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docs: separated interface

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@@ -3,26 +3,31 @@ title: Separated Interface
category: Structural
language: en
tag:
- Decoupling
- API design
- Decoupling
- Interface
---
## Also known as
* API Segregation
* Client-Server Interface
## Intent
Separate the interface definition and implementation in different packages. This allows the client
to be completely unaware of the implementation.
To define a client interface in a separate package from its implementation to allow for easier swapping of implementations and better separation of concerns.
## Explanation
Real world example
> An Invoice generator may be created with ability to use different Tax calculators that may be
> added in the invoice depending upon type of purchase, region etc.
> Consider a restaurant where the menu (interface) is separate from the kitchen operations (implementation).
>
> In this analogy, the menu lists the dishes customers can order, without detailing how they are prepared. Different restaurants (or even different chefs within the same restaurant) can use their own recipes and methods to prepare the dishes listed on the menu. This separation allows the restaurant to update its menu or change its chefs without disrupting the overall dining experience. Similarly, in software, the Separated Interface pattern decouples the interface from its implementation, allowing changes and variations in the implementation without affecting the client code that relies on the interface.
In plain words
> Separated interface pattern encourages to keep the implementations of an interface decoupled from
> the client and its definition, so the client is not dependent on the implementation.
> Defines a client interface separate from its implementation to allow for flexible and interchangeable components.
A client code may abstract some specific functionality to an interface, and define the definition of
the interface as an SPI ([Service Programming Interface](https://en.wikipedia.org/wiki/Service_provider_interface)
@@ -33,16 +38,47 @@ code at runtime (with a third class, injecting the implementation in the client)
**Programmatic Example**
**Client**
The Separated Interface design pattern is a software design pattern that encourages the separation of the definition of an interface from its implementation. This allows the client to be completely unaware of the implementation, promoting loose coupling and enhancing flexibility.
`InvoiceGenerator` class accepts the cost of the product and calculates the total
amount payable inclusive of tax.
In the given code, the `InvoiceGenerator` class is the client that uses the `TaxCalculator` interface to calculate tax. The `TaxCalculator` interface is implemented by two classes: `ForeignTaxCalculator` and `DomesticTaxCalculator`. These implementations are injected into the `InvoiceGenerator` class at runtime, demonstrating the Separated Interface pattern.
Let's break down the code:
First, we have the `TaxCalculator` interface. This interface defines a single method `calculate` that takes an amount and returns the calculated tax.
```java
public interface TaxCalculator {
double calculate(double amount);
}
```
Next, we have two classes `ForeignTaxCalculator` and `DomesticTaxCalculator` that implement the `TaxCalculator` interface. These classes provide the concrete logic for tax calculation.
```java
public class ForeignTaxCalculator implements TaxCalculator {
public static final double TAX_PERCENTAGE = 60;
@Override
public double calculate(double amount) {
return amount * TAX_PERCENTAGE / 100.0;
}
}
public class DomesticTaxCalculator implements TaxCalculator {
public static final double TAX_PERCENTAGE = 20;
@Override
public double calculate(double amount) {
return amount * TAX_PERCENTAGE / 100.0;
}
}
```
The `InvoiceGenerator` class is the client that uses the `TaxCalculator` interface. It doesn't know about the concrete implementations of the `TaxCalculator` interface. It just knows that it has a `TaxCalculator` that can calculate tax.
```java
public class InvoiceGenerator {
private final TaxCalculator taxCalculator;
private final double amount;
public InvoiceGenerator(double amount, TaxCalculator taxCalculator) {
@@ -53,84 +89,69 @@ public class InvoiceGenerator {
public double getAmountWithTax() {
return amount + taxCalculator.calculate(amount);
}
}
```
The tax calculation logic is delegated to the `TaxCalculator` interface.
Finally, in the `App` class, we create instances of `InvoiceGenerator` with different `TaxCalculator` implementations. This demonstrates how the Separated Interface pattern allows us to inject different implementations at runtime.
