OOAD Quick Guide
Jan 10, An example is the Ownership relation that connects objects in the . (if this association is not represented as a relationship) and similarly for the. OOAD Quick Guide - Learn Object Oriented Analysis and Design in simple and easy Association depicts the relationship between objects of one or more classes. . Concurrency in operating systems allows performing multiple tasks or . Association is relation between two separate classes which establishes In Object-Oriented programming, an Object communicates to other Object to use.
Since a derived class "is-a" base class, the same invariants must hold true. But these invariants aren't necessarily visible from the header, or even sometimes from the implementation file, so it is easy for the designer of the derived class to mess up the invariants she inherits.
Documentation and review are the keys.
Difference between Link and Association
Class bloat A class becomes unwieldy at some point: This can happen over time, and may indicate the need to split the class via inheritance or aggregation, or simply other classes. Too much functionality unrelated to the abstraction can creep into the class. When it does, people will rely on it, then you're stuck. This is too simple.
Some classes are obvious, have corresponding physical entities in the problem domain. Gamma says these sort are key to making designs flexible.Class Diagram: Association & Aggregation Relationships - Georgia Tech - Software Development Process
Many of the objects in the pattern catalog are of this type. This is why it's important to know the catalog. An interface is a collection of methods which an object responds to.
Different kinds of objects may share the same type. An object can have many types. An object's implementation is defined by its class.
The class tells you about the state an object maintains. It also tells you how an object implements the methods in its interface. Class inheritance is a means of sharing implementation both state and behavior between classes.
Interface inheritance subtyping only specifies when one object can be used in place of another.
The distinction is important because it is the type of an object that is important to flexible design. Interface is everything in good design; type is interface. Designing with an interface in mind rather than an implementation helps because: Clients remain unaware hence decoupled of the specific kinds of objects they use. Clients remain unaware of the classes of the objects they use.
They just know the interfaces. These points together describe what Gamma calls the first "principle of OO design": Program to an interface, not an implementation. Variables should not be concrete classes. Rather, they should be to abstract classes that just specify an interface.
Java gives us an even better means of doing this. There are creational patterns for instantiating objects of concrete classes since you have to get work done somewhere while remaining unaware of their class. You can do pure implementation inheritance via private inheritance. This distinction is important because the design patterns in Gamma often make it. Another interesting aspect of this issue is that it helps explain the features of other OO languages e. Inheritance and composition Class inheritance is great for implementation re-use.
But it goes against the general goal of decoupled classes. It's also a static, compile-time relationship. This makes it easier to understand and program to, but less flexible. Composition aka association is another means of achieving re-use. Put an object inside another object and the first object can re-use the code behind the composed object. The difference is that the composed object can be determined at run-time.
The only stipulation is that the composed object must be of the proper type not class. If you view composition as an alternative to inheritance then you can in effect change the inheritance or class of an object at run-time. Composition is done by reference reference or pointer, lifetimes de-coupled so that the object being referred to can change at run-time. Gamma's second principle of good OO design is to: Favor object composition over class inheritance.
Systems that follow this rule have fewer classes and more objects.
OOAD Object Model
Their power is in the ways that the objects can interact with each other. It would be important to have good dynamic models. Delegation Delegation is an important design pattern itself. If B is a subclass of A then sending B a message for something in A's interface means that B can re-use the implemtation found in A. Delegation uses composition to put re-use the implementation from A. B's interface can be extended to have some of the same functionality of A. But when a B object is sent a message corresponding to a method in A's type, it forwards the message to an A object.
The aggregation design we saw earlier for implementing a Set in terms of a List is an example of the use of delegation. Suppose you want to make an app that won't blow away unsaved work when the app is going to quit running due to logout. Subclass Application and add this functionality? Lots of work for minimal additional functionality.
Think of all the X and X' classes you'd have in your app. Design for change now by settling only on an interface, not an implementation. So Car will have access to all the functions, properties and so on of the base class Vehicle depending on the access modifiers defined for base class members. Here we have common properties like color and price in the base class that can be set depending on the derived class's requirements. Apart from these, it will add its specific property named Discount.
But this class will not have access to the base class property VehicleType, since it is a private type.
The Quality of Classes and OO Design | Atomic Object
The same will be applicable for any other class that derives from the Vehicle class and any other member defined in the base class. Generalization is represented by the following symbol in UML: Association Association is defined as a structural relationship, that conceptually means that the two components are linked to each other.
This kind of relation is also referred to as a using relationship, where one class instance uses the other class instance or vice-versa, or both may be using each other. But the main point is, the lifetime of the instances of the two classes are independent of each other and there is no ownership between two classes.
For example, consider the same example of a Student-Teacher relationship. Conceptually speaking, each student can be associated with multiple teachers and each teacher can be associated with multiple students. Now to explain this relationship in terms of Object Oriented Programming, see the code below: First, both of these instances are being created outside the student class. So their lifetime is independent of the lifetime of the instance of the student.
So we can also say that no instance of student is a parent of any instance of teacher. So there is no ownership of instances in the association. This association is represented by the following symbol in UML: So that described association. Now for the specialized cases of association, aggregation and composition. Aggregation is the same as association but with an additional point that there is an ownership of the instances, unlike association where there was no ownership of the instances.
To understand it better, let's add another class named Department to our example explained above. If we talk about the relation between Teacher and Department then conceptually, a Department can have multiple Teachers associated with it but each Teacher can belong to only one Department at a time. They cannot belong to any other instance like department2.
So here the ownership exists. Aggregation is also referred to as a Weak Association and is represented by the following symbol in UML representation: This is the same as that of aggregation, but with the additional point that the lifetime of the child instance is dependent on the owner or the parent class instance.
To the same code above, let's add another class named University. So in this case also, conceptually, a university can have multiple departments in it.