android數據庫

組織contract類的一個好方法是在類的根層級定義一些全局變量，然後爲每一個table來創建內部類。

Note：通過實現 BaseColumns 的接口，內部類可以繼承到一個名爲_ID的主鍵，這個對於Android裏面的一些類似cursor adaptor類是很有必要的。這麼做不是必須的，但這樣能夠使得我們的DB與Android的framework能夠很好的相容。

例如，下面的例子定義了表名與該表的列名：

public final class FeedReaderContract {

    public FeedReaderContract(){}
    public static abstract class FeedEntry implements BaseColumns{
        public static final String TABLE_NAME = "entry";
        public static final String COLUMN_NAME_ENTRY_ID = "entryid";
        public static final String COLUMN_NAME_TITLE = "title";
        public static final String COLUMN_NAME_SUBTITLE = "subtitle";
    }
}

繼承SQLiteOpenHelp抽象類，藉助該類可以對數據庫進行創建和升級，SQLite中有兩個重要的方法，

getReadableDatabase()和getWriteableDatabase(),這兩個方法都可以打開或者創建數據庫（如果數據庫存在就打開，否則就創建新的數據庫）

public class MyDataBaseHelper extends SQLiteOpenHelper {
    private static final String TEXT_TYPE = "text";
    private static final String SQL_CREATE_ENTRIES =
            "create table " + FeedReaderContract.FeedEntry.TABLE_NAME + "("
                    + FeedReaderContract.FeedEntry._ID + " integer primary key autoincrement,"
                    + FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID +" " +TEXT_TYPE + ","
                    + FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE + " "+TEXT_TYPE + ")";
    private static final String SQL_DELETE_ENTRIES="drop table is exists"+FeedReaderContract.FeedEntry.TABLE_NAME;
    public static final String DATABASE_NAME="FeedReader.db";
    private static final int VERSION = 1;
    public MyDataBaseHelper(Context context) {
        super(context,DATABASE_NAME,null,VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        sqLiteDatabase.execSQL(SQL_CREATE_ENTRIES);
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
        sqLiteDatabase.execSQL(SQL_DELETE_ENTRIES);
        onCreate(sqLiteDatabase);
    }
}

判斷數據庫是否創建

  /* file = getExternalCacheDir();*///這句可以獲得數據庫，            

file=getCacheDir();//

if (file != null && file.exists())

根據點擊的按鈕不同執行不同的操作

public void onClick(View view) {
    switch (view.getId()) {
        case R.id.create:
            //db爲SQLiteDatabase,藉助db可以對數據庫進行增刪改查操作
            
            db = dataBaseHelper.getWritableDatabase();
           /* file = getExternalCacheDir();*///這句可以獲得數據庫，
            file=getCacheDir();
            if (file.exists()) {
                Toast.makeText(this, "create success", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(this, "create failed", Toast.LENGTH_SHORT).show();
            }
            break;
        case R.id.delete:
            if (file != null && file.exists()) {
                db.delete(FeedReaderContract.FeedEntry.TABLE_NAME, FeedReaderContract.FeedEntry._ID + "=?", new String[]{"1"});
                Toast.makeText(this, "delete success", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(this, "delete failed", Toast.LENGTH_SHORT).show();
            }
            break;
        case R.id.query:
            if (file != null && file.exists()) {
                String[] columns = new String[]{
                        FeedReaderContract.FeedEntry._ID,
                        FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE,
                        FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID};
                Cursor cursor = db.query(FeedReaderContract.FeedEntry.TABLE_NAME, columns, null, null, null, null, null);
                if (cursor.moveToFirst()) {
                    do {
                        int _id = cursor.getInt(cursor.getColumnIndex(FeedReaderContract.FeedEntry._ID));
                        String title = cursor.getString(cursor.getColumnIndex(FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE));
                        String entryId = cursor.getString(cursor.getColumnIndex(FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID));
                        Log.d("test", "-id:" + _id + ",title:" + title + ",entryId:" + entryId);
                    } while (cursor.moveToNext());
                }


            } else {
                Toast.makeText(this, "query failed", Toast.LENGTH_SHORT).show();
            }
            break;
        case R.id.insert:
            if (file != null && file.exists()) {
                ContentValues values = new ContentValues();
                values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID,"li");
                values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE,"花海");
                db.insert(FeedReaderContract.FeedEntry.TABLE_NAME, null, values);
                values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID,"zhang");
                values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE, "超人不會飛");
                db.insert(FeedReaderContract.FeedEntry.TABLE_NAME, null, values);
                values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID, "hua");
                values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE, "說好的幸福呢");
                db.insert(FeedReaderContract.FeedEntry.TABLE_NAME, null, values);
                Toast.makeText(this,"insert success",Toast.LENGTH_SHORT).show();
            }else {
                Toast.makeText(this,"insert failed",Toast.LENGTH_SHORT).show();
            }
            break;

    }
}