Relationship is-a, has-a and uses-a
Hi all I am bit confusing with is-a, has-a and uses-a relationship of The difference between HAS-A and IMPLEMENTED-IN-TERMS-OF (or. In Python, there are two main types of relationships between classes: . objects which have some differences and some similarities in the way they work. My definition: One class can use features from another class to extend its functionality (an “Is a” relationship) i.e., a Car is a Automobile.
Inheritance doesn't provide this luxury. In order to test derived class, you must need its super class. Since unit testing is one of the most important thing to consider during software development, especially in test driven development, composition wins over inheritance. Another good example of using composition over inheritance is Decorator design pattern. In Decorator patternwe don't extend any class to add additional functionality, instead we keep an instance of the class we are decorating and delegates original task to that class after doing decoration.
This is one of the biggest proof of choosing composition over inheritance, since these design patterns are well tried and tested in different scenarios and withstand test of time, keeping there head high.
If sub class is depending on super class behavior for its operation, it suddenly becomes fragile. When behavior of super class changes, functionality in sub class may get broken, without any change on its part.
One example of inheritance making code fragile is method add and addAll from HashSet. Suppose, If addAll of HashSet is implemented by calling add method and you write a sub class of HashSet, which encrypt the content before inserting into HashSet. Since there are only one methods addwhich can insert object into HashSet you override these method and called your encrypt method by overriding add. This automatically covers addAll as well, because addAll is implemented using addit looks very enticing.Java IS-A vs. HAS-A Relationship Between Classes - Inheritance Example - APPFICIAL
An object which inherits from another object is considered to be a subtype of that object. 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. We can put all the functionality that the objects have in common in a base class, and then define one or more subclasses with their own custom functionality. Inheritance is also a way of reusing existing code easily.
If we already have a class which does almost what we want, we can create a subclass in which we partially override some of its behaviour, or perhaps add some new functionality. Here is a simple example of inheritance: We create a subclass to represent students and one to represent staff members, and then a subclass of StaffMember for people who teach courses as opposed to staff members who have administrative positions.
We represent both student numbers and staff numbers by a single attribute, number, which we define in the base class, because it makes sense for us to treat them as a unified form of identification for any person. We use different attributes for the kind of student undergraduate or postgraduate that someone is and whether a staff member is a permanent or a temporary employee, because these are different sets of options.
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We have also added a method to Student for enrolling a student in a course, and a method to Lecturer for assigning a course to be taught by a lecturer. Because we use the class of an exception to determine whether it should be caught by a particular except block, it is useful for us to define custom classes for exceptions which we want to raise in our code.
Using inheritance in our classes is useful because if an except block catches a particular exception class, it will also catch its child classes because a child class is its parent class.
That means that we can efficiently write except blocks which handle groups of related exceptions, just by arranging them in a logical hierarchy.
Write a simple program which loops over a list of user data tuples containing a username, email and age and adds each user to a directory if the user is at least 16 years old. You do not need to store the age. Write a simple exception hierarchy which defines a different exception for each of these error conditions: Whenever an exception occurs, your program should move onto the next set of data in the list.
UML Association vs Aggregation vs Composition
Print a different error message for each different kind of exception. Think about where else it would be a good idea to use a custom class, and what kind of collection type would be most appropriate for your directory. You can assume that the age is already an integer value.
At a real university, the divisions between staff and students and administrative and teaching staff are not always clear-cut. A student who tutors a course is also a kind of temporary staff member. A staff member can enrol in a course.
A staff member can have both an administrative role in the department and a teaching position. In Python it is possible for a class to inherit from multiple other classes.
We could, for example, create a class called Tutor, which inherits from both Student and StaffMember. This ambiguity is known as the diamond problem, and different languages resolve it in different ways. Fortunately the super function knows how to deal gracefully with multiple inheritance. We would then end up with a class which has all the attributes and methods found in both Student and StaffMember. One way of doing this is to split up optional functionality into mix-ins.
A Mix-in is a class which is not intended to stand on its own — it exists to add extra functionality to another class through multiple inheritance. For example, let us try to rewrite the example above so that each set of related things that a person can do at a university is written as a mix-in: Each mix-in is responsible for providing a specific piece of optional functionality. 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!
Inheritance, IS-A and Has-A
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. 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. In other words, a superclass has the most general attributes, operations, and relationships that may be shared with subclasses. A subclass may have more specialized attributes and operations.