CSC/ECE 517 Fall 2011/ch4 4h lp

Introduction

This wiki article discusses about some of the commonly used & easy to understand Desgin patterns[1] in the software industry. Specifically, we will be studying about the Singleton, Adapter, Command & Strategy patterns.

Case Study

Singleton

Singleton is a design pattern which imposes a restriction on the class to instantiate exactly one object of that class.

Implementation

• In Java, We can create a thread-safe & lazy version of Singleton as follows. The comments in the code help you understand the Lazy & thread-safe aspect of it.

public class Singleton {
        private static Singleton instance;
 
        private Singleton() {
                //Do nothing. Initialize the object only when the first time getInstance() is called.
        }
 
        public static synchronized Singleton getInstance() { 

        //The keyword "synchronized" makes it thread safe so that two threads invoking getInstance()  
        //at the same time cannot create two instances when a context switch happens so as to 
        //facilitate this scenario. In general this can happen even with 'n' threads invoking getInstance().
        //"synchronized" keyword is used to eliminate such scenarios.

                if (null == instance) {
                        instance = new Singleton();
                }
                return instance;
        }
}

• In Ruby, we can achieve singleton classes by mixing-in the 'Singleton' module as follows. Here, "SingletonClass" is the name of the singleton class that is being created.

require 'singleton'
class SingletonClass
    include Singleton
    ...
end

When should we use Singleton ? <ref>When should we use Singleton ?</ref>

There is a lot of criticism to the use of singleton pattern. So, how do we decide whether an object should be singleton or not ? For an object to be considered as a singleton object, they must satisfy three requirements:

1. controls concurrent access to a shared resource.
2. access to the resource will be requested from multiple, disparate parts of the system.
3. there can be only one object.

Advantages

• Singletons are often preferred to global variables as they don't pollute the global namespace with unnecessary variables & also can be instantiated only when needed unlike global variables which always consume resources.

Drawbacks

• Introduces a global state into the application.
• Can make unit testing classes in isolation difficult.

Checkout the this video from Misko Hevery which explains in detail when the usage of singletons is not exercised.<ref>Why Singletons are bad</ref>

Strategy

Definition

"In computer programming, the strategy pattern is a design pattern, whereby algorithms can be selected at runtime. Formally speaking, the strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it." - Wikipedia <ref>Stratey Pattern Wikipedia</ref>

Example code for Strategy pattern

Here we will see how strategy pattern can be used to sort a given list of real numbers using either BubbleSort() or QuickSort().

Below is an interface to describe the individual algorithms.

public interface SortingInterface {
      public void sort(double[] list);
}

Now implement both the Quicksort & Bubblesort strategies.

public class QuickSort implements SortingInterface {
      //QuickSort will implement the sort() with the quicksort version.
      public void sort(double[] list) {
             //Do quick sort here
      }
}


public class BubbleSort implements SortingInterface {
      
      //BubbleSort will implement the sort() with the bubblesort version.
      public void sort(double[] list) {


            //Do bubble sort here  
      }
}

Now define a Strategy context that maintains a reference to a Strategy object(Bubblesort or Quicksort) and forwards client requests to that strategy.

public class Context {
private SortingInterface algorithm = null;

public void sortGivenList(double[] list) {
       algorithm.sort(list);
}

//Getter method for algorithm
public SortingInterface getAlgorithm() {
       return algorithm;
}

//Setter method for algorithm
public void setAlgorithm(SortingInterface algorithm) {
       this.algorithm = algorithm;
}

}

Finally, the client picks one of the available strategies(Quicksort or Bubblesort) in the context and sorts the List according to that strategy as shown below.

public class Client {

      public static void main(String[] args) {
      
      //Here is the List
      double[] list = {1,2.4,7.9,3.2,1.2,0.2,10.2,22.5,19.6,14,12,16,17};

      //Now define the Context
      Context context = new Context();

      //Now pick your strategy i.e. Quicksort or Bubblesort
      context.setAlgorithm(new BubbleSort());

      ///Now, sort it using the strategy you picked
      context.sortGivenList(list);
      
 }

Applications

• Sorting a list: where the strategy can be to choose the sorting algorithm itself (QuickSort, HeapSort, etc.) at runtime depending on the input data.
• Layout Managers in UI toolkits: where you can decide at runtime what layout is to be chosen for the GUI to be displayed.<ref>Strategy Pattern Real world examples</ref>

Differences between Command and Strategy Pattern

• A Command Pattern encapsulates a single action/algorithm whereas the Strategy pattern provides us with a number of actions/algorithms which can be selected based on a number of factors.
• A Command Pattern object has a single method but with a generic signature whereas Strategy pattern may have as many different methods provided.

References

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