Introduction

"Clients should not be forced to depend on methods they do not use."

In simpler terms, ISP encourages the creation of small, specific interfaces rather than large, monolithic ones. A client (class or object) should only have to implement the methods that are relevant to its purpose.

ISP violation

Example: Printer System

Imagine you are building a system for managing various types of printers in an office.
Initially, you might have a general Printer interface that includes methods for printing, scanning, faxing, and copying.
However, not all printers support all of these functionalities — for instance, some printers may only print and copy but do not have fax or scan capabilities.

Violation of ISP code

        // Fat Interface: Violates ISP
    interface Printer {
        void print(String document);
        void scan(String document);
        void fax(String document);
        void copy(String document);
    }

    // SimplePrinter that does not need fax or scan functionality
    class SimplePrinter implements Printer {
        @Override
        public void print(String document) {
            System.out.println("Printing: " + document);
        }

        @Override
        public void scan(String document) {
            throw new UnsupportedOperationException("SimplePrinter does not support scanning");
        }

        @Override
        public void fax(String document) {
            throw new UnsupportedOperationException("SimplePrinter does not support faxing");
        }

        @Override
        public void copy(String document) {
            System.out.println("Copying: " + document);
        }
    }

    // MultiFunctionPrinter that supports all functionalities
    class MultiFunctionPrinter implements Printer {
        @Override
        public void print(String document) {
            System.out.println("Printing: " + document);
        }

        @Override
        public void scan(String document) {
            System.out.println("Scanning: " + document);
        }

        @Override
        public void fax(String document) {
            System.out.println("Faxing: " + document);
        }

        @Override
        public void copy(String document) {
            System.out.println("Copying: " + document);
        }
    }
    

Problem (ISP Violation)

The SimplePrinter class, which is a basic printer, does not support scanning or faxing. However, because it implements the Printer interface, it is forced to provide implementations for these methods. This leads to the unnecessary implementation of methods like scan() and fax(), which either throw exceptions or remain unimplemented.

How to achieve ISP

To comply with the Interface Segregation Principle, we should break the large, monolithic Printer interface into smaller, more specific interfaces. This way, each printer class only implements the interfaces relevant to its capabilities.

// Segregated Interfaces adhering to ISP
interface Printable {
    void print(String document);
}

interface Scannable {
    void scan(String document);
}

interface Faxable {
    void fax(String document);
}

interface Copyable {
    void copy(String document);
}

// SimplePrinter that only supports printing and copying
class SimplePrinter implements Printable, Copyable {
    @Override
    public void print(String document) {
        System.out.println("Printing: " + document);
    }

    @Override
    public void copy(String document) {
        System.out.println("Copying: " + document);
    }
}

// MultiFunctionPrinter that supports all functionalities
class MultiFunctionPrinter implements Printable, Scannable, Faxable, Copyable {
    @Override
    public void print(String document) {
        System.out.println("Printing: " + document);
    }

    @Override
    public void scan(String document) {
        System.out.println("Scanning: " + document);
    }

    @Override
    public void fax(String document) {
        System.out.println("Faxing: " + document);
    }

    @Override
    public void copy(String document) {
        System.out.println("Copying: " + document);
    }
}
    

Key Points in the refactored design:

1. Small, Specific Interfaces: We have now segregated the functionalities into smaller, more specific interfaces: Printable, Scannable, Faxable, and Copyable. This way, printer classes only implement the functionalities they actually support.
2. SimplePrinter Class: The SimplePrinter now implements only the Printable and Copyable interfaces, which are the functionalities it supports. It no longer needs to provide implementations for scan() and fax(), which are irrelevant to its purpose.
3. MultiFunctionPrinter Class: The MultiFunctionPrinter implements all four interfaces (Printable, Scannable, Faxable, Copyable), as it supports all the functionalities.