Grokking SOLID Design Principles
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Restructuring the code to follow ISP
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In the previous lesson, we saw how a bloated Printer interface violated the Interface Segregation Principle (ISP) by forcing classes to implement methods they didn’t need. To solve this, we will refactor the code to create smaller, more specific interfaces that only define the methods needed by each type of printer.

Step 1: Break the Large Interface into Smaller Interfaces

Instead of having a single large Printer interface, we can split it into more focused interfaces. Each interface will represent a specific functionality, allowing classes to implement only what they need.

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Now, we have four separate interfaces: Printer for printing, Scanner for scanning, Fax for faxing, and Stapler for stapling. This way, classes only need to implement the interfaces they require.

Step 2: Implementing BasicPrinter

Let’s refactor the BasicPrinter class so it only implements the Printer interface. Since the BasicPrinter only needs to print documents, we no longer have to worry about unnecessary methods like scanning or faxing.

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The BasicPrinter class is now clean and focused. It doesn’t need to implement any methods it doesn’t use, which simplifies the code and avoids throwing exceptions for unsupported operations.

Step 3: Implementing AdvancedPrinter

For an AdvancedPrinter that supports all functionalities (printing, scanning, faxing, and stapling), we can implement multiple interfaces. This allows the class to inherit only the behaviors it needs, without being forced to implement everything from a single large interface.

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The AdvancedPrinter now implements all the relevant interfaces (Printer, Scanner, Fax, and Stapler). This allows the class to handle each task while keeping the design clean and modular.

Step 4: Test the New Design

Here’s a quick test to see how the new design works with different types of printers.

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Why This Follows ISP?

By restructuring the code:

  • Smaller, Focused Interfaces: Each interface now represents a specific task (printing, scanning, faxing, or stapling). This allows classes to implement only what they need, following the Interface Segregation Principle.
  • More Flexible Design: Classes like BasicPrinter only need to implement the Printer interface, avoiding unnecessary methods. Meanwhile, AdvancedPrinter can implement multiple interfaces to support a range of functionalities.
  • Cleaner, Easier to Maintain Code: The code is easier to understand and maintain since each class focuses on the exact responsibilities it needs to fulfill.

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