Interface Segregation Principle¶

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

“YOU SHOULD ONLY DEFINE METHODS THAT ARE GOING TO BE USED”.

This principle basically says that it's better to create more specific interfaces rather than having a single generic interface.

Wrong:

interface IMultiFunction {
    public void print();
    public void getPrintSpoolDetails();
    public void scan();
    public void scanPhoto();
    public void fax();
    public void internetFax();
}

class CanonPrinter implements IMultiFunction {

    @Override
    public void print() {}

    @Override
    public void getPrintSpoolDetails() {}

    /* This machine can't scan */
    @Override
    public void scan() {}

    /* This machine can't scan photo */
    @Override
    public void scanPhoto() {}

    /* This machine can't fax */
    @Override
    public void fax() {}

     /* This machine can't fax on internet */
    @Override
    public void internetFax() {}

}

A class should not be required to implement interfaces that it will not use.

Correct:

interface Movie {
    public function play();
}
interface AudioControl {
    public function increaseVolume();
}

class TheLionKing implements Movie, AudioControl {
    public function play() {}
    public function increaseVolume() {}
}

class ModernTimes implements Movie {
    public function play() {}
}