* <p> * 增量更新的方式, This class will also look for update "delta" files in the same * directory, with file names that start the same way (up to the first period). * These files have the same format, and provide updated data that supersedes * what is in the main data file. This is a mechanism that allows an application * to push updates to {@link FileDataModel} without re-copying the entire data * file. *  * 同一个目录下,数字来区分 * Finds update delta files in the same directory as the data file. This finds * any file whose name starts the same way as the data file (up to first period) * but isn‘t the data file itself. For example, if the data file is * /foo/data.txt.gz, you might place update files at /foo/data.1.txt.gz, * /foo/data.2.txt.gz, etc. * </p> * * <p> * 表示删除的语法, 偏好为空 One small format difference exists. Update files must also be * able to express deletes. This is done by ending with a blank preference * value, as in "123,456,". * </p> * * <p> * 增量更新的文件中，删除和更新不能混合使用 Note that it‘s all-or-nothing -- all of the items in the * file must express no preference, or the all must. These cannot be mixed. Put * another way there will always be the same number of delimiters on every line * of the file! * </p>