In this session we will discuss
· Interface Segregation Principle
· Will look at a Case Study of Interface Segregation Principle
· And will implement Interface Segregation Principle with a simple example
In the first session of SOLID Introduction we have understood that "I" in SOLID is an acronym for Interface Segregation Principle
· The interface-segregation principle (ISP) states that "no client should be forced to depend on methods it does not use".
· This means, instead of one fat interface many small interfaces are preferred based on groups of methods with each one serving one sub-module.
· The ISP was first used and formulated by Robert C. Martin while consulting for Xerox.
Let us now understand how the ISP was evolved with a case study.
Case Study
Problem
Case Study
Problem
· As we all know Xerox Corporation manufactures printer systems. In their development process of new systems Xerox had created a new printer system that could perform a variety of tasks such as stapling and faxing along with the regular printing task.
· The software for this system was created from the ground up.
· As the software grew for Xerox, making modifications became more and more difficult so that even the smallest change would take a redeployment cycle of an hour, which made development nearly impossible.
· The design problem was that a single Job class was used by almost all of the tasks. Whenever a print job or a stapling job needed to be performed, a call was made to the Job class.
· This resulted in a 'fat' class with multitudes of methods specific to a variety of different clients.
Because of this design, a staple job would know about all the methods of the print job, even though there was no use for them.
Solution
Solution
· To overcome this problem Robert C Martin suggested a solution which is called the Interface Segregation Principle.
· Which means, Instead of one fat interface many small interfaces are preferred based on groups of methods with each one serving one sub-module.
Below example demonstrates how we can achieve Single Responsibility Principle
Code before Interface Segregation Principle.
Code before Interface Segregation Principle.
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namespace ISPDemoConsole
{
public interface IPrintTasks
{
bool PrintContent(string content);
bool ScanContent(string content);
bool FaxContent(string content);
bool PhotoCopyContent(string content);
bool PrintDuplexContent(string content);
}
}
namespace ISPDemoConsole.Client
{
class HPLaserJet : IPrintTasks
{
public bool FaxContent(string content)
{
Console.WriteLine("Fax Done"); return true;
}
public bool PhotoCopyContent(string content)
{
Console.WriteLine("PhotoCopy Done"); return true;
}
public bool PrintContent(string content)
{
Console.WriteLine("Print Done"); return true;
}
public bool PrintDuplexContent(string content)
{
Console.WriteLine("Print Duplex Done"); return true;
}
public bool ScanContent(string content)
{
Console.WriteLine("Scan Done"); return true;
}
}
}
namespace ISPDemoConsole.Client
{
class CannonMG2470 : IPrintTasks
{
public bool PhotoCopyContent(string content)
{
Console.WriteLine("PhotoCopy Done"); return true;
}
public bool PrintContent(string content)
{
Console.WriteLine("Print Done"); return true;
}
public bool ScanContent(string content)
{
Console.WriteLine("Scan Done"); return true;
}
public bool PrintDuplexContent(string content)
{
return false;
}
public bool FaxContent(string content)
{
return false;
}
}
}
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Code after Interface Segregation Principle
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namespace ISPDemoConsole.Client
{
class HPLaserJet : IPrintScanContent, IFaxContent, IPrintDuplex
{
public bool FaxContent(string content)
{
Console.WriteLine("Fax Done"); return true;
}
public bool PhotoCopyContent(string content)
{
Console.WriteLine("PhotoCopy Done"); return true;
}
public bool PrintContent(string content)
{
Console.WriteLine("Print Done"); return true;
}
public bool PrintDuplexContent(string content)
{
Console.WriteLine("Print Duplex Done"); return true;
}
public bool ScanContent(string content)
{
Console.WriteLine("Scan Done"); return true;
}
}
}
namespace ISPDemoConsole.Client
{
class CannonMG2470 : IPrintScanContent
{
public bool PhotoCopyContent(string content)
{
Console.WriteLine("PhotoCopy Done");
return true;
}
public bool PrintContent(string content)
{
Console.WriteLine("Print Done");
return true;
}
public bool ScanContent(string content)
{
Console.WriteLine("Scan Done");
return true;
}
}
}
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I believe this session has given you a good idea on how we can implement Interface segregation principle.