Interface Segregation

4 | The Interface Segregation Principle

The Interface Segregation Principle states the following:

No client should be forced to depend on methods it does not use.

This can be explained to mean the following - do not add new methods, with additional functionality to an existing interface if they are not going to be used. Alternatively create a new interface and allow objects to implement multiple interfaces as needed. Let’s take a look at some code to understand this further:

interface IPrinter
{
    void print(string text);
    string checkBlackInkLevels();
    string checkColorInkLevels();
}

Here we have an interface for a printer, IPrinter. It has 3 methods defined - print, checkBlackInkLevels and checkColorInkLevels. But what if I have BWLaserPrinter, a laser printer that prints in grayscale and only has black ink? The method checkColorInkLevels would be completely redundant, yet the interface would have forced the BWLaserPrinter object to implement a method which it has no use of. This would be in violation of the Interface Segregation Principle. Let’s fix this problem!

interface IPrinter
{
    void print(string text);
    string checkBlackInkLevels();
}

interface IColorPrinter
{
    string checkColorInkLevels();
}

The first step is to separate out the methods that do not apply to all implementations. Above I have created a new interface IColorPrinter which contains the checkColorInkLevels method signature. The original interface IPrinter now has two methods - print and checkBlackInkLevels, as all printers have these capabilities. Now let’s implement these interfaces:

public class BWLaserPrinter : IPrinter
{
    void print(string text)
    {
        //Send to LASER printer
    }
    string checkBlackInkLevels()
    {
        //Check BLACK ink
    }
}

Above the BWLaserPrinter object implements just one interface - IPrinter. It is a black and white printer, so only needs to have the two original methods.

public class InkJetColorPrinter : IPrinter, IColorPrinter
{
    void print(string text)
    {
        //Send to INKJET printer
    }
    string checkBlackInkLevels()
    {
        //Check BLACK ink
    }
    string checkColorInkLevels()
    {
        //Check COLOR ink
    }
}

Here is an example of using both interfaces. The InkJetColorPrinter object implements both IPrinter and IColorPrinter. It has the ability to check color and black ink levels. By creating a second interface, no methods are unused and the Interface Segregation Principle is followed!