SOLID are five basic principles which help to create good software architecture. SOLID is an acronym where:
1) S stands for SRP (Single responsibility principle)
2) O stands for OCP (Open closed principle)
3) L stands for LSP (Liskov substitution principle)
4) I stands for ISP (Interface segregation principle)
5) D stands for DIP (Dependency inversion principle)
These principles, when combined together, make it easy for a programmer to develop software that are easy to maintain and extend. They also make it easy for developers to avoid code smells, easily refactor code, and are also a part of the agile or adaptive software development.
Single responsibility principle:
A class should have one and only one reason to change, meaning that a class should have only one job.
There should never be more than one reason for a class to change. Basically, this means that your classes should exist for one purpose only. For example, let's say you are creating a class to represent a SalesOrder. You would not want that class to save to the database, as well as export an XML-based receipt. Why? Well if later on down the road, you want to change database type (or if you want to change your XML schema), you're allowing one responsibility's changes to possibly alter another. Responsibility is the heart of this principle, so to rephrase there should never be more than one responsibility per class.
The Open Closed Principle:
Objects or entities should be open for extension, but closed for modification.
Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification. At first, this seems to be contradictory: how can you make an object behave differently without modifying it? The answer: by using abstractions, or by placing behavior (responsibility) in derivative classes. In other words, by creating base classes with override-able functions, we are able to create new classes that do the same thing differently without changing the base functionality. Further, if properties of the abstracted class need to be compared or organized together, another abstraction should handle this. This is the basis of the "keep all object variables private" argument.
The Liskov Substitution Principle:
Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it.
In other words, if you are calling a method defined at a base class upon an abstracted class, the function must be implemented properly on the subtype class. Or, "when using an object through its base class interface, [the] derived object must not expect such users to obey preconditions that are stronger than those required by the base class." The ever-popular illustration of this is the square-rectangle example. Turns out a square is not a rectangle, at least behavior-wise.
The Interface Segregation Principle:
A client should never be forced to implement an interface that it doesn’t use or clients shouldn’t be forced to depend on methods they do not use.
Clients should not be forced to depend upon interfaces that they do not use. My favorite version of this is written as "when a client depends upon a class that contains interfaces that the client does not use, but that other clients do use, then that client will be affected by the changes that those other clients force upon the class." Kinda sounds like the inheritance specific single responsibility principle.
The Dependency Inversion Principle:
Entities must depend on abstractions not on concretions. It states that the high level module must not depend on the low level module, but they should depend on abstractions.
Depend on abstractions, not on concretions or High level modules should not depend upon low level modules. Both should depend upon abstractions. Abstractions should not depend upon details. Details should depend upon abstractions. (I like the first explanation the best.) This is very closely related to the open closed principle we discussed earlier. By passing dependencies (such as connectors to devices, storage) to classes as abstractions, you remove the need to program dependency specific. Here's an example: an Employee class that needs to be able to be persisted to XML and a database. If we placed ToXML() and ToDB() functions in the class, we'd be violating the single responsibility principle. If we created a function that took a value that represented whether to print to XML or to DB, we'd be hard-coding a set of devices and thus be violating the open closed principle. The best way to do this would be to:
1) Create an abstract class (named DataWriter, perhaps) that can be inherited from for XML (XMLDataWriter) or DB (DbDataWriter) Saving, and then
2) Create a class (named EmployeeWriter) that would expose an Output(DataWriter saveMethod) that accepts a dependency as an argument. See how the Output method is dependent upon the abstractions just as the output types are? The dependencies have been inverted. Now we can create new types of ways for Employee data to be written, perhaps via HTTP/HTTPS by creating abstractions, and without modifying any of our previous code! No rigidity--the desired outcome.