Practical Examples of Interface Segregation Principle

Interface Segregation Principle (ISP)

Practical Examples

Example 1: Good Design

Here is a code snippet showing an interface that adheres to ISP by having smaller, client-specific interfaces:

Java

interface Printer {
    void print(Document doc);
}

interface Scanner {
    void scan(Document doc);
}

class MultiFunctionPrinter implements Printer, Scanner {
    @Override
    public void print(Document doc) {
        System.out.println("Printing document");
    }

    @Override
    public void scan(Document doc) {
        System.out.println("Scanning document");
    }
}

class SimplePrinter implements Printer {
    @Override
    public void print(Document doc) {
        System.out.println("Printing document");
    }
}

JavaScript

class Printer {
    print(doc) {
        console.log("Printing document");
    }
}

class Scanner {
    scan(doc) {
        console.log("Scanning document");
    }
}

class MultiFunctionPrinter extends Printer, Scanner {
    print(doc) {
        console.log("Printing document");
    }

    scan(doc) {
        console.log("Scanning document");
    }
}

class SimplePrinter extends Printer {
    print(doc) {
        console.log("Printing document");
    }
}

Python

class Printer:
    def print(self, doc):
        print("Printing document")

class Scanner:
    def scan(self, doc):
        print("Scanning document")

class MultiFunctionPrinter(Printer, Scanner):
    def print(self, doc):
        print("Printing document")

    def scan(self, doc):
        print("Scanning document")

class SimplePrinter(Printer):
    def print(self, doc):
        print("Printing document")

Example 2: Violation

Here is a code snippet where a single interface is overloaded with methods, violating ISP:

Java

interface Machine {
    void print(Document doc);
    void scan(Document doc);
    void fax(Document doc);
}

class MultiFunctionPrinter implements Machine {
    @Override
    public void print(Document doc) {
        System.out.println("Printing document");
    }

    @Override
    public void scan(Document doc) {
        System.out.println("Scanning document");
    }

    @Override
    public void fax(Document doc) {
        System.out.println("Faxing document");
    }
}

class SimplePrinter implements Machine {
    @Override
    public void print(Document doc) {
        System.out.println("Printing document");
    }

    @Override
    public void scan(Document doc) {
        // Not implemented
    }

    @Override
    public void fax(Document doc) {
        // Not implemented
    }
}

JavaScript

class Machine {
  print(doc) {
    throw new Error('This method should be overridden')
  }

  scan(doc) {
    throw new Error('This method should be overridden')
  }

  fax(doc) {
    throw new Error('This method should be overridden')
  }
}

class MultiFunctionPrinter extends Machine {
  print(doc) {
    console.log('Printing document')
  }

  scan(doc) {
    console.log('Scanning document')
  }

  fax(doc) {
    console.log('Faxing document')
  }
}

class SimplePrinter extends Machine {
  print(doc) {
    console.log('Printing document')
  }

  scan(doc) {
    // Not implemented
  }

  fax(doc) {
    // Not implemented
  }
}

Python

class Machine:
    def print(self, doc):
        raise NotImplementedError("This method should be overridden")

    def scan(self, doc):
        raise NotImplementedError("This method should be overridden")

    def fax(self, doc):
        raise NotImplementedError("This method should be overridden")

class MultiFunctionPrinter(Machine):
    def print(self, doc):
        print("Printing document")

    def scan(self, doc):
        print("Scanning document")

    def fax(self, doc):
        print("Faxing document")

class SimplePrinter(Machine):
    def print(self, doc):
        print("Printing document")

    def scan(self, doc):
        # Not implemented
        pass

    def fax(self, doc):
        # Not implemented
        pass

Refactor Example 2

Refactor the violating example to split the interface into smaller, client-specific interfaces:

Java

interface Printer {
    void print(Document doc);
}

interface Scanner {
    void scan(Document doc);
}

interface Fax {
    void fax(Document doc);
}

class MultiFunctionPrinter implements Printer, Scanner, Fax {
    @Override
    public void print(Document doc) {
        System.out.println("Printing document");
    }

    @Override
    public void scan(Document doc) {
        System.out.println("Scanning document");
    }

    @Override
    public void fax(Document doc) {
        System.out.println("Faxing document");
    }
}

class SimplePrinter implements Printer {
    @Override
    public void print(Document doc) {
        System.out.println("Printing document");
    }
}

JavaScript

class Printer {
    print(doc) {
        console.log("Printing document");
    }
}

class Scanner {
    scan(doc) {
        console.log("Scanning document");
    }
}

class Fax {
    fax(doc) {
        console.log("Faxing document");
    }
}

class MultiFunctionPrinter extends Printer, Scanner, Fax {
    print(doc) {
        console.log("Printing document");
    }

    scan(doc) {
        console.log("Scanning document");
    }

    fax(doc) {
        console.log("Faxing document");
    }
}

class SimplePrinter extends Printer {
    print(doc) {
        console.log("Printing document");
    }
}

Python

class Printer:
    def print(self, doc):
        print("Printing document")

class Scanner:
    def scan(self, doc):
        print("Scanning document")

class Fax:
    def fax(self, doc):
        print("Faxing document")

class MultiFunctionPrinter(Printer, Scanner, Fax):
    def print(self, doc):
        print("Printing document")

    def scan(self, doc):
        print("Scanning document")

    def fax(self, doc):
        print("Faxing document")

class SimplePrinter(Printer):
    def print(self, doc):
        print("Printing document")

By refactoring the large interface into smaller, client-specific interfaces, we adhere to the Interface Segregation Principle, leading to a more modular, maintainable, and flexible design.