Input and Output Ports
Understand how Input and Output Ports (interfaces) define the boundaries for communication between Use Cases and external layers.
Input and Output Ports is a free Clean Architecture & Design Patterns in Practice lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Clean Architecture & Design Patterns in Practice learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What are Input & Output Ports?
In Clean Architecture, "Ports" are crucial interfaces that define how different layers communicate. They act like contracts.
Think of them as the "sockets" of your application's core logic. They specify what goes in (Input) and what comes out (Output).
Defining Input Ports
An Input Port is an interface defined by the Use Case layer. It declares the methods that an external layer (like a UI controller) can call to interact with the Use Case.
- It's the "what you can do" contract for the Use Case.
- It ensures the Use Case doesn't know about the UI or external callers.
Input Port: User Login
Here's a simple Input Port for a user login feature. It defines the operations the Use Case expects.
public interface UserLoginInputPort {
void login(String username, String password);
}
public class UserLoginUseCase implements UserLoginInputPort {
@Override
public void login(String username, String password) {
// Business logic for login
System.out.println("Attempting login for: " + username);
if (username.equals("admin") && password.equals("pass")) {
System.out.println("Login successful!");
} else {
System.out.println("Login failed.");
}
}
public static void main(String[] args) {
UserLoginInputPort loginService = new UserLoginUseCase();
loginService.login("admin", "pass");
loginService.login("user", "123");
}
}Defining Output Ports
An Output Port is also an interface, defined by the Use Case layer. It declares the methods that the Use Case will call to deliver results or interact with external services (like a database or UI presenter).
- It's the "what I need to tell you" contract.
- It keeps the Use Case unaware of specific implementations (e.g., how results are displayed).
Output Port: Login Presenter
This Output Port defines how the login result should be presented or handled. The Use Case calls these methods, but doesn't know how they are implemented.
public interface UserLoginOutputPort {
void presentLoginSuccess(String username);
void presentLoginFailure(String message);
}
public class ConsolePresenter implements UserLoginOutputPort {
@Override
public void presentLoginSuccess(String username) {
System.out.println("Welcome, " + username + "!");
}
@Override
public void presentLoginFailure(String message) {
System.out.println("Error: " + message);
}
public static void main(String[] args) {
UserLoginOutputPort presenter = new ConsolePresenter();
presenter.presentLoginSuccess("Alice");
presenter.presentLoginFailure("Invalid credentials.");
}
}How Use Cases Use Ports
The Use Case (or Interactor) depends on these port interfaces. It takes an Input Port as its public method and receives an Output Port through its constructor or method parameters.
This means the Use Case only knows about abstractions, not concrete implementations.
Use Case Connecting Ports
Here, our UserLoginUseCase now uses both ports. Notice it doesn't know if it's talking to a web UI, a mobile app, or a console!
public interface UserLoginInputPort {
void login(String username, String password);
}
public interface UserLoginOutputPort {
void presentLoginSuccess(String username);
void presentLoginFailure(String message);
}
public class UserLoginUseCase implements UserLoginInputPort {
private final UserLoginOutputPort outputPort;
public UserLoginUseCase(UserLoginOutputPort outputPort) {
this.outputPort = outputPort;
}
@Override
public void login(String username, String password) {
if (username.equals("admin") && password.equals("pass")) {
outputPort.presentLoginSuccess(username);
} else {
outputPort.presentLoginFailure("Invalid credentials.");
}
}
public static void main(String[] args) {
// In real app, this would be wired by a framework
UserLoginOutputPort presenter = new ConsolePresenter(); // From Scene 5
UserLoginInputPort loginService = new UserLoginUseCase(presenter);
loginService.login("admin", "pass");
loginService.login("user", "wrong");
}
}
class ConsolePresenter implements UserLoginOutputPort {
@Override
public void presentLoginSuccess(String username) {
System.out.println("Welcome, " + username + "!");
}
@Override
public void presentLoginFailure(String message) {
System.out.println("Error: " + message);
}
}Ports and the Dependency Rule
The use of Ports perfectly adheres to the Dependency Rule. The Use Case layer (core business logic) depends on interfaces (the Ports).
The outer layers (like UI or databases) then implement these interfaces, providing the concrete details without the core knowing about them.
Why Use Ports?
Ports offer powerful advantages:
- Decoupling: The Use Case is isolated from UI, database, or external services.
- Testability: You can easily mock or stub these interfaces for robust unit testing of Use Cases.
- Flexibility: You can swap out implementations (e.g., change database) without touching core logic.
- Clarity: They clearly define the boundaries and interactions of your application's core.
Quick Check: Port Roles
Consider the roles of Input and Output Ports in Clean Architecture.
Recap: Ports as Boundaries
We learned that Input Ports are interfaces defining what a Use Case expects, acting as entry points.
Output Ports are interfaces defining what a Use Case delivers or needs from external services.
Together, they enforce the Dependency Rule, ensuring our core business logic remains independent, testable, and flexible.
Frequently asked questions
Is the “Input and Output Ports” lesson free?
Yes — the full text of “Input and Output Ports” is free to read here on the web, and the Clean Architecture & Design Patterns in Practice course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Clean Architecture & Design Patterns in Practice course, upgrade to CoddyKit PRO.
What will I learn in “Input and Output Ports”?
Understand how Input and Output Ports (interfaces) define the boundaries for communication between Use Cases and external layers. You practise Clean Architecture & Design Patterns in Practice with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Clean Architecture & Design Patterns in Practice?
No prior experience is required. Clean Architecture & Design Patterns in Practice on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Input and Output Ports” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Clean Architecture & Design Patterns in Practice lesson?
Yes. Every Clean Architecture & Design Patterns in Practice lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.