f597fc1b07
* Fix languages * Missed change for version number * Add language field for presentation * Revert change in README for double buffer Co-authored-by: Jackie Nim <=>
140 lines
4.2 KiB
Markdown
140 lines
4.2 KiB
Markdown
---
|
|
layout: pattern
|
|
title: Separated Interface
|
|
folder: separated-interface
|
|
permalink: /patterns/separated-interface/
|
|
categories: Structural
|
|
language: en
|
|
tags:
|
|
- Decoupling
|
|
---
|
|
|
|
|
|
## Intent
|
|
|
|
Separate the interface definition and implementation in different packages. This allows the client
|
|
to be completely unaware of the implementation.
|
|
|
|
## 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.
|
|
|
|
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.
|
|
|
|
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)
|
|
is an API intended and open to be implemented or extended by a third party). Another package may
|
|
implement this interface definition with a concrete logic, which will be injected into the client
|
|
code at runtime (with a third class, injecting the implementation in the client) or at compile time
|
|
(using Plugin pattern with some configurable file).
|
|
|
|
**Programmatic Example**
|
|
|
|
**Client**
|
|
|
|
`InvoiceGenerator` class accepts the cost of the product and calculates the total
|
|
amount payable inclusive of tax.
|
|
|
|
```java
|
|
public class InvoiceGenerator {
|
|
|
|
private final TaxCalculator taxCalculator;
|
|
|
|
private final double amount;
|
|
|
|
public InvoiceGenerator(double amount, TaxCalculator taxCalculator) {
|
|
this.amount = amount;
|
|
this.taxCalculator = taxCalculator;
|
|
}
|
|
|
|
public double getAmountWithTax() {
|
|
return amount + taxCalculator.calculate(amount);
|
|
}
|
|
|
|
}
|
|
```
|
|
|
|
The tax calculation logic is delegated to the `TaxCalculator` interface.
|
|
|
|
```java
|
|
public interface TaxCalculator {
|
|
|
|
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
|
|
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());
|
|
```
|
|
|
|
## Class diagram
|
|
|
|
|
|
|
|
## Applicability
|
|
|
|
Use the Separated interface pattern when
|
|
|
|
* 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
|
|
|
|
* [Separated Interface Tutorial](https://www.youtube.com/watch?v=d3k-hOA7k2Y)
|
|
|
|
## 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)
|