# Multiple Classes with Single responsibility VS Single Class with Multiple Responsibilities

While taking a session with a set of mobile developers, one of them asked a question that made many of them curious.

Why multiple classes when single class can do?

1. The main reasoning for single class was that one would be able to save some lines of code.
2. Also, one would be required to create just one object for multiple functionalities.

Lets look at some of the key advantages of multiple classes, each one of them having single functionality.

• High in cohesion, and hence reduced coupling. This tends to make the class reusable.
• Lesser costly to change than single class. Following explains the cost structure related with a change:

Let’s say a class is performing multiple different functions. Mathematically, the class can be represented as following:

Class A = f(x) + f(y) + f(z)

Let’s say, one needs to fix a bug related with f(x). Thus, one fixed the code in the class A that performed f(x).  The code was, then, shipped to QA engineers. QA engineers used to spend, let’s say, 1hour for each function. Thus, for a change in one function he spent 3 hours.

The total cost of change is 3 X (let’s say $20/hour as rate for QA engineer) =$60.

• On the other hand, if there were three different classes for three different functions, the cost of change would have been just \$15. (Think How?)

Thus, it makes much more sense to split functionality into different classes as the change will be less costlier than following the approach of having multiple functionality in one class. The design leading to this approach (multiple class) could also be termed as “Cost-effective” design.