SOLID- Single Responsibility Principle

The Single Responsibility principle (SRP) states that:

There should never be more than one reason for a class to change.

We can relate the “reason to change” to “the responsibility of the class”. So each responsibility would be an axis for change. This principle is similar to designing classes which are highly cohesive. So the idea is to design a class which has one responsibility or in otherwords caters to implementing a functionality . I would like to clarify here that one responsibility doesnt mean that the class has only ONE method. A responsibility can be implemented by means of different methods in the class.

Why is that this principle is required?

Imagine designing classes with more than one responsibility/implementing more than one functionality. There’s no one stopping you to do this. But imagine the amount of dependency your class can create within itself in the due course of the development time. So when you are asked to change a certain functionality, you are not really sure how it would impact the other functionalities implemented in the class. The change might or might not impact other features, but you really can’t take risk, especially in production applications. So you end up testing all the dependent features.

You might say, we have automated tests, and the number of tests to be checked are low, but imagine the impact over time. These kind of changes get accumulate owing to the viscosity of the code making it really fragile and rigid.

One way to correct the violation of SRP is to decompose the class functionalities into different classes, each of which confirms to SRP.

An example to clarify this principle:

Suppose you are asked to implement a UserSetting service where in the user can change the settings but before that the user has to be authenticated. One way to implement this would be:

public class UserSettingService
{
  public void changeEmail(User user)
  {
    if(checkAccess(user))
    {
       //Grant option to change
    }
  }
  public boolean checkAccess(User user)
  {
    //Verify if the user is valid.
  }
}

All looks good, until you would want to reuse the checkAccess code at some other place OR you want to make changes to the way checkAccess is being done OR you want to make change to the way email changes are being approved. In all the later 2 cases you would end up changing the same class and in the first case you would have to use UserSettingService to check for access as well, which is unnecessary.
One way to correct this is to decompose the UserSettingService into UserSettingService and SecurityService. And move the checkAccess code into SecurityService.

public class UserSettingService
{
  public void changeEmail(User user)
  {
    if(SecurityService.checkAccess(user))
    {
       //Grant option to change
    }
  }
}
public class SecurityService
{
  public static boolean checkAccess(User user)
  {
    //check the access.
  }
}

Another example would be:

Suppose there is a requirement to download the file- may be in csv/json/xml format, parse the file and then update the contents into a database or file system. One approach would be to-

public class Task
{
  public void downloadFile(location)
  {
    //Download the file
  }
  public void parseTheFile(file)
  {
    //Parse the contents of the file- XML/JSON/CSV
  }
  public void persistTheData(data)
  {
    //Persist the data to Database or file system.
  }
}

Looks good, all in one place easy to understand. But what about the number of times this class has to be updated? What about the reusability of parser code? or download code? Its not good design in terms of reusabiltiy of different parts of the code, in terms of cohesiveness.
One way to decompose the Task class is to create different classes for downloading the file- Downloader, for parsing the file- Parser and for persisting to the database or file system.

Even in JDK you must have seen that Rectangle2D or other Shape classes in java.awt package dont really have information regarding how it has to be drawn on the UI. The drawing information has been embedded in the Graphics/Graphics2D package.

A detailed description can be found here.