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 ].
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