CSC/ECE 517 Fall 2007/wiki3 3 qq

Topic

Take the principle of Separation of Responsibility and catalog the information on it available on the Web. Find good descriptions and good, concise, understandable examples. Tell which you consider the best to present to a class.

Defination of Separation of Responsibility (SOR)

In Object-Oriented Design (OOD)

• One class should be responsible for knowing and maintaining a set of data, even if that data is used by many other classes.[1]
• Different responsibilities should be divided among different objects. Ideally, each individual object has one single responsibility, which we call Single Responsibility Principle (SRP).

Why Separation of Responsibility

• Encapsulation: Data are kept in only one place.

• Reusability: Ready to be used in more than one application or environment.

• Flexibility: An object with few responsibilities can be modified for use in applications or environments other than those for which it was specifically designed.

• Maintainability: An object with few responsibilities can be easily modified to correct faults, improve performance, or other attributes, or adapt to a changed environment without affecting other functionality.[2]

• Once and Only Once(The Dry Principle): The separation of responsibility can help to eliminate duplicated declarations of behavior, typically by merging them or replacing multiple similar implementations with a unifying abstraction.[3]

• Decreases the coupling between two objests: One class do not need to know what happened and implementation details in the other class (e.x.,what kind of object the pointer points to). [3]

Applications of Separation of Responsibility

An Example Not using SOR

I am thinking maybe it is better for us to first take a look at what is NOT an Separation of Responsibility. The SOR principle says that a class should have one, and only one, reason to change. As an example, imagine the following class:

class Employee
{
  public Money calculatePay()
  public void save()
  public String reportHours()
}

This class violates the SOR because it has three reasons to change:

• The business rules having to do with calculating pay.
• The database schema.
• The format of the string that reports hours.

Obviously, we don't want a single class to be impacted by these three completely different forces. We don't want to modify the Employee class every time the accounts decide to change the format of the hourly report.

Example 2

Let’s look into the matter with another example. Consider following code:

public void createCustomer(Map requestParameters) {
	Customer customer = new Customer();
	customer.setName = requestParameters.get("name"); 
            //Check if a customer was already registered with that name
	if (customerService.getCustomerByName(customer.getName()) != null) {
		System.out.println("Customer already exists");
		return;
	}
	customer.setShoppingCart(new ShoppingCart());	
        customerService.save(customer);
}

The above method name is save. So I expect that the code only does some saving. Instead, it creates a new customer, checks if it’s valid (no double names) and then saves it. So the method is actually performing 3 things.

This method should be refactored into 4 methods. The first method, which I would call bindValidateAndSave is the algorithm method. It tells what to do, not how it’s done. Its responsibility is to provide the algorithm. The 3 other methods would be called validateCustomer, bindCustomer (bind and add new shoppingCart),and saveCustomer respectively.[4] The code after using Separation of Responsibility is shown below:

public void bindValidateAndSave(Map requestParameters) {
	if (!validateCustomer(requestParameters))
                return;
	customer.setShoppingCart(new ShoppingCart());	
        
}
public Customer validateCustomer(Map requestParameters) {
       Customer customer = new Customer();
       customer.setName = requestParameters.get("name"); 
       if (customerService.getCustomerByName(customer.getName()) != null) {
		System.out.println("Customer already exists");
		return 0;
	}
        return 1;
}
public void bindCustomer(Customer customer) {
       customer.setShoppingCart(new ShoppingCart());
}
public void saveCustomer(Customer customer) {
       customerService.save(customer);
}

Example 3

Let's think about a example shown below: Each node consists of data and a link (next) and the LinkedList implemented from Nodes.

    class Node { 
　　    Object data; 
　　    Node next; //point to next node 
　　    } 

    class Linklist implement Node{
        ...
        ...
       public Object currentNode() 
　　　　{ 
　　        Node temp=cursor(); 
　　        return temp.data; 
　　    }
    }

Then, we make a object client responsible for traversing a list which and keeping track of where it is.

    class client{
        Linklist a   
    }

Reference

[1] http://courses.ncsu.edu/csc517/common/lectures/notes/lec20.doc

[2] Programming Life and the Zen of Computers

[3] http://www.codinghorror.com/blog/archives/000805.html

[4] Java by experience