Interface Segregation Principle

The Interface Segregation Principle (ISP) says that a class shouldn’t be forced to implement interfaces it doesn’t use.

In other words, it’s better to have smaller, specific interfaces instead of one large interface with methods that aren’t relevant to all implementations.

ISP violation example

Let’s look at an example of an ISP violation using a Device interface.

interface Device {
  powerOn(): void;
  reboot(): void;
  print(): void;
}

class Printer implements Device {
  powerOn() {...}

  reboot() {...}

  print() {...}
}

class Monitor implements Device {
  powerOn() {...}

  reboot() {...}

  print() {
    throw new Error("Monitor doesn't support printing!");
  }
}

The Device interface has method print() that only makes sense for devices that can print.

The Monitor class is now forced to implement print() method, which can either lead to errors or, in best case, unnecessary code.

Split the Interfaces

We can follow ISP by extracting methods from the Device interface into another interface:

interface Device {
  powerOn(): void;
  reboot(): void;
}

interface PrinterDevice extends Device {
  print(): void;
}

class Printer implements PrinterDevice {
  powerOn() {...}

  reboot() {...}

  print() {...}
}

class Monitor implements Device {
  powerOn() {...}

  reboot() {...}
}

Printer can implement both interfaces, while Monitor only implements the basic Device interface. This keeps the classes clean and avoids forcing them to implement unnecessary methods.

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