Exporting "IsA" and "HasA" relationships to excel
Inheritance (is-a) relationships are derived child classes that inherit attributes and methods from their parent class. Composition (has-a) relationships are classes. Only the Is-a relation is related to inheritance. The OO concept of the Is-a relationship is called specialization. Most OO languages use classes to implement the. A relation on itself is like a hook in the air ;) So you will likely export the two objects the connector links as well. You might look into the VB script.
If we want to use the data stored in an object to perform an action or calculate a derived value, we define a method associated with the object which does this. Then whenever we want to perform this action we call the method on the object.
We consider it bad practice to retrieve the information from inside the object and write separate code to perform the action outside of the object.
Encapsulation is a good idea for several reasons: We could switch to using another object which is completely different on the inside, and not have to change any code because both objects have the same interface. Some languages have features which allow us to enforce encapsulation strictly. In Java it is also considered good practice to write setters and getters for all attributes, even if the getter simply retrieves the attribute and the setter just assigns it the value of the parameter which you pass in.
It is also customary to set and get simple attribute values directly, and only write setter and getter methods for values which require some kind of calculation. In Python, there are two main types of relationships between classes: We saw in the previous chapter how we can make a datetime.
We can say that a person has a birthdate — if we can express a relationship between two classes using the phrase has-a, it is a composition relationship. Relationships like this can be one-to-one, one-to-many or many-to-many, and they can be unidirectional or bidirectional, depending on the specifics of the the roles which the objects fulfil. According to some formal definitions the term composition implies that the two objects are quite strongly linked — one object can be thought of as belonging exclusively to the other object.
If the owner object ceases to exist, the owned object will probably cease to exist as well.
If the link between two objects is weaker, and neither object has exclusive ownership of the other, it can also be called aggregation. Here are four classes which show several examples of aggregation and composition: This is an example of the way that translation of real-life concepts into objects in your code may not always be as straightforward as it appears.
Would it have made sense to have a single course object which has both description, code and department attributes and a list of students?
We have represented these two concepts by two separate classes which are linked to each other. Course is the theoretical description of a course, and CourseRunning is the concrete instance of a course.
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We have defined several relationships between these classes: A student can be enrolled in several courses CourseRunning objectsand a course CourseRunning can have multiple students enrolled in it in a particular year, so this is a many-to-many relationship.
A student knows about all his or her courses, and a course has a record of all enrolled students, so this is a bidirectional relationship. A department offers multiple courses Course objectsbut in our implementation a course can only have a single department — this is a one-to-many relationship. It is also bidirectional.
Furthermore, these objects are more strongly coupled — you can say that a department owns a course. The course cannot exist without the department. You should include classes for songs, artists, albums and playlists.
You can also assume that each song is associated with a single album, but that multiple copies of the same song which are included in different albums can exist.
Write a simple implementation of this model which clearly shows how the different classes are composed. Write some example code to show how you would use your classes to create an album and add all its songs to a playlist.
An object which inherits from another object is considered to be a subtype of that object.
Inheritance, IS-A and Has-A
As we saw in the previous chapter, all objects in Python inherit from object. We can say that a string, an integer or a Person instance is an object instance.
When we can describe the relationship between two objects using the phrase is-a, that relationship is inheritance. We also often say that a class is a subclass or child class of a class from which it inherits, or that the other class is its superclass or parent class. We can refer to the most generic class at the base of a hierarchy as a base class.
Inheritance can help us to represent objects which have some differences and some similarities in the way they work. Similar way is used to show aggregation and composition between two classes.IS-A and Has-A relationship in java in hindi by alok sir
Here are UML notations for different kind of dependency between two classes. As I said all three denotes relationship between object and only differ in their strength, you can also view them as below, where composition represents strongest form of relationship and association being the most general form.
What is the difference between IS -A relationship and HAS-A relationship in Java? - Stack Overflow
Association vs Composition vs Aggregation Here is the list of differences between Composition and Aggregation in point format, for quick review. An example is Car and Engine. While if A and B are associated with each other, such that B can exist without being associated with A, then this association in known as Aggregation.
Organization uses People as an employee. Since in Composition, Owner object expects a part object to be available and functions, by making it final, your provide guarantee that, when Owner will be created, this part object will exist. This is actually a Java idiom to represent a strong form of association i. If one object is-part-of another object e. Delete the Class and the Students still exist.
Composition implies a relationship where the child cannot exist independent of the parent. House parent and Room child. Rooms don't exist separate to a House. We should be more specific and use the composition link in cases where in addition to the part-of relationship between Class A and Class B - there's a strong lifecycle dependency between the two, meaning that when Class A is deleted then Class B is also deleted as a result Aggregation Example: It's important to note that the aggregation link doesn't state in any way that Class A owns Class B nor that there's a parent-child relationship when parent deleted all its child's are being deleted as a result between the two.
Actually, quite the opposite!
Summing it up - To sum it up association is a very generic term used to represent when on class used the functionalities provided by another class.
We say it's a composition if one parent class object owns another child class object and that child class object cannot meaningfully exist without the parent class object.
Has-a - Wikipedia
If it can then it is called Aggregation. Generalization vs Specialization Generalization is a mechanism for combining similar classes of objects into a single, more general class. Generalization identifies commonalities among a set of entities. The commonality may be of attributes, behavior, or both.