界面隔離原理是什麼？

接口隔离原则（ISP）要求不强制客户端依赖未使用的接口。其核心是用多个小而精的接口替代大而全的接口。违反该原则的表现包括：类实现接口时抛出未实现异常、存在大量无效方法实现、无关功能被强行归入同一接口。应用方法包括：按常用方法组划分接口、依据客户端使用拆分接口、必要时使用组合替代多接口实现。例如将包含打印、扫描、传真方法的Machine接口拆分为Printer、Scanner和FaxMachine。在小型项目或所有客户端均使用全部方法时可适当放宽规则。

The Interface Segregation Principle (ISP) is one of the five SOLID principles of object-oriented design. Simply put, it states that clients should not be forced to depend on interfaces they don’t use. In practical terms, this means it’s better to have many smaller, specific interfaces rather than one large, general-purpose interface.

Why It Matters in Real-World Code

Think of a scenario where you have a class that implements an interface with ten methods, but your class only needs three of them. If you're forced to implement all ten, you end up writing empty or throwaway implementations for the rest — which clutters your code and makes maintenance harder.

This principle helps reduce side effects from changes, keeps dependencies clean, and improves maintainability by ensuring classes only interact with what they need.

Signs You’re Violating ISP

Here are some common symptoms of violating the Interface Segregation Principle:

• Your class implements an interface but throws NotImplementedException for some methods.
• You see a lot of “dummy” method implementations just to satisfy an interface.
• Multiple unrelated features are grouped into one interface, forcing clients to deal with unnecessary dependencies.

If any of these sound familiar, it might be time to break that big interface into smaller, more focused ones.

How to Apply ISP Effectively

Applying ISP doesn’t mean creating a new interface for every single method — that would overcomplicate things. Instead, focus on grouping related behaviors logically. Here’s how:

• Identify groups of methods in a large interface that are commonly used together.
• Split the interface based on client usage — if two clients use different parts, split them into two interfaces.
• Use composition when needed — instead of one class implementing multiple interfaces, consider combining smaller services.

For example, instead of having one Machine interface with print(), scan(), and fax(), create separate Printer, Scanner, and FaxMachine interfaces. That way, a basic printer doesn’t have to pretend it can fax.

When It’s Okay to Bend the Rules

Like most design principles, ISP isn’t absolute. Sometimes merging interfaces makes sense for simplicity or performance reasons — especially in small projects or prototypes where over-engineering adds overhead. Also, if all clients truly do use all the methods, there’s no harm keeping a larger interface.

But as a rule of thumb: if you notice certain methods being ignored or faked in implementations, it’s a good time to revisit your interface design.

Basically, that's the idea behind ISP — keep interfaces lean, relevant, and respectful of the classes that use them.

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