CSC/ECE 517 Fall 2009/wiki2 7 cn

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Problem Statement

Different languages have different mechanisms for "code reuse." Assembly and low level languages have macros, C/C++ has includes, Ruby has mixins and modules, Java has packages, and there are many other concepts around reuse and extension. Here is a page that describes the various mechanisms, groups the mechanisms into logical categories, and discusses the advantages and disadvantages of the various schemes, in terms of simplicity, performance, reliability, or other factors.

Code Reuse - A Brief Overview

Code reuse, also called software reuse, is the use of existing software, or software knowledge, to build new software. Code reuse in software development is something assumed literally through re-using bits of code in more than one place. It is truly achieved through using common libraries and interfaces accessible to both the entire application and to other applications in the domain. Here are some of the features of code reuse.

Reuse is not duplication

Duplicating code is the result of a copy-paste scenario. The moment you have copy-pasted a module of code to two locations in your application you’ve just created an additional maintenance point. Also if you have coded a bug into the original code & duplicated it in multiple locations, the bug has now been instantiated in more than one place. Reuse is different from duplication. In code reuse the logic or work that a module does in multiple areas of the application can be reused from one place.The code doesn’t have to physically reside in all of those places to get reused.

Interfaces

Reuse is not always functional code that needs to be applied in more than one place. It can be obtained through the use of interfaces and generics as well. By using a solid interface design to allow any type of object with a compatible set of methods to be used in a piece of your application, you allow for everything on both sides of that interface to be easily interchangeable. Other type of interface-like methods that promote architectural reuse are generics, tag libraries, and dozens of frameworks out there that allow pieces to be easily decoupled and injected into the application.

Advantages of Code Reuse

  • Reusing code saves programming time, which reduces costs.
  • Sharing code can help prevent bugs by reducing the amount of code that needs to be written to perform a task. The shared code can also be tested separately from the end applications.
  • Separating code into common libraries lets programmers specialize in their particular strengths. A security library, for example, can be built by security experts while a user interface which uses the library can let UI experts focus on their tasks.
  • Delegation of tasks into shared modules allows offloading of some functionality onto separate systems.

Drawbacks

  • Maintenance nightmare – If for any reason one to make changes in the copied section, changes have to be made ALL the copies.
  • Unorganized – Reused code tends to be unorganized .It may generate create massive amounts of code which is difficult to decipher after a while.

Examples of Effective Code Reuse

  • Google Chrome is an excellent example of code reuse, it uses at least 25 different software Libraries.
  • Matlab programs are also a good example of code reuse where one reuses different functions from the matlab tool box.

General Methods for Code Reuse in Object Oriented Languages

The types of Code Reuse used for the Object Oriented Framework are as follows:

  • Architected Reuse: The identification, development, and support of large-scale, reusable assets via enterprise architecture. Your enterprise architecture may define reusable domain components, collections of related domain/business classes that work together to support a cohesive set of responsibilities, or service domains which bundle a cohesive collection of services together.
  • Pattern Reuse: The use of documented approaches to solving common problems within a specified context. With pattern reuse you’re not reusing code, instead you are reusing the thinking that goes behind the code. Patterns can be a multiple levels such as analysis, design, and architecture are the most common.
  • Framework Reuse: The use of collections of classes that together implement the basic functionality of a common technical or business domain.Horizontal frameworks, such as a security framework or user interface framework such as the Java Foundation Class (JFC) library and vertical frameworks, such as a financial services framework, are common.
  • Artifact Reuse: The use of previously created development artifacts such as use of cases, standards documents, domain-specific models, procedures and guidelines, and even other applications such as a commercial off the shelf (COTS) package to give one a kick start on a new project.
  • Module Reuse. The use of pre-built, fully encapsulated "modules", components, services, or code libraries in the development of an application. A module is self sufficient and encapsulates only one concept. Module reuse differs from code reuse as one don’t to have access to the source code.
  • Template Reuse. The practice of using a common set of layouts for key development artifacts such as documents, models, and source code within an organization.
  • Code Reuse. The reuse of source code within sections of an application and potentially across multiple applications. At its best code reuse is accomplished through the sharing of common classes and/or collections of functions and procedures. At its worst code reuse is accomplished by copying and then modifying existing code causing a maintenance nightmare.

The Page now talks about reuse in different languages such as C, C++, Java & Ruby.

Code Reuse in C/C++

Functions

(Applicable for both C/C++)

Instead of writing a same function multiple times, we can call the function, thus making the code more concise, readable & modular. Another way of using functions is to write them in header files & then including the header files in the code. Later the functions written in the Header File can be called directly from the main program.

