SOLID Principles



SOLID Principles

Many of the old programmers have adopted SOLID principles and it is taught in lots of schools these days. Yet I find many people not clearly understanding SOLID principles. I think these principles are fundamental to creating good object oriented software and every developer should learn them. So I have series of posts for these principles

Single Responsibility : A piece of software always have a single responsibility!
What this basically translates into is that a piece of code should have one and only one functionality. The corollary to this would be that  a piece of code should have only a single reason to change. Some advantages of this would be to ensure minimal change to existing classes in case of a change in requirements, which avoids larger testing efforts for a larger number of classes changed, which cuts down on testing efforts and increase turnaround times, which leads to lower overall cost of ownership for the code, which saves $$.

Take the example of a logging class. As a software developer, chances are pretty high that you’ve all at some point used some form of a logger, be it log4j, its .Net port log4net, Serilog, Elmah or a number of other logging frameworks that are out there. At their core, one thing all these libraries have in common is their focus one one thing. Logging messages. Be it to a flat file, RDBMS, other or other forms of persistence. At no point do these libraries attempt to do anything more than just that. Now a lot of you have also used the same libraries to do something seemingly different, such as  sending out emails for example. The beauty of their design however is that at their core the implementations of these libraries themselves do things at a slightly higher level of abstraction. Namely, they take data, including the message level / severity, to be logged (from a source) and deliver it to a sink (which in most cases happens to be a flat file). While writing to flat files is usually the default implementation of the data sink provided by the frameworks, at their true level of abstraction, they are simply conduits for data, providing solid low level implementations for supporting different logging levels based on configuration. Even the basic things that we take for granted, such as file lock management, rolling log files etc, aren’t truly in the hands of the core framework. These auxiliary features are handled by specific implementations of the log sinks, from DMBS connectors, to file writers, and even SMTP appenders. This brings us to the second principle.

Open-Closed Principle : Software should be open for extension and closed for modification. This weird sounding principle basically attempts to convey that code should be designed such that allows its behavior to be modified without actually modifying the source code for the class itself

Now, historically, this principle has been interpreted in a few different ways, but they have all relied on inheritance to achieve the purported goal.

One interpretation of this principle suggests that a class is ‘open’ for extension if its structure enables us to add new properties or functions in addition to the existing ones provided by the structure. The ‘closed’ part of this interpretation applies when the module by itself is available for other modules for use through its publicly available interface.

The other interpretation of the open closed principle (based on a polymorphic view) refers to the use of abstractions and interfaces to provide and extend certain behaviors, while keeping other (often core) implementation details hidden, and closed to change.

The logger example from the first solid principle is a prime example of this approach, where the implementations of the data sinks (often referred to as appenders) are often supported purely through configuration without touching the core conduit between the source and the sink. The fie appender writing to flat files however, still maintains the Single Responsibility principle by assuming responsibility for only writing to flat files, while also taking care of features and issues specific to flat files such as managing file locks, and often providing out of the box support for rolling log files that prevent log files from becoming too large to manage and view using ordinary text editors.

Liskov’s Substitution Principle : Child class object can be substituted for a base class variable. This one is a slightly more theoretical concept than the others, as unlike the others, it tries more to enforce consistency of behavior between parent and child classes than the syntax of the classes themselves.

The gist of the matter is that if S is a sub type of base type T, then all instances of T in a program should be replaceable with instances of type S without altering the desirable properties of the program. Note the use of the term ‘desirable’, which has a more semantic implication than, say the easier to observe consistency of method signatures that is often associated with overriding / hiding of method implementations in class hierarchies. Without going into the technical details of the precise rules proposed under this rule, it is safe to say that if you see a method or property in a sub type behaving drastically differently from the implementation of its parent, chances are that this principle is being violated in some form.

Interface Segregation: Many interfaces (client specific) is better than the single monolithic interface.

Dependency Inversion : Just depend on abstractions and not implementations.

Learning SOLID principles have changed my programming life and the real one. We shall see each of these principles in more detail in the coming posts.

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