Skeletal Implementation
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| Prefer Skeletal implementation..captain Jack sparrow |
The skeletal implementation is a design, by which we can use the benefits of interface and abstract class together.
In Java Collection API has adopted this kind of design, AbstractSet , AbstractMap etc. are the example of Skeletal implementation . Also Joshua Bloch mentions Skeletal implementation into his book “Effective java”
In this article, we will see how we can efficiently design our system so it can use the feature of interface and Abstract class both.
Let try to understand it by a problem.
Suppose we want to create different types of Vending machines say candy vending, Soft drink vending etc. To get products from vending we need to start the vending machine then choose the product, pay for the same and collect it.
After that Vending machine should be stopped.
First Approach:
We can create a vending machine interface then for different product type we will create a concrete implementation of vending machine.
Code:
package com.example.skeletal;
public interface Ivending {
void start();
void chooseProduct();
void stop();
void process();
}
package com.example.skeletal;
public class CandyVending implements Ivending{
@Override
public void start() {
System.out.println("Start Vending machine");
}
@Override
public void chooseProduct() {
System.out.println("Produce diiferent candies");
System.out.println("Choose a type of candy");
System.out.println("pay for candy");
System.out.println("collect candy");
}
@Override
public void stop() {
System.out.println("Stop Vending machine");
}
@Override
public void process() {
start();
chooseProduct();
stop();
}
}
package com.example.skeletal;
public class DrinkVending implements Ivending{
@Override
public void start() {
System.out.println("Start Vending machine");
}
@Override
public void chooseProduct() {
System.out.println("Produce diiferent soft drinks");
System.out.println("Choose a type of soft drinks");
System.out.println("pay for drinks");
System.out.println("collect drinks");
}
@Override
public void stop() {
System.out.println("stop Vending machine");
}
@Override
public void process() {
start();
chooseProduct();
stop();
}
}
package com.example.skeletal;
public class VendingManager {
public static void main(String[] args) {
Ivending candy = new CandyVending();
Ivending drink = new DrinkVending();
candy.process();
drink.process();
}
}
Output :
Start Vending machine
Produce diiferent candies
Choose a type of candy
pay for candy
collect candy
Stop Vending machine
*********************
Start Vending machine
Produce diiferent soft drinks
Choose a type of soft drinks
pay for drinks
collect drinks
stop Vending machine
For simplicity, I do not divide each step as an individual method. In chooseProduct() I merge some steps.
Although it looks goods, the above code has some problems, if we see the codes carefully we can see There is a lot of duplicate codes. start(), stop() and process() methods do same thing in
Each concrete implementation.
Code duplication increases 3 times when the number of concrete implementation increases.
We can create a utility class and put common code into the same but that will break the single responsibility principal. Can introduce Shotgun surgery code smell.
Disadvantage of Interface:
As the interface is a contract and does not contain method body, each implementation has to fulfill the contract and provide an implementation of all methods. May some of the methods duplicate across the concrete implementation.
Approach 2:
We can overcome it through Abstract class.
Code:
package com.example.skeletal;
public abstract class AbstractVending {
public void start()
{
System.out.println("Start Vending machine");
}
public abstract void chooseProduct();
public void stop()
{
System.out.println("Stop Vending machine");
}
public void process()
{
start();
chooseProduct();
stop();
}
}
package com.example.skeletal;
public class CandyVending extends AbstractVending{ //implements Ivending{
@Override
public void chooseProduct() {
System.out.println("Produce diiferent candies");
System.out.println("Choose a type of candy");
System.out.println("pay for candy");
System.out.println("collect candy");
}
}
package com.example.skeletal;
public class DrinkVending extends AbstractVending{ //implements Ivending{
@Override
public void chooseProduct() {
System.out.println("Produce diiferent soft drinks");
System.out.println("Choose a type of soft drinks");
System.out.println("pay for drinks");
System.out.println("collect drinks");
}
}
package com.example.skeletal;
public class VendingManager {
public static void main(String[] args) {
AbstractVending candy = new CandyVending();
AbstractVending drink = new DrinkVending();
candy.process();
System.out.println("*********************");
drink.process();
}
}
Here, I provide common code implementation in to the abstract class. And Candyvending and DrinkVending extends this AbstractVending. This implementation gets rid of duplicate code but adds a new problem.
