Enum

 Enum使用经验：

1.Enum类型本身实现了序列化，可以随着对象放到memcache;

2.通常每个Enum需要有一个int类型的属性。DB中保存的字段可以是一个enum的值，也可以多个enum的与或的结果。比字符串的扩展性更好。

2.1 type的子类型的设计
注意，通常权限都是1, 2, 4, 8.....      二进制值11仍然可以表示为10的仅有的子类型。101,110,111可以表示100的三种子类型。

如果type的值存在子类型值，那么sql中就不适合通过type = someType 来获取包含子类型的所有记录
假如库中，type=1表示想去，type=2表示去过, type=1 ｜2表示从想去到运过的状态（但属于去过的一种子类型），如果想查询所有的去过的记录,type & 2 >0就是正确的办法。

2.2 在sql中使用 type & allType=someType与type=someType效果类似。

3.使用Map代替switch case 返回对应的枚举。一种好的方式是在enum中的static块中构造map。

 public static enum PostCategory {
        NO(0, "_post_category_no", ""), TRIP(1, "_post_category_lvyouluxian", "Lvyouluxian"), FOOD(
                2, "_post_category_meishi", "Meishi"), SHOPPING(3, "_post_category_gouwu",
                "Gouwu"), ENTAINMENT(4, "_post_category_yule", "Yule"), OTHER(5,
                "_post_category_qita", "Qita");

        private static final Map<Integer, PostCategory> mLookup = new HashMap<Integer, PostCategory>();

        static
        {
            for (PostCategory category : EnumSet.allOf(PostCategory.class))
            {
                mLookup.put(category.getFlag(), category);
            }
        }

        public static PostCategory get(int nCategoryId)
        {
            return mLookup.get(nCategoryId);
        }

        private int    m_nFlag;
        private String m_sCaption;
        private String m_sPinyin;

        private PostCategory(int nFlag, String sCaption, String sPinyin)
        {
            m_nFlag = nFlag;
            m_sCaption = sCaption;
            m_sPinyin = sPinyin;
        }

        public int getFlag()
        {
            return m_nFlag;
        }

        public String getCaption()
        {
            return m_sCaption;
        }

        public String getPinyin()
        {
            return m_sPinyin;
        }
    }

4.对于一些存在关联关系的enum，static块中也可以使用map去映射关系。

    public enum SubCategory {
        YAN_TU_JING_DIAN(PostCategory.TRIP, "subcategory6_tag", "subcategory_yantujingdian", 6), XIAN_LU_TE_SE(
                PostCategory.TRIP, "subcategory7_tag", "subcategory_xianlutese", 7), TUI_JIAN_CAN_GUAN(
                PostCategory.FOOD, "subcategory8_tag", "subcategory_tuijiancanguan", 8), DANG_DI_MEI_SHI(
                PostCategory.FOOD, "subcategory9_tag", "subcategory_dangdimeishi", 9), GOU_WU_CHANG_SU(
                PostCategory.SHOPPING, "subcategory10_tag", "subcategory_gouwuchangsu", 10), SU_GOU_SHANG_PIN(
                PostCategory.SHOPPING, "subcategory11_tag", "subcategory_sugoushangpin", 11), YU_LE_CHANG_SUO(
                PostCategory.ENTAINMENT, "subcategory12_tag", "subcategory_yulechangsuo", 12);

        private static Map<PostCategory, List<SubCategory>> CATEGORY_MAP = new HashMap<DaoDaoTourismMetadata.PostCategory, List<SubCategory>>();
        private static List<String>                         NAME_SET     = new ArrayList<String>();
        static
        {

            for (SubCategory category : EnumSet.allOf(SubCategory.class))
            {
                {
                    if (CATEGORY_MAP.get(category.getParentCategory()) == null)
                    {
                        List<SubCategory> scl = new ArrayList<SubCategory>();
                        scl.add(category);
                        CATEGORY_MAP.put(category.getParentCategory(), scl);
                    }
                    else
                    {
                        CATEGORY_MAP.get(category.getParentCategory()).add(category);
                    }
                    NAME_SET.add(category.getName());
                }
            }

        }
        private PostCategory                                parentCategory;
        private String                                      formField;
        private String                                      key;
        private int                                         id;

        private SubCategory(PostCategory parent, String field, String name, int id)
        {
            this.parentCategory = parent;
            this.formField = field;
            this.key = name;
            this.id = id;
        }

        public String getFieldName()
        {
            return this.formField;
        }

        public String getName()
        {
            return UserInterfaceUtil.getInstance().localize(this.key);
        }

        public int getId()
        {
            return this.id;
        }

        public PostCategory getParentCategory()
        {
            return this.parentCategory;
        }

        public static List<SubCategory> getSubCategoryBy(PostCategory category)
        {
            return CATEGORY_MAP.get(category);
        }

        public static List<String> getNameSet()
        {
            return NAME_SET;
        }

    }

 

 

5.注意Enum类的equal方法是不希望用户去复盖，使用了final，默认是比较引用。