```java
public interface TaxCalculator {
public class App {
public static final double PRODUCT_COST = 50.0;
double calculate(double amount);
}
```
**Implementation package**
In another package (which the client is completely unaware of) there exist multiple implementations
of the `TaxCalculator` interface. `ForeignTaxCalculator` is one of them which levies 60% tax
for international products.
```java
public class ForeignTaxCalculator implements TaxCalculator {
public static final double TAX_PERCENTAGE = 60;
@Override
public double calculate(double amount) {
return amount * TAX_PERCENTAGE / 100.0;
}
}
```
Another is `DomesticTaxCalculator` which levies 20% tax for international products.
```java
public class DomesticTaxCalculator implements TaxCalculator {
public static final double TAX_PERCENTAGE = 20;
@Override
public double calculate(double amount) {
return amount * TAX_PERCENTAGE / 100.0;
}
}
```
These both implementations are instantiated and injected in the client class by the ```App.java```
class.
```java
public static void main(String[] args) {
var internationalProductInvoice = new InvoiceGenerator(PRODUCT_COST, new ForeignTaxCalculator());
LOGGER.info("Foreign Tax applied: {}", "" + internationalProductInvoice.getAmountWithTax());
var domesticProductInvoice = new InvoiceGenerator(PRODUCT_COST, new DomesticTaxCalculator());
LOGGER.info("Domestic Tax applied: {}", "" + domesticProductInvoice.getAmountWithTax());
}
}
```
In this way, the Separated Interface pattern allows us to decouple the interface of a component from its implementation, enhancing flexibility and maintainability.
## Class diagram
![alt text](./etc/class_diagram.png "Separated Interface")
![Separated Interface](./etc/class_diagram.png "Separated Interface")
## Applicability
Use the Separated interface pattern when
* Use when you want to decouple the interface of a component from its implementation.
* Useful in large systems where different teams work on different parts of the system.
* Ideal when the implementation might change over time or vary between deployments.
* You are developing a framework package, and your framework needs to call some application code through interfaces.
* You have separate packages implementing the functionalities which may be plugged in your client code at runtime or compile-time.
* Your code resides in a layer that is not allowed to call the interface implementation layer by rule. For example, a domain layer needs to call a data mapper.
## Tutorial
## Tutorial
* [Separated Interface Tutorial - YouTube](https://www.youtube.com/watch?v=d3k-hOA7k2Y)
* [Separated Interface Tutorial](https://www.youtube.com/watch?v=d3k-hOA7k2Y)
## Known Uses
* Java's JDBC (Java Database Connectivity) API separates the client interface from the database driver implementations.
* Remote Method Invocation (RMI) in Java, where the client and server interfaces are defined separately from the implementations.
## Consequences
Benefits:
* Enhances flexibility by allowing multiple implementations to coexist.
* Facilitates testing by allowing mock implementations.
* Improves maintainability by isolating changes to specific parts of the code.
Trade-offs:
* Initial setup might be more complex.
* May lead to increased number of classes and interfaces in the codebase.
## Related Patterns
* Adapter: Adapts one interface to another, which can be used alongside Separated Interface to integrate different implementations.
* Dependency Injection: Often used to inject the implementation of a separated interface, promoting loose coupling.
* Bridge: Separates an object’s interface from its implementation, similar to Separated Interface but usually applied to larger-scale architectural issues.
## Credits
* [Martin Fowler](https://www.martinfowler.com/eaaCatalog/separatedInterface.html)
* [Patterns of Enterprise Application Architecture](https://www.amazon.com/gp/product/0321127420/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=0321127420&linkId=e08dfb7f2cf6153542ef1b5a00b10abc)
* [Design Patterns: Elements of Reusable Object-Oriented Software](https://amzn.to/3w0pvKI)
* [Effective Java](https://amzn.to/4cGk2Jz)
* [Pattern-Oriented Software Architecture Volume 1: A System of Patterns](https://amzn.to/3xZ1ELU)
* [Patterns of Enterprise Application Architecture](https://amzn.to/3WfKBPR)
* [Separated Interface - Martin Fowler](https://www.martinfowler.com/eaaCatalog/separatedInterface.html)