Interface Segregation Principle (ISP) – Complete Guide

What is ISP?

The Interface Segregation Principle (ISP) is a core concept in SOLID programming. It states that clients should not be forced to depend on methods they do not use. This ensures that interfaces are specific and cohesive, making the system easier to maintain and extend.

Why ISP is Important

• Prevents large, unwieldy interfaces.
• Reduces unnecessary dependencies.
• Makes code easier to maintain and test.
• Supports modular and clean design.

Stepwise Implementation of ISP

1. Step 1 – Identify Fat Interfaces: Find interfaces that have methods unrelated to all clients.
2. Step 2 – Split into Smaller Interfaces: Break the large interface into multiple specific interfaces.
3. Step 3 – Implement Only Needed Interfaces: Update classes to implement only relevant interfaces.
4. Step 4 – Test Behavior: Ensure the system works as expected with the new interfaces.
5. Step 5 – Refactor if Necessary: Continuously improve interfaces for better cohesion.

Example: ISP in Action

Consider a multifunction printer:

• Separate interfaces: Printer, Scanner, Fax
• Classes implement only what they need:

interface Printer {
    void print(Document doc);
}

interface Scanner {
    void scan(Document doc);
}

interface Fax {
    void fax(Document doc);
}

class BasicPrinter implements Printer {
    public void print(Document doc) {
        // print logic
    }
}

class MultiFunctionMachine implements Printer, Scanner, Fax {
    public void print(Document doc) { /* print logic */ }
    public void scan(Document doc) { /* scan logic */ }
    public void fax(Document doc) { /* fax logic */ }
}
      

This approach keeps interfaces lean and classes focused, following ISP principles.