Different Types of Header Files used in C/C++ are as follows:

  • assert.h, math.h, string.h, stdio.h, time.h etc

Simple Program to Illustrate C Functions: 

#include <stdio.h>
void print_message(void);
{
printf("Hello World \n");
}

Main Program 

int main(void)
{
print_message();
return 0;
}

Classes

(Applicable only for C++, Java)

Classes define methods which process the data present in multiple objects. Thus the same method code gets re-used for all the objects of the class, thus providing code re-use. C++ also defines other features for classes which promote code reuse. Eg: Funtion overloading helps the same function to perform different operations based on the parameters passed to it, thus allowing the use of one function to do multiple tasks.

An simple example of an class is as follows:- 

// classes example
#include <iostream>
using namespace std;
class CRectangle {
 int x, y;
public:
 void set_values (int,int);
 void set_values (int);
 int area (int x) {return (x*x);} 
 int area (int x, int y) {return (x*y);}
 };

void CRectangle::set_values (int a, int b) {
x = a;
y = b;
}
void CRectangle::set_values (int a) {
x = a;
}

int main () {
CRectangle rect;
rect.set_values (5,4);
cout << "area: " << rect.area();  //Output = 20
rect.set_values (3,4);
cout << "area: " << rect.area();  //Output = 12
CRectangle sqr;
sqr.set_values (7);
cout << "area: " << sqr.area(); //Output = 49 : Example of Function Overloading & Classes.
return 0;
}

Inheritence

(Applicable for C++, Java)

Derived Classes can reuse code from super class hierarchy. Variables and methods from the super class get added to the derived class, thus preventing duplication of code & enabling code re-use. The derived class can also have some extra methods/variables added to it if required. C++ also supports multiple inheritance where a base class extends more than one class. 

class exforsys
 {
public:
exforsys(void) { x=0; }
void f(int n1)
 {
x= n1*5;
 }
void output(void) { cout<<x; }
private: int x;
 };

class sample: public exforsys
 {
public:
sample(void) { s1=0; }
void f1(int n1)
 {
s1=n1*10;
 }
void output(void)
 {
exforsys::output();
cout << s1;
 }
private: int s1;
 };

int main(void)
 {
sample s;
s.f(10);
s.output();
s.f1(20);
s.output();
 }

Libraries

(Aplicable for C++ only)

Standard C++ Libraries are a collection of functions, constants, classes, objects & templates which extend the C++ language providing basic functionality to perform several tasks. Eg: Classes to interact with OS, Data containers, Iterators to operate them & algorithms commonly needed.

Some of the few C++ Standard Template Libraries(STL) are as as follows:

  • Containers Library eg: Bitset, Deqeue, List, Map, Queue, Vector.
  • Iterators Library eg: Iterator

Code Reuse in Java

Some of the code reuse techniques such as Classes, Function Overloading & Inheritance are common for the Java Programming Language as well. Though there will some differences such as: Java doesn't support multiple inheritance like C++. Java makes use of Interfaces & Abstract Classes to achieve Multiple Inheritance.

Re-using Java Classes

The code for a class can now be reused in other class by making use of the new method. Here is an example of the same: 

public class helloworld_use {
public static void main(String args[]) {
helloworld myhello = new helloworld();
myhello.greeting_text = "Hello again";
System.out.println(myhello.greeting());
}
}

The code can be compiled and run: 

javac helloworld_use.java
java helloworld_use

The end result is the words "Hello again" written to the screen, but the key thing here is that the new application requires no new code to be written for that to happen.

Inheritance and Interfaces

Interfaces are a definition which decide the methods that need to be declared/used in a class. Interfaces are essential to the Java language. They provide for multiple inheritance as well as information hiding and code re-use. Interfaces have advantages over classes because of improved information hiding and code re-use. A very clean design might use interfaces instead of base classes.Inheritance & Interfaces can be used together to implement code reuse.Here is an generic example of an interfaces. 


interface MinMax<T extends Comparable<T>> { 
 T min(); 
 T max(); 
} 

class MyClass<T extends Comparable<T>> implements MinMax<T> { 
 T[] vals; 

