What is the Interface Segregation Principle (ISP)?

Definition
Interface Segregation Principle (ISP) states that clients should not be forced to depend on interfaces they do not use. Instead of one fat interface, prefer multiple smaller, focused interfaces.
ISP is the fourth principle in SOLID. It addresses the problem of bloated interfaces that force implementing classes to provide empty or meaningless implementations for methods they do not need.
Core Problem It SolvesWhen an interface grows too large with many methods, classes that implement it must provide all methods even if they only need a subset. This creates several issues:
First, implementing classes carry unnecessary baggage with stub methods that throw exceptions or do nothing.
Second, changes to unused methods still require recompilation of all implementing classes.
Third, clients that depend on the interface see methods they cannot or should not call, violating the Principle of Least Surprise.
The SolutionSplit large interfaces into smaller, role-specific interfaces. Each interface represents a cohesive set of behaviors. Implementing classes choose only the interfaces relevant to their responsibilities.
Interview Tip: Always explain ISP in terms of client perspective, not just implementer perspective. The principle protects clients from seeing methods they should not call.

💡 Key Takeaways

✓Clients should not depend on interfaces they do not use

✓Split fat interfaces into multiple focused interfaces

✓Each interface should represent one cohesive role or capability

✓Implementing classes implement only interfaces they actually need

✓Protects clients from coupling to irrelevant methods

📌 Examples

1A Robot class should not implement an Eat interface meant for animals

2A PaymentProcessor should not implement scheduling methods if it only processes payments

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