Understand Source Code by Reading Code and Refactoring for Maintaining Large Projects

This list consists of one-liner tips to be used for easy lookup and reference.

Rewriting Code for Better Understanding (Coding Horror)

• What I cannot create, I do not understand. [Richard Feynman]
• "What I do not create, I do not understand."[Chris]
• "It's easier to understand 600 tables than 100,000 lines of code." [PaulC on comp.databases.theory/2005]
• Create a new project with blocks of code copied from old code. Refactor to reduce "old code smell"
• Start refactoring by adding small tests to better understand code and act as regression tests.
• Just scan through huge amounts of code till it starts making sense - "understand by constant exposure".
• Create a text file to capture your understanding - briefly describe how the component works, realtions to other components, what it persists, its states, and source browsing/debugging tips you found useful.
• Manually reformat a copy of the code - white space, and indented as per your personal style.

References:

1. Working Effectively with Legacy Code
2. Object Oriented Reengineering Patterns
3. Refactoring: Improving the Design of Existing Code

 Noble Art of Maintainance Programming (Coding Horror)

• The Daily WTF is "a very enjoyable web site where people relate short stories about the crazy code they've worked with, usually with examples." (http://thedailywtf.com/)
• On taking over foreign code
  

Tips for Increasing Code Readability (StackOverFlow)

• Top-Down: Look for indicative class names and modules
• Bottom-Up: Navigate to a feature implementation and browse/annotate to build standalone chunks of knowledge.
• Use whitespace for increasing code-readability
• Scan the system packages and classes looking for entities you can identify.

References:

1. Code Reading: The Open Source Perspective
2. Code Quality: The Open Source Perspective
3. Implementation Patterns by Kent Beck

Some tips from StackOverflow

• Debug to understand basic data/control flows in source.
• Add Logging/print statements to source to understand basic data/control flows in source.
• Write unit tests for the code
• Annotate a printout of the source.
• Do code walk-through with someone knows how the code works.
• Code review
• Fixing bugs in source slowly exposes you to small parts of the whole source.
• Going through bug histories will give you background info esp. on the "smelly parts" of source.
• Refactor code to improve code quality and testability esp. regression tests.
• Add annotations to a copy of the code as you push the edges of your ignorance.
• Reverse engineer source to design document (needs heavy manual editing)
• Get familiar with language idioms to quickly understand source code.
• Make a paper Diagram of the application to understand flow and dependencies

This is a list of good tips para-phrased from Coding Horror and StackOverflow (linked) blogs and many user suggestions scattered across some of the useful comments.