In our daily development work, these two terms often appear together. Sometimes they seem interchangeable, while other times they feel quite different. This ambiguity stems from the fact that while conceptually closely related, they represent different perspectives. Simply put, "interface" is a broader, more fundamental concept, while "API" is a specific, typical implementation of an interface in software development and system interaction.

To clarify this relationship, we need to take a step back and examine the true meaning of "interface." In the real world, any boundary point where one object connects with another can be considered an interface. A power outlet is the interface between an appliance and the power grid, and a USB port is the interface between a peripheral device and a computer. Its core function is to define a clear and finite set of interaction rules. As long as both parties adhere to these rules, collaboration can proceed smoothly without needing to understand the other's complex internal structure. For example, when charging your phone, you only need to know to plug in the Type-C cable; you don't need to understand the phone's internal battery chemistry.

Transferring this concept to the software world, the essence of an interface remains unchanged. It remains a communication contract agreed upon between two independent parts. This "independent part" can be very small, such as a class, or very large, such as a complete operating system. At the programming language level, an interface (such as `interface` in Java or `interface` in Go) is a syntactic feature that mandates which methods a class or struct must implement, thus making code depend on the contract rather than specific implementations. This improves the flexibility and maintainability of the program. Below is a simple example of a Java interface.

Java

// This is an "interface" (language feature)

public interface DataStore {

void save(String data);

String load(String id);

}

// This is a class that implements the above interface

public class DatabaseStore implements DataStore {

@Override

public void save(String data) {

// The specific database storage logic

System.out.println("Saving to database: " + data);

}

@Override

public String load(String id) {

// The specific database read logic

return "Data from DB for id: " + id;

}

}

In this example, the `DataStore` interface defines two functions: `save` and `load`. Any class that implements this interface, such as `DatabaseStore`, must provide the concrete code for these two methods. Other parts of the program can rely solely on the `DataStore` interface type for operation. Whether it uses a database, files, or memory behind the scenes can be changed at any time; this is the decoupling benefit of interfaces.

So, what is an API? API is short for "Application Programming Interface." It specifically refers to the interface provided by a software system or component to external developers or programs. The key words are "application" and "programming," meaning that APIs are typically at the code level, allowing programs (not end users) to call services. When we talk about APIs, we are most often referring to interfaces that are remotely invoked over a network, such as HTTP-based REST APIs or TCP-based RPC interfaces. It encapsulates complex internal business logic, exposing only a set of clearly defined operation endpoints for integration by external systems.

A typical Web API, such as a weather information service, might provide an HTTP endpoint.

GET https://api.weather.com/v3/forecast?location=beijing&apikey=YOUR_KEY

After the external program sends this request, the weather service internally performs complex data collection, calculations, and formatting, but ultimately only returns a concise JSON data packet to the caller. The caller doesn't need to know how the weather model works; they only need to understand how to construct the request and how to parse the returned JSON. This is the core value of an API: hiding complexity and providing standardized access capabilities.

Therefore, when we compare APIs with interfaces in the general sense, several key differences emerge. First is the level of abstraction and use cases. An interface is a fundamental abstract concept in computer science, existing at all levels from hardware to software. A network card has a hardware interface, an operating system has a system call interface, and a class library has a function interface. APIs typically refer to higher-level, application- or service-oriented interfaces, especially those used for cross-process and cross-network calls, such as Web APIs, operating system APIs (like the Windows API), or interfaces provided by third-party service SDKs.

Second is the scope of communication boundaries. An interface defines a contract that can be used between modules within a system (such as calls between Java classes as mentioned above) or between external systems. While an API can theoretically be used internally, the term emphasizes its "external" and "open" attributes; that is, a system consciously exposes some of its functionality for others to integrate. For example, when we say "WeChat Pay API," we are referring to the set of interfaces that the WeChat Pay system exposes to merchant applications.

Furthermore, there's the difference in technical form. Interfaces in programming languages ​​have specific syntax keywords (`interface`, `protocol`). APIs, however, take many forms: they might be a detailed document (such as the OpenAPI specification), an SDK toolkit, or simply a specific HTTP URL and its request format. You can think of a well-designed API document as a detailed description of that "interaction contract."

Understanding these differences allows us to use terminology more precisely in daily communication and design. When we discuss how a class or module should be called by other code, we are designing its interface. When we discuss how to securely and stably expose internal company data services to partner applications for integration, we are designing an API. The latter, besides considering functional contracts, also needs to address a series of challenges unique to distributed systems, such as network protocols, authentication, rate limiting, version management, and monitoring and maintenance.

In summary, interfaces are the "way," the core idea in software engineering for achieving modularity and reducing coupling. APIs are the "techniques," the specific practice and product of this idea in cross-system and cross-network integration scenarios. All APIs are interfaces, but not all interfaces can be called APIs. APIs are the members of the interface family specifically responsible for handling "long-distance relationships." The next time you see these two terms, you can understand them from this perspective: they discuss the same type of problem, but the context of "interface" leans more towards design and constraints, while the context of "API" leans more towards implementation, integration, and openness.