考虑为 GUI 文本编辑器管理文件文本的类，这在 第六章：通用模块更深入 和 第七章：不同的层，不同的抽象 中讨论过。该类提供了将文件从磁盘读入内存、查询和修改文件在内存中的副本以及将修改后的版本写回磁盘的方法。当学生必须实现这个类时，他们中的许多人选择了一个面向行的接口，该接口具有读取、插入和删除整行文本的方法。这导致了类的简单实现，但也为更高级别的软件带来了复杂性。在用户界面级别，操作很少涉及整行。例如，击键会导致在现有行中插入单个字符;复制或删除选择项可以修改几个不同行的部分。使用面向行的文本界面，为了实现用户界面，高级软件必须分割和连接行。
面向字符的界面（如 6.3. 更通用的 API 中所述）降低了复杂性。用户界面软件现在可以插入和删除任意范围的文本，而无需分割和合并行，因此变得更加简单。文本类的实现可能会变得更加复杂：如果内部将文本表示为行的集合，则必须拆分和合并行以实现面向字符的操作。这种方法更好，因为它封装了在文本类中拆分和合并的复杂性，从而降低了系统的整体复杂性。
Consider the class that manages the text of a file for a GUI text editor, which was discussed in Chapters 6 and 7. The class provides methods to read a file from disk into memory, query and modify the in-memory copy of the file, and write the modified version back to disk. When students had to implement this class, many of them chose a line-oriented interface, with methods to read, insert, and delete whole lines of text. This resulted in a simple implementation for the class, but it created complexity for higher level software. At the level of the user interface, operations rarely involve whole lines. For example, keystrokes cause individual characters to be inserted within an existing line; copying or deleting the selection can modify parts of several different lines. With the line-oriented text interface, higher-level software had to split and join lines in order to implement the user interface.
A character-oriented interface such as the one described in Section 6.3 pulls complexity downward. The user interface software can now insert and delete arbitrary ranges of text without splitting and merging lines, so it becomes simpler. The implementation of the text class probably becomes more complex: if it represents the text internally as a collection of lines, it will have to split and merge lines to implement the character-oriented operations. This approach is better because it encapsulates the complexity of splitting and merging within the text class, which reduces the overall complexity of the system.