2011.09.26（4）——— android sample之Notepad(BaseColums)

2011.09.26（4）——— android sample之Notepad(BaseColums)

一个小知识
参考：http://book.51cto.com/art/200907/140494.htm

NotePad .java

/* * Copyright (C) 2007 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package com.example.android.notepad;import android.net.Uri;import android.provider.BaseColumns;/** * Convenience definitions for NotePadProvider */public final class NotePad {    public static final String AUTHORITY = "com.google.provider.NotePad";    // This class cannot be instantiated    private NotePad() {}        /**     * Notes table     */    public static final class Notes implements BaseColumns {        // This class cannot be instantiated        private Notes() {}        /**         * The content:// style URL for this table         */        public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/notes");        /**         * The MIME type of {@link #CONTENT_URI} providing a directory of notes.         */        public static final String CONTENT_TYPE = "vnd.android.cursor.dir/vnd.google.note";        /**         * The MIME type of a {@link #CONTENT_URI} sub-directory of a single note.         */        public static final String CONTENT_ITEM_TYPE = "vnd.android.cursor.item/vnd.google.note";        /**         * The default sort order for this table         */        public static final String DEFAULT_SORT_ORDER = "modified DESC";        /**         * The title of the note         * <P>Type: TEXT</P>         */        public static final String TITLE = "title";        /**         * The note itself         * <P>Type: TEXT</P>         */        public static final String NOTE = "note";        /**         * The timestamp for when the note was created         * <P>Type: INTEGER (long from System.curentTimeMillis())</P>         */        public static final String CREATED_DATE = "created";        /**         * The timestamp for when the note was last modified         * <P>Type: INTEGER (long from System.curentTimeMillis())</P>         */        public static final String MODIFIED_DATE = "modified";    }}

BaseColumns 是一个接口，里边有两个变量，一个是_ID="_id"，一个是_COUNT="_ count" 。在Android当中，每一个数据库表至少有一个字段，而且这个字段是_id。所以当我们构造列名的辅助类时，直接实现BaseColumns ，这样我们便默认地拥有了_id字段。

源码如下：

package android.provider;public interface BaseColumns{    /**     * The unique ID for a row.     * <P>Type: INTEGER (long)</P>     */    public static final String _ID = "_id";    /**     * The count of rows in a directory.     * <P>Type: INTEGER</P>     */    public static final String _COUNT = "_count";}

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