在这种情况下如何使用 HashMap？

Posted 2023-02-19

【中文标题】在这种情况下如何使用 HashMap？

【英文标题】：How to use HashMap for this scenario?

【发布时间】：2019-11-29 02:05:46

【问题描述】：

给定客户要求的任务。

• 创建 Vehicle 类，其中包括车辆 Id(Integer) 作为车辆 number 和 lot(Integer) 作为停车场号码。实现 getId() 和 getLot() 返回车辆 Id 和 Lot。

创建包含 parkedVehicle、vehicle_lot_data 并实现以下内容的 ParkingLot 类

HashMap<Integer, Vehicle> parkedVehicle = new HashMap<>(); 
HashMap<Integer, Integer> vehicle_lot_data = new HashMap<>(); 

• parkVehicle(Vehicle, lot) 将车辆停在停车场，检查是否有效 停车，还要检查同一辆车不允许停在 多个批次。如果批次已被预订，则返回“批次已被占用” 如果我们要预订，则某人和“车辆已经存在” 同一辆车的多个批次。 • isLotBooked(int lot) 返回 boolean - 它检查已经预订的批次。返回假 对于空批次，对于预订批次为 True。 • isVehicleExists(int vehicleId) return boolean - 它检查已经存在的车辆。 如果车辆存在则返回 True 否则为不存在则返回 False 车辆。

第 3 步：（Database.java）

创建类数据库来存储停车场数据。

创建本地 SQLite 数据库 (parking_database.db) • 创建 DATABASE_NAME 变量并存储parking_database.db。 • 实施 onCreate, onUpgrade 用于数据库创建和数据库删除。 • ParkVehicle(Vehicle) - 将停放的车辆保存到数据库。 • getParkedVehicle()、getParkedVehicle(int vehicleId) - 全部停放 车辆返回（光标） • getAllBookedLots() - 获取所有预订 停车场返回（光标）

具有以下架构的设计表

CREATE TABLE parking_lots (id integer PRIMARY KEY, 
lot_number INTEGER, 
vehicle_number INTEGER UNIQUE); 

• 实现 ParkVehicleNow()，它将从视图中获取用户输入数据， 并将数据存储在这些变量 vehicle_id_field_txt 和 将调用 park(int) 的 vehicle_lot_number_field_txt vehicle_id_field_txt, int vehicle_lot_number_field_txt) 函数 内部。 • park(int vehicle_id_field_txt, int vehicle_lot_number_field_txt) - 将使用 ParkingLot 类和 使用 ParkingLot 实例 pl 停放车辆并将该车辆保存到 使用 Databaseclass 实例 db 的数据库。 • 如果您无法停车 然后祝酒“车辆已停放选择不同的地段或 车辆”，否则“车辆停放”消息将被烘烤。 • 另外，不要 忘记对“Lot already taken”和“Vehicle”进行验证 已经存在”然后不要停放车辆。

但我没有得到第 2 步如何在 ParkingLot 中实现哈希图。

我累的是，这里基于场景，我无法在任何地方使用hashmap。

public class ParkingLot 
    HashMap<Integer, Vehicle> parkedVehicle = new HashMap<>();
    HashMap<Integer, Integer> vehicle_lot_data = new HashMap<>();

    public boolean isLotBooked(int lot, Context context) 
        Database db = new Database(context);
        return db.isAlreadyLotsExists(String.valueOf(lot));
    

    public boolean isVehicleExists(int vehicleId, Context context) 
        Database db = new Database(context);
        return db.isAlreadyVehicleExists(String.valueOf(vehicleId));
    




  public Vehicle()

    

    public Vehicle(int primaryId,int id, int lot)
        this.id = id;
        this.lot = lot;
        this.primaryId = primaryId;
    
    public Vehicle(int id, int lot)
        this.id = id;
        this.lot = lot;
    
    public int id;

    public int getPrimaryId() 
        return primaryId;
    

    public void setPrimaryId(int primaryId) 
        this.primaryId = primaryId;
    

    public int primaryId;

    public int getId() 
        return id;
    

    public void setId(int id) 
        this.id = id;
    

    public int getLot() 
        return lot;
    

    public void setLot(int lot) 
        this.lot = lot;
    

    public int lot;




public class MainActivity extends AppCompatActivity 

    @Override
    protected void onCreate(Bundle savedInstanceState) 
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Database db = new Database(this);
        db.ParkVehicle(new Vehicle(1000,1));
        db.ParkVehicle(new Vehicle(2000,2));

        // Reading all contacts
        Log.e("Reading: ", "Reading all contacts..");
        List<Vehicle> contacts = db.getParkedVehicle(); //all parked vehicle
        List<String> bookedSlots = db.getAllBookedLots(); //all booked slots

        for (Vehicle cn : contacts) 
            String log = "Id: " + cn.getPrimaryId() + " ,Vehicle: " + cn.getId() + " ,Lot: " +
                    cn.getLot();
            // Writing Contacts to log
            Log.e("ContactsAllVehicles: ", log);
        

        for (String cn : bookedSlots) 
            // Writing Contacts to log
            Log.e("ContactsSlots: ", cn);
        

        Vehicle vehicle= db.getParkedVehicle(1);
        Log.e("ContactsLots",""+vehicle.getId());

        //checking if lots already exisitng..
        boolean alreadyExist=db.isAlreadyLotsExists("0");
        Log.e("contactsslotsTest",""+alreadyExist);
    



public class Database extends SQLiteOpenHelper 

    private static final int DATABASE_VERSION = 1;
    private static final String DATABASE_NAME = "vehicles";
    private static final String KEY_ID = "id";
    private static final String LOT_NUMBER = "lot_number";
    private static final String VEHICLE_NUMBER = "vehicle_number";
    private static final String PARKING_LOTS = "parking_lots"; //table name

    public Database(Context context) 
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
        //3rd argument to be passed is CursorFactory instance
    

