Cohesion and Decoupling, what do they represent?

问题:

What are Cohesion and Decoupling? I found information about coupling but not about decoupling.

回答1:

That article from Aaron is very good for understanding, also I'd recommend that you read manning publications Spring in Action book, they give very good examples on how the spring solves that problem it will definitely improve your understanding of this.

EDIT :

I came accross this in this great book called Growing object oriented software guided by tests :

Coupling :

Elements are coupled if a change in one forces a change in the other. For example, if two classes inherit from a common parent, then a change in one class might require a change in the other. Think of a combo audio system: It’s tightly coupled because if we want to change from analog to digital radio, we must rebuild the whole system. If we assemble a system from separates, it would have low coupling and we could just swap out the receiver. “Loosely” coupled features (i.e., those with low coupling) are easier to maintain.

Cohesion:

An element’s cohesion is a measure of whether its responsibilities form a meaningful unit. For example, a class that parses both dates and URLs is not coherent, because they’re unrelated concepts. Think of a machine that washes both clothes and dishes—it’s unlikely to do both well.2 At the other extreme, a class that parses only the punctuation in a URL is unlikely to be coherent, because it doesn’t represent a whole concept. To get anything done, the programmer will have to find other parsers for protocol, host, resource, and so on. Features with “high” coherence are easier to maintain.

回答2:

Cohesion - related to the principle that a class/method should be responsible for one thing only i.e there are no stray methods that don't belong in the encapsulation; a method only does one thing. High/Low cohesion is the degree to which this holds.

Coupling - how interdependent different parts of the system are. e.g how and where there are dependencies. If two classes make calls to methods of each other then they are tightly coupled, as changing one would mean having to change the other. Decoupling is the process of making something that was tightly coupled less so, or not at all.

Flexible systems have High Cohesion and Loose Coupling.

回答3:

For coupling, this Wikipedia article should answer all your questions. This article deals with cohesion.

"Decoupling" is just another name for "little/low coupling".

So these terms answer these questions:

• How much does each part of your project depend on another part?
• If you wanted to use just a part of your project (like to solve a specific problem) how much do you need to know about all the rest of the project?
• Is every part of your project focused on a single solution to a specific problem or do solutions "leak" to other parts?

回答4:

Here are my thoughts on cohesion. Imagine there is a module. Inside that module, we have some tasks. When those tasks are highly related to each other, we say it has high cohesion. When those tasks are not related, we say it has low cohesion. My best attempt to explain decoupling is that decoupling is the act of removing coupling.

Low Coupling helps us get to high cohesion! Remember that we want our module to have related tasks and one single responsibility. But what is coupling? Coupling is the degree of dependency on other modules to achieve our single responsibility for that module. So by low coupling, we are saying that we are not very dependent on external modules hence we have high cohesion.

However, if we have many dependencies to external modules, we would have high coupling and low cohesion. Get it?

Other more decorated thinkers and groups say:

Cohesion is the degree to which the tasks performed by a single module are functionally related." IEEE, 1983 "Cohesion is the "glue" that holds a module together. It can be thought of as the type of association among the component elements of a module. Generally, one wants the highest level of cohesion possible." Bergland, 1981

A software component is said to exhibit a high degree of cohesion if the elements in that unit exhibit a high degree of functional relatedness. This means that each element in the program unit should be essential for that unit to achieve its purpose. Sommerville, 1989

回答5:

decoupling allows the separation of object interaction from classes and inheritance into distinct layers of abstraction used to polymorphic-ally decouple the encapsulation which is the practice of using re-usable code to prevent discrete code modules from interacting with each other.