Written by
on June 2, 2023

Interface Segregation Principle: SOLID principles

Introduction:

In this tutorial, we’ll be discussing the Interface Segregation Principle, one of the SOLID principles. Representing the “I” in “SOLID”, interface segregation simply means that we should break larger interfaces into smaller ones. Thus ensuring that implementing classes need not implement unwanted methods.

What is the Interface Segregation Principle?

The Interface Segregation Principle states that clients should not be forced to depend on interfaces they do not use. In other words, we should strive to keep interfaces focused and cohesive, catering to specific sets of behaviors, rather than bundling unrelated methods together. This principle promotes loose coupling and improves the flexibility, maintainability, and reusability of the codebase.

Example Scenario: To better comprehend the ISP, let’s consider a hypothetical scenario of a document processing application. We have various types of documents, such as Word documents, PDFs, and Spreadsheets, each requiring different operations. Initially, we create a single interface, Document, to handle all document types.

    public interface Document {
        void open();
        void close();
        void save();
        void print();
    }

The Document interface contains methods for opening, closing, saving, and printing a document. However, this approach violates the ISP since not all document types support the same operations. For instance, a PDF document cannot be edited and should not have a save() method.

Applying the Interface Segregation Principle:

To adhere to the ISP, we should segregate the Document interface into more specialized interfaces based on the behaviors each document type supports. Let’s refactor our code accordingly:

    public interface Openable {
        void open();
        void close();
    }

    public interface Savable {
        void save();
    }

    public interface Printable {
        void print();
    }

    public interface Document {
        // No methods here
    }

    public class WordDocument implements Document, Openable, Savable, Printable {
        // Implement methods specific to Word documents
    }

    public class PDFDocument implements Document, Openable, Printable {
        // Implement methods specific to PDF documents
    }

    public class Spreadsheet implements Document, Openable, Savable {
        // Implement methods specific to Spreadsheets
    }

Now, we have separate interfaces that cater to the specific behaviors needed by each document type. The Openable interface defines methods for opening and closing a document, while the Savable interface provides the save() method for saving a document. Similarly, the Printable interface includes the print() method for printing a document.

Benefits of Applying the ISP:

By adhering to the Interface Segregation Principle, we achieve several advantages:

Modularity: Specialized interfaces make it easier to understand and reason about the code. Each interface represents a clear set of behaviors, promoting modularity.
Reduced Dependencies: Clients can depend on interfaces that contain only the methods they require. This reduces unnecessary dependencies and minimizes the impact of changes in other interfaces.
Flexibility: The ISP allows for easier extensibility by adding new document types or behaviors without affecting existing implementations.
Testability: Interfaces focused on specific behaviors facilitate unit testing, as clients can mock or stub only the required methods for testing.

Conclusion:

The Interface Segregation Principle (ISP) is a vital aspect of the SOLID principles in software development. By segregating interfaces into cohesive units based on behaviors, we can create flexible, maintainable, and reusable code. Applying the ISP enhances modularity, reduces dependencies, and improves the overall design of our applications.

← → Top