    @Override
    public void onCreate(SQLiteDatabase db) 
        String CREATE_CONTACTS_TABLE = "CREATE TABLE " + PARKING_LOTS + "("
                + KEY_ID + " INTEGER PRIMARY KEY," + LOT_NUMBER + " INTEGER,"
                + VEHICLE_NUMBER + " INTEGER UNIQUE" + ")";
        db.execSQL(CREATE_CONTACTS_TABLE);

    

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) 
        // Drop older table if existed
        db.execSQL("DROP TABLE IF EXISTS " + PARKING_LOTS);

        // Create tables again
        onCreate(db);
    

    void ParkVehicle(Vehicle contact) 
        SQLiteDatabase db = this.getWritableDatabase();

        ContentValues values = new ContentValues();
        values.put(LOT_NUMBER, contact.getLot()); // Lot number
        values.put(VEHICLE_NUMBER, contact.getId()); // Vehicle number

        // Inserting Row
        db.insert(PARKING_LOTS, null, values);
        //2nd argument is String containing nullColumnHack
        db.close(); // Closing database connection
    

    Vehicle getParkedVehicle(int id)  //single vehicle
        SQLiteDatabase db = this.getReadableDatabase();

        Cursor cursor = db.query(PARKING_LOTS, new String[]KEY_ID,
                        LOT_NUMBER, VEHICLE_NUMBER, KEY_ID + "=?",
                new String[]String.valueOf(id), null, null, null, null);
        if (cursor != null)
            cursor.moveToFirst();

        Vehicle contact = new Vehicle(Integer.parseInt(cursor.getString(0)),
                Integer.parseInt(cursor.getString(1)), Integer.parseInt(cursor.getString(2)));
        // return contact
        return contact;
    

    public List<Vehicle> getParkedVehicle() 
        List<Vehicle> contactList = new ArrayList<Vehicle>();
        // Select All Query
        String selectQuery = "SELECT  * FROM " + PARKING_LOTS;

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to list
        if (cursor.moveToFirst()) 
            do 
                Vehicle contact = new Vehicle();
                contact.setPrimaryId(Integer.parseInt(cursor.getString(0)));
                contact.setLot(Integer.parseInt(cursor.getString(1)));
                contact.setId(Integer.parseInt(cursor.getString(2)));
                // Adding contact to list
                contactList.add(contact);
             while (cursor.moveToNext());
        

        // return contact list
        return contactList;
    

    public List<String> getAllBookedLots()
        List<String> contactList = new ArrayList<>();
        // Select All Query
        String selectQuery = "SELECT  * FROM " + PARKING_LOTS;

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);
        cursor.moveToFirst();

        // looping through all rows and adding to list
        if (cursor!=null && cursor.moveToFirst()) 
            while(cursor.isAfterLast() == false)
                contactList.add(cursor.getString(cursor.getColumnIndex(LOT_NUMBER)));
                Log.d("contactslotnumber",contactList.toString());

                cursor.moveToNext();
            
        return contactList;
    

    public boolean isAlreadyLotsExists(String name)
            SQLiteDatabase sqLiteDatabase = this.getReadableDatabase();
            String query="SELECT * FROM "+PARKING_LOTS+" WHERE "+LOT_NUMBER+" = "+name;
            Cursor cursor = sqLiteDatabase.rawQuery(query,null);
            if(cursor.getCount()>0)
                return true;
            else
                return false;

    

    public boolean isAlreadyVehicleExists(String name)
        SQLiteDatabase sqLiteDatabase = this.getReadableDatabase();
        String query="SELECT * FROM "+PARKING_LOTS+" WHERE "+VEHICLE_NUMBER+" = "+name;
        Cursor cursor = sqLiteDatabase.rawQuery(query,null);
        if(cursor.getCount()>0)
            return true;
        else
            return false;

    

        // return contact list



    public int updateContact(Vehicle contact) 
        SQLiteDatabase db = this.getWritableDatabase();

        ContentValues values = new ContentValues();
        values.put(LOT_NUMBER, contact.getLot());
        values.put(VEHICLE_NUMBER, contact.getId());

        // updating row
        return db.update(PARKING_LOTS, values, KEY_ID + " = ?",
                new String[]String.valueOf(contact.getPrimaryId()));
    

    // Deleting single contact
    public void deleteContact(Vehicle contact) 
        SQLiteDatabase db = this.getWritableDatabase();
        db.delete(PARKING_LOTS, KEY_ID + " = ?",
                new String[]String.valueOf(contact.getPrimaryId()));
        db.close();
    

    // Getting contacts Count
    public int getContactsCount() 
        String countQuery = "SELECT  * FROM " + PARKING_LOTS;
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(countQuery, null);
        cursor.close();

        // return count
        return cursor.getCount();
    

这是我所做的，但要求中缺少一些步骤。例如：第 2 步我不确定在哪里使用 hashmap 来使用 ParkingLot 类中的 parkVehicle 方法，同样在 MainActivity 中使用第 4 步。

【参考方案1】：

可能在缓存数据库数据的上下文中给出了哈希图。当您写入与车辆/批次相关的数据时，您使用 hashmaps 作为缓存（使用回写或直写策略，以符合您的要求或需要为准），当您读取数据时，您总是从 hashmaps 中读取它们，即缓存

回想一下，回写意味着您将新数据写入缓存而不是 DB（数据稍后写入 DB），而直写意味着无论何时写入新数据，它都会同时写入缓存和数据库