashset无序 只能有一个null
        HashSet<String> hashSet = new HashSet<>(listArray);
        for (String hash : hashSet) {
            System.out.print(hash);
        }
        System.out.println(hashSet.add(null));
        System.out.println(hashSet.add(null));
        System.out.println("----------------------------------------------");

//      treeSet从小到大排序，不可重复，不能有null
        TreeSet<String> treeSet = new TreeSet<>(listArray);
        for (String s1 : treeSet) {
            System.out.print(s1);
        }
        System.out.println("----------------------------------------------");

//      保留原始顺序，只能有一个null
        LinkedHashSet<String> linkedHashSet = new LinkedHashSet<>(listArray);
        System.out.println("判断linkehashset添加null值");
        System.out.println(linkedHashSet.add(null));
        System.out.println(linkedHashSet.add(null));
        for (String s : linkedHashSet) {
            System.out.print(s);
        }