As, Candyvending and DrinkVending extends the Abstract class we can’I have provision to extends another class or does not support Multiple inheritance.
Say I want to add VendingServicing class which will clean and check the vending machine.
In this scenario I can’t extend VendingServicing as I have already extends AbstractVending. One thing I can do, create a composition but again we have to pass vending Machine into it which will strongly couple VendingServicing and Vending machine.
Disadvantage of Abstract class:
We can’t support Multiple inheritance due to diamond problem.
So It will be good if we can use advantages of both interface and Abstract class.
We call it Abstract Interface or Skeletal Implementation .
To achieve Skeletal Implementation ,
Step 1: Create an interface.
Step 2: Create an Abstract class implement that interface and provide the implementation of Common method.
Step 3. In subclass create a private inner class which extends the Abstract class, Now this class can extend class implement any interfaces while using the common method by delegation call to Abstract class.
Let check the code,
package com.example.skeletal;
public interface Ivending {
void start();
void chooseProduct();
void stop();
void process();
}
package com.example.skeletal;
public abstract class AbstractVending implements Ivending{
public void start()
{
System.out.println("Start Vending machine");
}
//public abstract void chooseProduct();
public void stop()
{
System.out.println("Stop Vending machine");
}
public void process()
{
start();
chooseProduct();
stop();
}
}
package com.example.skeletal;
public class CandyVending implements Ivending{
private class AbstractVendingDelegator extends AbstractVending
{
@Override
public void chooseProduct() {
System.out.println("Produce diiferent candies");
System.out.println("Choose a type of candy");
System.out.println("pay for candy");
System.out.println("collect candy");
}
}
AbstractVendingDelegator delegator = new AbstractVendingDelegator();
@Override
public void start() {
delegator.start();
}
@Override
public void chooseProduct() {
delegator.chooseProduct();
}
@Override
public void stop() {
delegator.stop();
}
@Override
public void process() {
delegator.process();
}
}
package com.example.skeletal;
package com.example.skeletal;
public class DrinkVending extends VendingService implements Ivending {
private class AbstractVendingDelegator extends AbstractVending
{
@Override
public void chooseProduct() {
System.out.println("Produce diiferent soft drinks");
System.out.println("Choose a type of soft drinks");
System.out.println("pay for drinks");
System.out.println("collect drinks");
}
}
AbstractVendingDelegator delegator = new AbstractVendingDelegator();
@Override
public void start() {
delegator.start();
}
@Override
public void chooseProduct() {
delegator.chooseProduct();
}
@Override
public void stop() {
delegator.stop();
}
@Override
public void process() {
delegator.process();
}
}
package com.example.skeletal;
public class VendingManager {
public static void main(String[] args) {
Ivending candy = new CandyVending();
Ivending drink = new DrinkVending();
//AbstractVending candy = new CandyVending();
//AbstractVending drink = new DrinkVending();
candy.process();
System.out.println("*********************");
drink.process();
if(drink instanceof VendingService)
{
VendingService vs = (VendingService)drink;
vs.service();
}
}
}
Please look the above design I create an interface then create an abstract class where I define all common implementation, then for each subclass implement a delegator class
And using that delegator we forward the call to AbstractVending.
Benefits of Skeletal Implementation :
1. A subclass can extend other class like DrinkVending.
2. Get rid of duplicate code by delegation call to Abstract class.
3. If a subclass needs a new implementation of interface it can do so.
Conclusion: when your interface has some common method always create an Abstract class
And in a subclass, you can use this subclass as a delegator. Always try to use Skeletal Implementation
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