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

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";
}