Android 主要概念总结归纳

核心提示：your data any way you want, so long as you follow the calling and return value conventions.4. Define the column names that you will return to your clients. If y

your data any way you want, so long as you follow the calling and return value conventions.

4. Define the column names that you will return to your clients. If you are using an underlying database, these column names are typically identical to the SQL database column names they represent. In any case, you should include an integer column named _id to define a specific record number. If using the SQLite database, this should be type INTEGER PRIMARY KEY AUTOINCREMENT. The AUTOINCREMENT descriptor is optional, but by default, SQLite autoincrements an ID counter field to the next number above the largest existing number in the table. If you delete the last row, the next row added will have the same ID as the deleted row. To avoid this by having SQLite increment to the next largest value whether deleted or not, then assign your ID column the following type: INTEGER PRIMARY KEY AUTOINCREMENT. (Note You should have a unique _id field whether or not you have another field (such as a URL) that is also unique among all records.) Android provides the SQLiteOpenHelper class to help you create and manage versions of your database.

5. If you are exposing byte data, such as a bitmap file, the field that stores this data should actually be a string field with a content:// URI for that specific file. This is the field that clients will call to retrieve this data. The content provider for that content type (it can be the same content provider or another content provider — for example, if you're storing a photo you would use the media content provider) should implement a field named _data for that record. The _data field lists the exact file path on the device for that file. This field is not intended to be read by the client, but by the ContentResolver. The client will call ContentResolver.openOutputStream() on the user-facing field holding the URI for the item (for example, the column named photo might have a value content://media/images/4453). The ContentResolver will