Interface Segregation Principle (ISP) in SOLID with Java Example

 The Interface Segregation Principle (ISP) is one of the five SOLID principles of object-oriented design and development. In Java, the ISP promotes the idea that:

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

This means interfaces should be specific and fine-grained rather than large and general. Clients should not be required to implement methods they don't need.

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🔧 Why It Matters

If an interface has too many methods, implementing classes may end up with empty or meaningless method implementations, which leads to rigid, fragile, and hard-to-maintain code.

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✅ Good Example – Following ISP

interface Printer {
    void print(String content);
}

interface Scanner {
    void scan(String document);
}

class CanonPrinter implements Printer {
    @Override
    public void print(String content) {
        System.out.println("Printing: " + content);
    }
}

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

Here, a class only implements the interface it actually needs, promoting separation of concerns.

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❌ Bad Example – Violating ISP

interface MultiFunctionDevice {
    void print(String content);
    void scan(String document);
    void fax(String document);
}

class OldPrinter implements MultiFunctionDevice {
    @Override
    public void print(String content) {
        System.out.println("Printing: " + content);
    }

    @Override
    public void scan(String document) {
        // Not supported
        throw new UnsupportedOperationException("Scan not supported");
    }

    @Override
    public void fax(String document) {
        // Not supported
        throw new UnsupportedOperationException("Fax not supported");
    }
}

Here, OldPrinter is forced to implement methods it doesn't support. This violates ISP.

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✅ Solution with ISP using Interface Composition

interface Printer {
    void print(String content);
}

interface Scanner {
    void scan(String document);
}

interface Fax {
    void fax(String document);
}

class ModernPrinter implements Printer, Scanner, Fax {
    @Override
    public void print(String content) {
        System.out.println("Printing: " + content);
    }

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

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

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Summary

• ISP encourages creating smaller, specific interfaces.
• Helps in building decoupled, modular, and easy-to-maintain code.
• Supports flexibility and clean architecture.

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Thanks for reading! 🎉 I'd love to know what you think about the article. Did it resonate with you? 💭 Any suggestions for improvement? I’m always open to hearing your feedback so that I can improve my posts! 👇🚀. Happy coding! 💻✨