 MyClass(T[] o) { vals = o; } 

 public T min() { 
   T v = vals[0]; 
   for(int i=1; i < vals.length; i++) 
     if(vals[i].compareTo(v) < 0) v = vals[i]; 

   return v; 
 } 

 public T max() { 
   T v = vals[0]; 

   for(int i=1; i < vals.length; i++) 
     if(vals[i].compareTo(v) > 0) v = vals[i]; 

   return v; 
 } 
} 

 public class GenIFDemo { 
 public static void main(String args[]) { 
   Integer inums[] = {3, 6, 2, 8, 6 }; 
   Character chs[] = {'b', 'r', 'p', 'w' }; 

   MyClass<Integer> iob = new MyClass<Integer>(inums); 
   MyClass<Character> cob = new MyClass<Character>(chs); 

   System.out.println("Max value in inums: " + iob.max()); 
   System.out.println("Min value in inums: " + iob.min()); 

   System.out.println("Max value in chs: " + cob.max()); 
   System.out.println("Min value in chs: " + cob.min()); 
 } 
}

Here you can see that MyClass can be used to find the max value of either an Integer array or a character array based on data type used during the declaration of variables iob & cob, hence implementing re-use of code.

Code Reuse in Ruby

Ruby provides the concept of a module, a container for grouping one or more pieces of code. Modules may hold class definitions, as well as methods and variables that are not part of a specific class. Importantly, they can vary in scale from small snippets to complex software libraries.

You may not only call specific items of code from a module, but also use an include statement at any point to in your code mix-in all of the code from a named module at that position, as if it had been written at the line where the include statement appears. Code within a module may also call or mix-in from any other modules that are available.

Combined with class inheritance, modules enable you to organize and reuse code very efficiently both within and between software. Ruby only permits a class to inherit an initial set of methods and attributes from one parent class, but you may freely use module mix-ins within a class definition to supplement the properties and methods that the child class inherits from the parent. When you write your own classes you commonly mix-in the content of built-in Ruby modules such as Enumerable to provide your new class with standard methods.

The load path global variable defines the complete set of directories that the runtime should check for source files. This variable is automatically generated at run-time, and you may modify it like any other variable.

RubyGems

The RubyGems component of the platform massively extends the code reuse facilities offered by the Ruby language itself. Every library, application, or other collection of files that are configured as a gem may be deployed and upgraded across all systems that have a version of Ruby installed. The public servers at RubyForge provide a global distribution point for Open Source Ruby applications, and you may also offer your own RubyGems server for public or private use, using the supplied tools.

Modules & Mixins

Module is a collection of methods & constants. The methods in a module maybe instance methods or module methods. Instance methods appear as methods in a class when the module is included, module methods do not. Conversely, module methods may be called an encapsulating object, while instance methods may not.

In the Ruby language a mixin is a class that is mixed with a module. In other words the implementation of the class and module are joined, intertwined, combined, etc. A mixin is a different mechanism to the extend construct used to add concrete implementation to a class. With a mixin you can extend from a module instead of a class.

Here is an example of the usage of module & mixins 

module HealingProperties
def heal_wounds
#do something that heal wounds
end
end
 
class Wolverine
include HealingProperties
def kill_something_with_my_claws!
#Arrr!
end
end
 
logan = Wolverine.new
logan.kill_something_with_my_claws!
logan.heal_wounds


Module "HealingProperties" has been instantiated in the class "Wolverine". Then code of the Module "HealingProperties" is being used with all the objects of Class "Wolverine", thus providing Code Re-use.

Conclusion

In the end, reuse is definitely more than the literal meaning of the term. It most certainly implies more than just a quick copy-paste job against other modules. There’s even more to it than simply putting the code in a commonly accessible method, though that’s usually enough for most scenarios.

Unfortunately code reuse is not always practical in every case. Many times it is not even considered when developing a system. This approach results in a system that is too specific and has too many internal dependencies to be used in other applications. Real code reuse is also often short circuited by schedule constraints, limited resources and budgetary concerns. Many well-intentioned development teams know they should start with a reusable design, but the payback for this effort won't be seen until much later.

It's also hard to quantify how much code reuse is saving time and money. You can use tools to count lines of code before and after you implement reused code, but the end result can be misleading. How do you account for lines you have removed? Many times you have to rely on developer's opinions as to what percentage of their code was reused.

References

http://www.wikipedia.org
http://www.geocities.com/learnprogramming123/Clesson11.htm
http://www.ambysoft.com/essays/typesOfReuse.html
http://www.java2s.com/Code/Java/Language-Basics/Agenericinterfaceexample.htm
http://java-programming.suite101.com/article.cfm/how_to_write_and_reuse_a_java_class
http://www.rubyfleebie.com/an-introduction-to-modules-part-2/
http://www.retrospector.com/2006/05/23/what-code-reuse-really-means/

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