在知道的情况下将 Arraylist 存储在 Room 中的最佳方法是什么
那我要不是只有一个Arraylist来了？
可以说，没有办法是最好的办法。
ArrayList 不只是出现，数据和元素结构（如果每个元素存在多个值）来自某个地方，即它们只是容器，而不是分组数据的持久容器。由于最终结果似乎是持久的结构化数据集，因此主要利用数据库可能会更简单。
我正在尝试为 Cardgame 构建一个 Deckbuilder。不同的甲板
是基于我想在本地安全的一个 Arraylists。
这听起来像是卡片的基础，即那些可用于游戏的卡片。所以听起来您想要在数据库中为 cards 提供一个表。
这意味着你可以创建你的套牌，而不是我希望你能够
保护好它，然后创建一个新牌组，
听起来您想要在数据库中为 套牌 提供一个表，并且套牌可以有一张卡片列表。那么有多少卡呢？ 10、20、3000？好吧，如果您利用 Realtional Database Manager（其中 SQLite 和 Room（因为后者是 SQLite 上的抽象层））的关系功能。因此，很可能是一个被称为映射（同一事物的引用、关系和其他名称）的表。
这主要存储由可以识别关系的一部分的列和可以识别另一部分的另一列组成的关系。将此应用于您的情况，DECK 将与多张卡片有关系，一张卡片可能会出现在许多 DECK 中。这是映射表满足的多对多关系。所以你可能需要一个映射表。
为了进一步解释，假设我们谈论的是纸牌（黑桃 A、红心 Q 等）。
所以我们需要三个表，一个 Card 表、一个 Deck 表和一个将卡牌映射到牌组的表（反之亦然）一个 Card_Deck_Map 表格。
Card 表为简单起见，将有一列用于 cardname。
Deck 表将有一列用于 deckname
为了高效起见，将使用作为特殊列 rowid 别名的标识符进行映射。因此，上面的表格每个都有一个额外的列，称为 _id（将列命名为 _id，这对 Android 可能是有益的）。
假设您不希望卡片名称或套牌名称重复，因此将应用 UNIQUE 约束，不允许名称重复。
简而言之，这是在 SQL 中的样子（这最终是大多数数据存储、操作和提取完成的方式）：-
-- Delete the tables if they exist (just in case)
DROP TABLE IF EXISTS card_deck_map;
DROP TABLE IF EXISTS card;
DROP TABLE IF EXISTS deck;
-- Create the tables
CREATE TABLE IF NOT EXISTS card (_id INTEGER PRIMARY KEY, cardname UNIQUE);
CREATE TABLE IF NOT EXISTS deck (_id INTEGER PRIMARY KEY, deckname UNIQUE);
CREATE TABLE IF NOT EXISTS card_deck_map (
card_reference INTEGER REFERENCES card(_id),
deck_reference INTEGER REFERENCES deck(_id),
PRIMARY KEY(card_reference,deck_reference)
);
-- Add 3 cards to the card table
INSERT INTO card (cardname) VALUES ('CARD001'),('CARD002'),('CARD003');
-- Add 3 decks to the deck table
INSERT INTO deck (deckname) VALUES ('DECK001'),('DECK002');
-- Create some mapping entries (aka put some cards into each deck)
INSERT INTO card_deck_map VALUES
(1,2), -- _id value for CARD001 should be 1, _id value for DECK002 should be 2
(3,2), -- CARD003 is in DECK002
(2,1), -- CARD002 is in DECK001
(1,1) -- CARD001 is also in DECK002
;
-- Have a look at what we have (ignore the id values they mean little to the user)
SELECT deckname, cardname
FROM deck
JOIN card_deck_map ON deck._id = deck_reference
JOIN card ON card_deck_map.card_reference = card._id
ORDER BY deckname, cardname
;
上面的输出将是：-
所以现在数据库设计看起来很适合，那么现在可以转换它以供 ROOM 使用。
首先是 3 个实体 Defining data using Room entities
Card.java
:-
@Entity (indices = {@Index(value = {"cardname"}, unique = true)})
public class Card {
@PrimaryKey(autoGenerate = true)
public long _id;
@ColumnInfo(name = "cardname")
public String cardname;
}
Deck.java
:-
@Entity(indices = {@Index(value = "deckname", unique = true)})
public class Deck {
@PrimaryKey(autoGenerate = true)
public long _id;
@ColumnInfo(name = "deckname")
public String deckname;
}
Card_Deck_Map.java
:-
@Entity(
primaryKeys = {"card_reference","deck_reference"},
foreignKeys = {
@ForeignKey(entity = Card.class,parentColumns = "_id",childColumns = "card_reference"),
