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Some of the terms programmers frequently use. Part Two

Message

An object in isolation is of no use. Its value comes from its interactions with other objects.
This interaction takes place through messages. A message consists of three parts
(i)   A receiver object
(ii)   A method the receiver knows how to execute
(iii)   Parameters, if any.

Cohesion


Cohesion is a measure of how much an item, say for e.g. a class or method, makes sense. Objects with low cohesion are more likely to be changed. A method is highly cohesive if it does one thing and only one thing.  High cohesion is desirable.

Coupling


Object coupling describes the degree of inter relationships among the objects that make up a system. The more any one object knows about any other object in the system, the tighter the coupling is between those objects. In other words, coupling is a measure of how two items, such as classes or methods, are interrelated. When one class depends on another class they are said to be coupled. When one class interacts with another class, but does not have any knowledge of the implementation details of the other class they are said to be loosely coupled. When one class relies on the implementation of another class, they are said to be tightly coupled.  Always try for loose coupling among objects.

Abstract classes and Interfaces


 

Abstract Class


Abstract are classes that cannot be instantiated, and are frequently either partially
implement end, or not at all implemented. Abstract classes are useful when creating
components because they allow you specify an invariant level of functionality in some methods, but leave the implementation of other methods until a specific implementation of that class is needed. When implementing an abstract class, you must implement each abstract method in that class, and each implemented method must receive the same number and type of arguments, and have the same return value, as the method specified in the abstract class. Classes can inherit from only one abstract class.

Interface


An interface is a reference type and it contains only  abstract members . We cannot have any implementation code in the interface. The interface contains only declaration for its members.  Interface’s members can be events, methods, properties and indexers. An interface is also known as a “contract”.  The entire client which uses the interface must fulfil the contract by implementing the interface. A class can implement multiple interfaces.

How to decide between Abstract class and Interface


If you want to provide common, implemented functionality among all implementations of your component, use an abstract class. Abstract classes allow you to partially implement your class, whereas interfaces contain no implementation  for any members.


If the functionality you are creating will be useful across a wide range of disparate objects, use an interface. Abstract classes should be used primarily for objects that are closely related, whereas interfaces are best suited for providing common functionality to unrelated classes.

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