與Android資料庫一起工作

   

一個好的習慣是建立一個輔助類來簡化你的資料庫互動。

 

考慮建立一個資料庫適配器，來添加一個與資料庫互動的封裝層。它應該提供直觀的、強型別的方法，如添加、刪除和更新項目。資料庫適配器還應該處理查詢和對建立、開啟和關閉資料庫的封裝。

 

它還常用靜態資料庫常量來定義表的名字、列的名字和列的索引。

 

下面的程式碼片段顯示了一個標準資料庫適配器類的架構。它包括一個SQLiteOpenHelper類的擴充類，用於簡化開啟、建立和更新資料庫。

 

import android.content.Context;

import android.database.*;

import android.database.sqlite.*;

import android.database.sqlite.SQLiteDatabase.CursorFactory;

import android.util.Log;

public class MyDBAdapter

{

private static final String DATABASE_NAME = “myDatabase.db”;

private static final String DATABASE_TABLE = “mainTable”;

private static final int DATABASE_VERSION = 1;

 

// The index (key) column name for use in where clauses.

public static final String KEY_ID=”_id”;

 

// The name and column index of each column in your database.

public static final String KEY_NAME=”name”;

public static final int NAME_COLUMN = 1;

 

// TODO: Create public field for each column in your table.

// SQL Statement to create a new database.

private static final String DATABASE_CREATE = “create table “ +

DATABASE_TABLE + “ (“ + KEY_ID + “ integer primary key autoincrement, “ +

KEY_NAME + “ text not null);”;

 

// Variable to hold the database instance

private SQLiteDatabase db;

 

// Context of the application using the database.

private final Context context;

 

// Database open/upgrade helper

private myDbHelper dbHelper;

 

public MyDBAdapter(Context _context) {

context = _context;

dbHelper = new myDbHelper(context, DATABASE_NAME, null, DATABASE_VERSION);

}

 

public MyDBAdapter open() throws SQLException {

db = dbHelper.getWritableDatabase();

return this;

}

 

public void close() {

db.close();

}

 

public long insertEntry(MyObject _myObject) {

ContentValues contentValues = new ContentValues();

// TODO fill in ContentValues to represent the new row

return db.insert(DATABASE_TABLE, null, contentValues);

}

 

public boolean removeEntry(long _rowIndex) {

return db.delete(DATABASE_TABLE, KEY_ID + “=” + _rowIndex, null) > 0;

}

public Cursor getAllEntries () {

return db.query(DATABASE_TABLE, new String[] {KEY_ID, KEY_NAME},

null, null, null, null, null);

}

public MyObject getEntry(long _rowIndex) {

MyObject objectInstance = new MyObject();

// TODO Return a cursor to a row from the database and

// use the values to populate an instance of MyObject

return objectInstance;

}

public int updateEntry(long _rowIndex, MyObject _myObject) {

String where = KEY_ID + “=” + _rowIndex;

ContentValues contentValues = new ContentValues();

// TODO fill in the ContentValue based on the new object

return db.update(DATABASE_TABLE, contentValues, where, null);

}

 

private static class myDbHelper extends SQLiteOpenHelper

{

public myDbHelper(Context context, String name, CursorFactory factory, int version) {

super(context, name, factory, version);

}

 

// Called when no database exists in

// disk and the helper class needs

// to create a new one.

@Override

public void onCreate(SQLiteDatabase _db) {

_db.execSQL(DATABASE_CREATE);

}

 

// Called when there is a database version mismatch meaning that

// the version of the database on disk needs to be upgraded to

// the current version.

@Override

public void onUpgrade(SQLiteDatabase _db, int _oldVersion, int _newVersion) {

// Log the version upgrade.

Log.w(“TaskDBAdapter”, “Upgrading from version “ +

_oldVersion + “ to “ + _newVersion +

“, which will destroy all old data”);

// Upgrade the existing database to conform to the new version.

// Multiple previous versions can be handled by comparing

// _oldVersion and _newVersion values.

// The simplest case is to drop the old table and create a

// new one.

_db.execSQL(“DROP TABLE IF EXISTS “ + DATABASE_TABLE);

 

// Create a new one.

onCreate(_db);

}

}

}