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In many codebases, classes are forced to implement methods they don’t actually need. These methods may remain empty, throw exceptions, or exist only to satisfy an interface contract. While the code may compile, the design slowly becomes harder to understand, maintain, and evolve.
The Interface Segregation Principle (ISP) addresses this problem by stating that clients should not be forced to depend on methods they do not use. Instead of large, general-purpose interfaces, ISP encourages designing smaller, focused interfaces that serve specific client needs.
This principle shifts the focus from how classes are implemented to how they are used. By aligning interfaces with actual client behavior, ISP reduces unnecessary coupling and makes systems easier to change and extend.
In this part of our SOLID Principles blog series, we explore how overly broad interfaces lead to fragile designs, how ISP helps avoid this, and how it works alongside principles like LSP and OCP to create clean, maintainable software systems.
The Interface Segregation Principle (ISP) states that “Clients should not be forced to depend on interfaces they do not use”. In simpler terms, a class should only be required to implement methods that are actually relevant to it. Large, “all-in-one” interfaces often force classes to include unnecessary methods, leading to confusing designs and fragile code.
ISP encourages breaking down broad interfaces into smaller, more specific interfaces, each focused on a single responsibility from the client’s point of view. Now, let’s understand this with an example. Consider a media player application that needs to handle different types of players. An initial design might use a single interface for both audio and video functionality:
interface MediaPlayer {
playAudio(): void;
recordAudio(): void;
}A class representing an audio player might implement this interface:
class VideoPlayer implements MediaPlayer {
playAudio() {
// Play audio track of video
}
recordAudio() {
// Irrelevant — video player does not record audio
}
}Here, the VideoPlayer class is forced to implement recordAudio(), even though it doesn’t make sense for its behavior. This violates the Interface Segregation Principle because the class depends on a method it doesn’t use.
To fix this, we create smaller, more focused interfaces that match each client’s real needs
interface AudioPlayable {
playAudio(): void;
}
interface AudioRecordable {
recordAudio(): void;
}
interface VideoPlayable {
playVideo(): void;
}Now each class only implements the interfaces it truly cares about:
class AudioPlayer implements AudioPlayable, AudioRecordable {
playAudio() {
// Play audio
}
recordAudio() {
// Record audio
}
}
class VideoPlayer implements VideoPlayable {
playVideo() {
// Play video content
}
}With this structure:
This example clearly shows how an ISP helps prevent unnecessary dependencies and creates cleaner abstractions.
The Interface Segregation Principle is a powerful guideline for designing clean and flexible interfaces. However, applying ISP everywhere without context can lead to unnecessary complexity. Knowing when not to apply ISP is essential for making good design decisions.
If an interface contains only a few closely related methods and all implementing classes genuinely use them, splitting it further provides little value. Over-segmentation can make the design harder to understand rather than clearer. For example, an interface with two tightly related methods used by all implementations does not need to be broken down further.
If the interface represents a well-defined contract that is unlikely to evolve, introducing multiple smaller interfaces may be unnecessary. In such cases, a single interface can be easier to maintain and reason about.
ISP becomes more valuable when different clients evolve at different speeds.
In early development or educational projects, clarity and speed often matter more than perfect abstraction. Applying ISP too early can slow down progress and distract from understanding the core problem.
It’s often better to refactor toward ISP once real usage patterns emerge.
Too many tiny interfaces can overwhelm readers, especially students or new team members. If applying ISP makes the design harder to follow, it may be better to keep the interface slightly broader.
Good design improves readability, not just theoretical purity.
ISP exists to prevent clients from depending on unnecessary methods. If no client is being forced to implement or depend on unused behavior, introducing segmentation may not solve a real problem.
The Interface Segregation Principle is a design guideline, not a strict rule. Apply it when interfaces grow too large or serve multiple unrelated clients, but avoid it when it introduces complexity without clear benefits. Effective software design is about responding to real problems, not anticipating imaginary ones.
At its core, the Interface Segregation Principle reminds us that clients shouldn’t be burdened with methods they never use.
By designing smaller, focused interfaces, we reduce unnecessary coupling, improve flexibility, and make systems easier to evolve. ISP helps keep our codebase clean by ensuring each interface has a clear purpose and serves a specific client need. Nothing more, nothing less.
At the same time, ISP reminds us that good design is about balance. Over-segmentation can be just as harmful as bloated interfaces. The real skill lies in recognizing when an interface is becoming a liability and refactoring it at the right moment.
So far in the SOLID series, we’ve focused on how responsibilities are structured and how contracts are designed. But there’s still one crucial question left: “Who should depend on whom?” This is where the Dependency Inversion Principle (DIP) comes in. Read our next blog to understand what DIP is and how it helps you in system design.
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