List、Collections

private static void addAll() {
        ArrayList<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        list.add(3);
        System.out.println("list1"+list);
        Collections.addAll(list, 4, 5, 6);
        System.out.println("list2"+list);
    }
//list1[1, 2, 3]
//list2[1, 2, 3, 4, 5, 6]

   /**
     * 取字符串第一个字符按降序排列
     */
    private static void collecyionsSort() {
        ArrayList<String> list = new ArrayList<>();
        Collections.addAll(list, "哈哈", "哭哭", "嘻嘻");
        Collections.sort(list, new Comparator<String>() {
            @Override
            public int compare(String o1, String o2) {
                return o2.charAt(0)-o1.charAt(0);
            }
        });
        System.out.println("list = " + list);
    }
// list = [嘻嘻, 哭哭, 哈哈]