Clean Code – Chapter 6 Objects and Data Structures

Data Abstraction

Hiding implementation

Data/Object Anti-Symmetry

Objects hide their data behind abstractions and expose function that operate on that data. Data structure expose their data and hava no meaningful functions.

Procedural code(code using data structure) makes it easy to add new functions without changing the existing data structures. OO code, on the other hand, makes it easy to add new classes without changing existing functions.

Procedural code makes it hard to add new data structures because all functions must change. OO code makes it hard to add new functions because all the classes must change.

The Law of Demeter

A module should not know about the innards of the objects it manipulates.

A method f of a class C should only call the methods of these:

• C
• An object created by f
• An object passed as an argument to f
• An object held in an instance variable of C

推荐一篇博文：笛米特法则详解，通过一个简单的代码示例解释了上述概念。

• Train Wrecks

  Bad code:

  final String outputDir = ctxt.getOptions().getScratchDir().getAbsolutePath();

  Or

  Options opts = ctxt.getOptions();
  File scratchDir = opts.getScratchDir();
  final String outputDir = scratchDir.getAbsolutePath();

  Only used for data structures

  final String outputDir = ctxt.options.scratchDir.absolutePath;

• Hybrids

  Avoid hybrid structures that are half object and half data structure.

• Hiding Structure

  Tell an object to do something, not ask it about its internals.

Data Transfer Objects (DTO)

The quintessential form of a data structure is a class with public variables and no functions.

• Active Record

  Special form of DTO. Hava navigational methods like save and find.

  Typically direct translations from database tables, or other data sources.