@ForeignKey(entity = Deck.class, parentColumns = "_id",childColumns = "deck_reference")}
)
public class Card_Deck_Map {
@ColumnInfo (name="card_reference")
public long card_reference;
@ColumnInfo(name="deck_reference")
public long deck_reference;
}
现在您需要数据访问对象定义Accessing data using Room DAOs
DeckBuildeDao
:-
@道
公共接口 DeckBuilderDao {
@Insert(onConflict = OnConflictStrategy.IGNORE)
public long[] insertCards(Card... cards);
@Insert(onConflict = OnConflictStrategy.IGNORE)
public long insertCard(Card card);
@Update
public int updateCardBaseEntries(Card... cards);
@Update
public int updateCardBaseEntry(Card card);
@Delete
public int deleteCardBaseEntried(Card... cards);
@Delete
public int deleteCardBaseEntry(Card card);
@Query("SELECT * FROM card")
public Card[] getAllCards();
@Query("SELECT * FROM card WHERE _id = :id")
public Card getACard(long id);
@Insert(onConflict = OnConflictStrategy.IGNORE)
public long[] insertDecks(Deck... decks);
@Insert(onConflict = OnConflictStrategy.IGNORE)
public long insertDeck(Deck deck);
@Update
public int updateDeckEntries(Deck... decks);
@Update
public int updateDeckEntry(Deck deck);
@Delete
public int deleteDeckEntries(Deck... decks);
@Delete
public int deleteDeckEntry(Deck deck);
@Query("SELECT * FROM deck")
public int getAllDecks();
@Query("SELECT * FROM deck WHERE _id = :id")
public Deck getADeck(long id);
@Insert(onConflict = OnConflictStrategy.IGNORE)
public long[] addCardDeckEntries(Card_Deck_Map... cardDeckMaps);
@Insert(onConflict = OnConflictStrategy.IGNORE)
public long addCardDeckEntry(Card_Deck_Map cardDeckMap);
@Query("SELECT Deck._id,Card.cardname, Deck.deckname " +
"FROM deck " +
"JOIN card_deck_map ON deck._id = card_deck_map.deck_reference " +
"JOIN card ON card_deck_map.card_reference = card._id " +
"ORDER BY deckname, cardname")
public Cursor getAllDecksWithCards();
}
将实体和 DAO 联系在一起的数据库类
DeckBuilderDatabase.java
:-
@Database(entities = {Card.class, Deck.class, Card_Deck_Map.class}, version = 1)
public abstract class DeckBuilderDatabase extends RoomDatabase {
public abstract DeckBuilderDao deckBuilderDao();
}
现在是一个使用数据库的活动。
在这个工作示例中；
-
数据库将填充 2 副牌（Deck001 和 Deck002），根据一包扑克牌减去 Jokers 的牌库。
- 卡片将命名为黑桃 A、红桃 2。
-
套牌将加载一些卡片（映射）
- Deck002 包含所有 52 张卡片。
- Deck001 有 3 张卡片。
将从数据库中提取牌组和卡片并用于填充 ListView。
MainActivity.java
public class MainActivity extends AppCompatActivity {
public static final String[] SUITS = new String[]{"Spades","Hearts","Clubs","Diamons"};
public static final int CARDS_IN_A_SUIT = 13;
DeckBuilderDatabase mDBDB;
SimpleCursorAdapter mSCA;
ListView mDecks_and_Cards_List;
Cursor mCursor;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mDecks_and_Cards_List = this.findViewById(R.id.decksandcards);
mDBDB = Room.databaseBuilder(this,DeckBuilderDatabase.class,"deckbuilder.db").build();
populateDB();
}
/**
* Populate the DB with some data, extract the data in the DB and setup the ListView
