Types Of Relationships In Object Oriented Programming (OOPS)
In OOP, IS - A relationship is completely inheritance. This means, that the child class is a type of parent class. For example If you just want to reuse the code and you know that the two are not of same kind use composition. These are the building blocks of object oriented programming and very Just wanted to refresh these terms and explain in simpler terms. Association is a relationship between two objects. At a very broader level you can understand this as inheritance. .. There are four kinds of Class relationships. The ranks are Ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen, and King. If we want to define a new object to represent a playing card, it is obvious what the attributes should be: .. This relationship is called IS-A, as in, “a Hand is a kind of a Deck.
Inheritance (object-oriented programming)
In Python we do this by splitting code up into modules. The main additional advantage of object orientation, as we saw in the previous chapter, is that it combines data with the functions which act upon that data in a single structure. We will discuss this principle, which we call encapsulation, in the next section.
Some people believe that OOP is a more intuitive programming style to learn, because people find it easy to reason about objects and relationships between them. OOP is thus sometimes considered to be a superior approach because it allows new programmers to become proficient more quickly.
The age function we saw in the previous chapter is a good example of this philosophy. 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?
What is the difference between IS -A relationship and HAS-A relationship in Java? - Stack Overflow
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. 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. As we saw in the previous chapter, all objects in Python inherit from object.
Chapter 18 Inheritance
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.Inheritance, Money and Toxic Parents
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.
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: In some cases, the inheritance structure reflects the natural structure of the problem, which makes the design easier to understand.
On the other hand, inheritance can make programs difficult to read. When a method is invoked, it is sometimes not clear where to find its definition. The relevant code may be spread across several modules.
Also, many of the things that can be done using inheritance can be done as well or better without it. These diagrams represent a snapshot in the execution of a program, so they change as the program runs.
They are also highly detailed; for some purposes, too detailed. A class diagram is a more abstract representation of the structure of a program. Instead of showing individual objects, it shows classes and the relationships between them. There are several kinds of relationship between classes: Objects in one class might contain references to objects in another class. For example, each Rectangle contains a reference to a Point, and each Deck contains references to many Cards.
This kind of relationship is called a dependency. A class diagram is a graphical representation of these relationships.
The arrow with a hollow triangle head represents an IS-A relationship; in this case it indicates that Hand inherits from Deck. A multiplicity can be a simple number, like 52, a range, like There are no dependencies in this diagram. They would normally be shown with a dashed arrow. Or if there are a lot of dependencies, they are sometimes omitted. A more detailed diagram might show that a Deck actually contains a list of Cards, but built-in types like list and dict are usually not included in class diagrams.
Suppose you are writing a function that works with Hand objects.
This behavior is usually a good thing, but it can be confusing. Any time you are unsure about the flow of execution through your program, the simplest solution is to add print statements at the beginning of the relevant methods.
As an alternative, you could use this function, which takes an object and a method name as a string and returns the class that provides the definition of the method: It should take the same parameters, return the same type, and obey the same preconditions and postconditions.
If you follow this rule, you will find that any function designed to work with an instance of a parent class, like a Deck, will also work with instances of child classes like a Hand and PokerHand. We identified objects we needed—like Point, Rectangle and Time—and defined classes to represent them. In each case there is an obvious correspondence between the object and some entity in the real world or at least a mathematical world. But sometimes it is less obvious what objects you need and how they should interact.
In that case you need a different development plan. In the same way that we discovered function interfaces by encapsulation and generalization, we can discover class interfaces by data encapsulation. If you download my code from http: If we read two texts, their prefixes and suffixes would be added to the same data structures which makes for some interesting generated text.
Object-oriented programming — Object-Oriented Programming in Python 1 documentation
To run multiple analyses, and keep them separate, we can encapsulate the state of each analysis in an object. This example suggests a development plan for designing objects and methods: Start by writing functions that read and write global variables when necessary. Once you get the program working, look for associations between global variables and the functions that use them.
Encapsulate related variables as attributes of an object. Transform the associated functions into methods of the new class. As an exercise, download my Markov code from http: To represent one set of values using another set of values by constructing a mapping between them. An attribute associated with a class object. Class attributes are defined inside a class definition but outside any method.
An attribute associated with an instance of a class. A method or function that provides a different interface to another function without doing much computation.
The ability to define a new class that is a modified version of a previously defined class. The class from which a child class inherits. A relationship between a child class and its parent class. A relationship between two classes where instances of one class contain references to instances of the other. A relationship between two classes where instances of one class use instances of the other class, but do not store them as attributes.
A diagram that shows the classes in a program and the relationships between them. A notation in a class diagram that shows, for a HAS-A relationship, how many references there are to instances of another class. A program development plan that involves a prototype using global variables and a final version that makes the global variables into instance attributes.
It should create the appropriate number of Hand objects, deal the appropriate number of cards per hand, and return a list of Hands.