Open/Closed Principle (SOLID 2/6)

"Software entities should be open for extension, but closed for modification" is the definition of open/closed principle. I learned a lot while writing this blog post. For example I learned what relationship does open/closed principle have with strategy pattern. I will explain also it beside I explain what is the open/closed principle. This is the 2/5 blog post from SOLID serie. It was worth to write!