Principles of Package Design
Principles of Package Design
Preparing your code for reuse
About the Book
In Principles of Package Design Matthias Noback tells you everything about designing software components, also known as packages. In the first part you'll revisit the SOLID principles of class design. They will help you prepare your classes for use in packages. The second part covers the important, yet lesser known package design principles. When you've finished this book, you'll be ready to design packages that have high cohesion, low coupling and are at the same time user- and maintainer-friendly.
This book has been revised and then republished by Apress. From now on you can order it from them: https://www.apress.com/us/book/9781484241189
- The situation
- Overview of the contents
- Notes about the code samples
- Thank you
- SOLID principles
- Why follow the principles?
- Prepare for change
The Single responsibility principle
- A class with too many responsibilities
- Responsibilities are reasons to change
- Refactoring: using collaborator classes
- Advantages of having a single responsibility
- A small peek ahead: common closure
The Open/closed principle
- A class that is closed for extension
- Refactoring: abstract factory
- Refactoring: making the abstract factory open for extension
- Refactoring: polymorphism
- Packages need classes that are open for extension
The Liskov substitution principle
- Violation: a derived class does not have an implementation for all methods
- Violation: different substitutes return things of different types
- Violation: a derived class is less permissive with regard to method arguments
- Violation: secretly programming a more specific type
The Interface segregation principle
Violation: leaky abstractions
Refactoring: create separate interfaces and use multiple inheritance
- What did we do?
- Refactoring: create separate interfaces and use multiple inheritance
Violation: multiple use cases
- Refactoring: separate interfaces and multiple inheritance
Violation: no interface, just a class
- Implicit changes in the implicit interface
- Refactoring: add header and role interfaces
- Violation: leaky abstractions
The Dependency inversion principle
Example of dependency inversion: the FizzBuzz generator
FizzBuzzclass open for extension
Removing the specificness from the
- Making the
Violation: a high-level class depends upon a low-level class
- Refactoring: abstractions and concretions both depend on abstractions
Violation: a class depends upon a class from another package
- Solution: add an abstraction and remove the dependency using composition
- Peeking ahead: abstract dependencies
- Example of dependency inversion: the FizzBuzz generator
Principles of cohesion
- Becoming a programmer
- The hardest part
- Class design principles benefit cohesion
- Package design principles, part I: Cohesion
The Release/reuse equivalence principle
- Keep your package under version control
- Add a package definition file
Use semantic versioning
Design for backward compatibility
Rules of thumb
- Don’t throw anything away
- When you rename something, add a proxy
- Only add parameters at the end and with a default value
- Methods should not have side effects
- Dependency versions should be permissive
- Use objects instead of primitive values
- Use private object properties and methods for information hiding
- Use object factories
- And so on
- Rules of thumb
- Design for backward compatibility
Add meta files
- Installation and configuration
- Extension points
- Limitations (optional)
- Change log
- Quality from the user’s point of view
What the package maintainer needs to do
- Add tests
The Common reuse principle
Signs that the principle is being violated
- Obvious stratification
- Obfuscated stratification
Classes that can only be used when … is installed
- Suggested refactoring
- A package should be “linkable”
- Cleaner releases
- Suggested refactoring
- Feature strata
- Guiding questions
- When to apply the principle
- When to violate the principle
- Why not to violate the principle
- Signs that the principle is being violated
The Common closure principle
A change in one of the dependencies
A change in an application layer
A change in the business
- The tension triangle of cohesion principles
- A change in one of the dependencies
Principles of coupling
The Acyclic dependencies principle
Coupling: discovering dependencies
Different ways of package coupling
- Not to be considered: global state
- Visualizing dependencies
- Different ways of package coupling
The Acyclic dependencies principle
- Nasty cycles
Cycles in a package dependency graph
- Dependency resolution
- Release management
- Is it all that bad?
Solutions for breaking the cycles
- Some pseudo-cycles and their dissolution
Some real cycles and their dissolution
- Dependency inversion
Inversion of control
- Chain of responsibility
- Mediator and chain of responsibility combined: an event system
- Coupling: discovering dependencies
The Stable dependencies principle
- Not every package can be highly stable
- Unstable packages should only depend on more stable packages
- Measuring stability
- Decreasing instability, increasing stability
Violation: your stable package depends on a third-party unstable package
- Solution: use dependency inversion
- A package can be both responsible and irresponsible
The Stable abstractions principle
- Stability and abstractness
How to determine if a package is abstract
- Abstract things belong in stable packages
- Abstractness increases with stability
- The main sequence
Types of packages
- Concrete, instable packages
- Abstract, stable packages
- Strange packages
Creating packages is hard
- Embracing software diversity
- Component reuse is possible, but requires more work
Creating packages is doable
- Reducing the impact of the first rule of three
- Reducing the impact of the second rule of three
- Creating packages is easy?
- Creating packages is hard
- Principles of cohesion
Appendix I: The full
- Appendix I: The full
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