*/
private void populateDB() {
new Thread(new Runnable() {
@Override
public void run() {
Card_Deck_Map currentcdm = new Card_Deck_Map();
Deck[] decks_to_add = new Deck[]{new Deck(), new Deck()};
decks_to_add[0].deckname = "DECK001";
decks_to_add[1].deckname= "DECK002";
mDBDB.deckBuilderDao().insertDecks(decks_to_add);
// Build Card base pack of 52 (no Jokers)
Card[] cardstoadd = new Card[CARDS_IN_A_SUIT * SUITS.length];
int counter = 0;
for (int suit = 0; suit < SUITS.length; suit++) {
for (int cardval = 0; cardval < CARDS_IN_A_SUIT; cardval++) {
Card thiscard = new Card();
String thiscardname = generateCardValueDescription(cardval+1,suit);
thiscard.cardname = thiscardname;
cardstoadd[counter++] = thiscard;
}
}
mDBDB.deckBuilderDao().insertCards(cardstoadd);
// Populate the decks with cards Deck002 has full pack of 52 Deck001 has 3 cards
Card_Deck_Map[] mappings = new Card_Deck_Map[55];
for (int cardid = 1; cardid < 53; cardid++) {
Card_Deck_Map cdm = new Card_Deck_Map();
cdm.deck_reference = 2;
cdm.card_reference = cardid;
mappings[cardid-1] = cdm;
}
Card_Deck_Map cdm53 = new Card_Deck_Map();
cdm53.card_reference = 19;
cdm53.deck_reference = 1;
mappings[52] = cdm53;
Card_Deck_Map cdm54 = new Card_Deck_Map();
cdm54.card_reference = 10;
cdm54.deck_reference = 1;
mappings[53] = cdm54;
Card_Deck_Map cdm55 = new Card_Deck_Map();
cdm55.card_reference = 23;
cdm55.deck_reference = 1;
mappings[54] = cdm55;
mDBDB.deckBuilderDao().addCardDeckEntries(mappings);
// Get the Decks and cards in the decks
mCursor = mDBDB.deckBuilderDao().getAllDecksWithCards();
setupOrRefeshListView();
}
}).start();
}
/**
* Handles the ListView (also write data to the log for debugging)
*/
private void setupOrRefeshListView() {
int rowcount = mCursor.getCount();
Log.d("ROWS","Number of rows in the Cursor is " + String.valueOf(rowcount));
while (mCursor.moveToNext()) {
Log.d(
"ROWS",
"Row " +
String.valueOf(mCursor.getPosition()) +
" Has a deck called " +
mCursor.getString(mCursor.getColumnIndex("deckname")) +
" and a card called " +
mCursor.getString(mCursor.getColumnIndex("cardname"))
);
}
if (mSCA == null) {
mSCA = new SimpleCursorAdapter(
this,
android.R.layout.simple_list_item_2,
mCursor,
new String[]{
"deckname",
"cardname"},
new int[]{
android.R.id.text1,
android.R.id.text2},
0
);
mDecks_and_Cards_List.setAdapter(mSCA);
} else {
mSCA.swapCursor(mCursor);
}
}
/**
* Converts numeric cardvalue (1-13) and suit to a decriptive name
* @param cardvalue
* @param suit
* @return
*/
private String generateCardValueDescription(int cardvalue, int suit) {
String rv;
switch (cardvalue) {
case 1:
rv = "Ace of " + SUITS[suit];
break;
case 11:
rv = "Jack of " + SUITS[suit];
break;
case 12:
rv = "Queen of " + SUITS[suit];
break;
case 13:
rv = "King of " + SUITS[suit];
break;
default:
rv = String.valueOf(cardvalue) + " of " + SUITS[suit];
}
return rv;
}
}
结果迷你应用：-