Interface Segregation Principle

// ❌ ISP VIOLATION: Fat interface forces unwanted methods

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

// Old printer that only prints - forced to implement everything
class OldPrinter implements IMultiFunctionDevice {
    @Override
    public void print(Document doc) {
        // Can print
        System.out.println("Printing: " + doc.getName());
    }

    @Override
    public void scan(Document doc) {
        // Cannot scan - what to do?
        throw new UnsupportedOperationException("Cannot scan!");
    }

    @Override
    public void fax(Document doc) {
        // Cannot fax - forced to implement anyway
        throw new UnsupportedOperationException("Cannot fax!");
    }

    @Override
    public void photocopy(Document doc) {
        // Cannot photocopy
        throw new UnsupportedOperationException("Cannot photocopy!");
    }
}

// Client code might call any method - runtime errors!
public void processDocument(IMultiFunctionDevice device, Document doc) {
    device.print(doc);
    device.scan(doc);  // Crash if OldPrinter!
}

// ✅ ISP COMPLIANT: Segregated interfaces

interface IPrinter {
    void print(Document doc);
}

interface IScanner {
    void scan(Document doc);
}

interface IFax {
    void fax(Document doc);
}

// Old printer implements only what it can do
class OldPrinter implements IPrinter {
    @Override
    public void print(Document doc) {
        System.out.println("Printing: " + doc.getName());
    }
    // No need to implement scan/fax!
}

// Modern device implements multiple interfaces
class MultiFunctionPrinter implements IPrinter, IScanner, IFax {
    @Override
    public void print(Document doc) {
        System.out.println("Printing: " + doc.getName());
    }

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

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

// Clients depend only on what they need
class PrintService {
    private IPrinter printer;  // Only needs print capability

    public PrintService(IPrinter printer) {
        this.printer = printer;
    }

    public void printDocument(Document doc) {
        printer.print(doc);  // Safe - guaranteed to work
    }
}

// Usage
public static void main(String[] args) {
    // Both work as printers
    IPrinter oldPrinter = new OldPrinter();
    IPrinter modernPrinter = new MultiFunctionPrinter();

    PrintService service = new PrintService(oldPrinter);
    service.printDocument(new Document("report.pdf"));

    // Scanner service only accepts IScanner
    ScanService scanService = new ScanService(new MultiFunctionPrinter());
    // scanService = new ScanService(oldPrinter);  // Compile error! Good!
}