'SOLID': Five Principles for Object-Oriented Software Quality.

In Object-Oriented Programming & Design, SOLID is a mnemonic acronym for five principles intended to make source code & design more understandable, flexible, and maintainable.

This means faster software construction/modification/debugging and lower overall project's costs (programmers are paid for their work-hours).

Software engineer and instructor Robert C. Martin introduced the basic principles of SOLID design in his 2000 paper: 'Design Principles and Design Patterns about software rot'. The SOLID acronym was coined around 2004 by Michael Feathers.


1. Single responsibility principle.

Every class should have only one responsibility, which means that a class should be responsible for only ONE 'thing'.

Importance:

Maintainability: When classes have a single, well-defined responsibility, they're easier to understand and modify.
Testability: It's easier to write unit tests for classes with a single focus.
Flexibility: Changes to one responsibility don't affect unrelated parts of the system.


2. Open–closed principle.

The open–closed principle (OCP) states that software entities should be open for extension, but closed for modification.

In other words, when we define a subclass, we can ask for less or same (less or same preconditions, as defined in class/method contract), and/or provide more or same (we still must provide what is defined in contract at the minimum, but we can also provide more).

Importance:

Extensibility: New features can be added without modifying existing code.
Stability: Reduces the risk of introducing bugs when making changes.
Flexibility: Adapts to changing requirements more easily.


3. Liskov substitution principle.

LSP requirement states that we must ensure that it's possible for a class to be replaced by any of its children (subclasses). Children classes inherit parent's behaviours, MUST NOT break the contract defined for the parent's class.

Importance:

Polymorphism: Enables the use of polymorphic behavior, making code more flexible and reusable.
Reliability: Ensures that subclasses adhere to the contract defined by the superclass.
Predictability: Guarantees that replacing a superclass object with a subclass object won't break the program.


4. Interface segregation principle.

The interface segregation principle (ISP) states that no code should be forced to depend on methods it does not use. ISP splits interfaces that are very large into smaller and more specific ones so that clients will only have to know about the methods that are of interest to them.

Importance:

Decoupling: Reduces dependencies between classes, making the code more modular and maintainable.
Flexibility: Allows for more targeted implementations of interfaces.
Avoids unnecessary dependencies: Clients don't have to depend on methods they don't use.


5. Dependency inversion principle.

The dependency inversion principle (DIP) states to depend upon abstractions, not upon concretes.

Importance:

Loose coupling: Reduces dependencies between modules, making the code more flexible and easier to test.
Flexibility: Enables changes to implementations without affecting clients.
Maintainability: Makes code easier to understand and modify.


See also:
> [ Design by Contract ],
> [ A few thoughts on code quality... ],
> [ Software Development & Quality ].

A few thoughts on code quality - mostly for Java, but can be abstracted and used with different technologies.

In my opinion java code of quality should have following properties:

1. Proper naming of classess, methods, variables and constants.
2. Single, properly defined and documented responsibility of each class and method. No unneccessary code (to remove code duplication you can use constants (final keyword, UPPERCASE_NAMES) and split methods with large chunks of code into few smaller methods so you can reuse them. To ensure single responsibility of method or class move some code into another method or class. Single responsibility class or method is more reausable and easier to document, read, test and modify. [or to break if someone wants to try firing quality coder].). Single responsibility of method may involve calling more than one instruction as long as it is considered atomic. For example: changeStateWithSideEffect(...);
3. Documented methods headers (first lines of methods and all information therein, including variable names) according to javadoc documentation.
4. Use of 'assert' keyword, software contracts, preconditions, postconditions and invariants.
5. Fitting 'a complete code part' on a 'single screen', if possible and worthwhile; ... easier thinking, less scrolling, perhaps more.

Later JUnit/Easymock automated tests can be added to build test harness.

Also, code should be properly formatted.

Commits / to code repository, using technologies such as SVN or Git / should be commented.

Methods should be abstract, empty or final.

Ideally, methods should consists of three instructions. init(...); transform(...); return [(...)]; Complex instruction counts as single instruction;

There can be more requirements / for example: coding in idiomatic way / , but in practice it's almost perfect if these are used. Professionals after all have no time to comment code, or they want to be priceless and unfirable by bosses, in a not-so-nice, unfair way.


See also:
> [ Design by Contract ],
> [ 'SOLID': Five Principles for Object-Oriented Software Quality ],
> [ Software Development & Quality ].

Software Development & Quality.

Introduction.

There are many methods for ensuring that software development proceeds smoothly, and there are methods for ensuring and measuring software artifacts' quality.

Artifacts are documents:
- design model,
- application code,
- automated tests code,
- documentation,
- configuration files,
- resource files as art or other data,
- ...


Code's Quality.

For a code quality's practices, see if You wish:
- A few thoughts on code quality...


Teamwork vs. Showoff.

Teamwork is Professional, needless showoff is NOT Professional.

Code should be written & documented in a simple way, so other co-workers can understand it with ease.

This friendly approach - of making teamwork easier helps the most newcomers - either beginner programmers, or programmers who worked with different Programming Language(s), Tools, Ecosystem, Technology(-ies).


Automated Testing.

For automated testing article, see if You wish:
- Automated Tests.


Design & Modelling.

Software models, often presented in a graphical way, are tools to look at software's design from a 'bird's view', to grasp whole picture.

Software modelling and design's goal is to abstract & simplify project.

Designs should be as simple and as abstract as possible - as this allows for easier code modifications as reality and customer's needs change.

... but of course modelling a single object with name 'idea' is too unreasonable interpretation of above words.

There are 'views' of a model as well - each view serves a purpose - each of views is a transformed model, both reduced and filled with details, to emphasize it's purpose.

For example we can have model consisting of every of car's parts, then we can produce a view of engine - fuel interaction. Unneccessary parts for this view are omitted, and more details about heat, and other aspects can be provided in view's documentation.

Models and views can be more or less exhaustive and detailed, and always should be unambiguous.