Types Of Relationships In Object Oriented Programming (OOPS)
In this article you will learn about the types of Relationships in Object Oriented Object oriented programming generally support 4 types of relationships that are: inheritance, association, It is just like saying that “A is type of B”. Simply composition means mean use of instance variables that are. However, it will probably be a while before you actually do anything with As an example, suppose you want to write a program that plays the card game, . The relationship between subclass and superclass is sometimes shown by a The program could use a class named Vehicle to represent all types of vehicles. It is just like saying "A is a B type of thing". Composition(HAS-A) simply mean the use of instance variables that are references to other objects.
This is due to inheritance. The only problem we have now is the fact that the output of "print y " is not the same as the "print y. We could write it like this: We could, for example, add a new attribute age in Person: Method overriding is an object-oriented programming feature that allows a subclass to provide a different implementation of a method that is already defined by its superclass or by one of its superclasses. The implementation in the subclass overrides the implementation of the superclass by providing a method with the same name, same parameters or signature, and same return type as the method of the parent class.
Overwriting is not a different concept but usually a term wrongly used for overriding! In the context of object-oriented programming, you might have heard about "overloading" as well.
Overloading is the ability to define the same method, with the same name but with a different number of arguments and types.
Javanotes , Section -- Inheritance, Polymorphism, and Abstract Classes
It's the ability of one function to perform different tasks, depending on the number of parameters or the types of the parameters. Let's look first at the case, in which we have the same number of parameters but different types for the parameters.
When we define a function in Python, we don't have to and we can't declare the types of the parameters. So if we define the function "successor" in the following example, we implicitly define a family of function, i. Of course, there are types which will lead to an error if used: The function f can be called with either one or two integer arguments: The second definition of f with two parameters redefines or overrides the first definition with one argument.
By default the subclass retains all of the operations of the base class, but the subclass may override some or all operations, replacing the base-class implementation with its own. The base class comprises operations to compute the sum of the squares between two integers. The subclass re-uses all of the functionality of the base class with the exception of the operation that transforms a number into its square, replacing it with an operation that transforms a number into its square and cube respectively.
An alternative technique, explicit delegationrequires more programming effort, but avoids the substitutability issue. Whereas public inheritance represents an " is-a " relationship and delegation represents a " has-a " relationship, private and protected inheritance can be thought of as an "is implemented in terms of" relationship. The parent class can be a combination of implemented operations and operations that are to be implemented in the child classes.
Often, there is no interface change between the supertype and subtype- the child implements the behavior described instead of its parent class. Subtyping Inheritance is similar to but distinct from subtyping. The relationship can be expressed explicitly via inheritance in languages that support inheritance as a subtyping mechanism. Inheritance, even in programming languages that support inheritance as a subtyping mechanism, does not necessarily entail behavioral subtyping.
Inheritance (object-oriented programming)
It is entirely possible to derive a class whose object will behave incorrectly when used in a context where the parent class is expected; see the Liskov substitution principle.
In some OOP languages, the notions of code reuse and subtyping coincide because the only way to declare a subtype is to define a new class that inherits the implementation of another.
Using inheritance extensively in designing a program imposes certain constraints. For example, consider a class Person that contains a person's name, date of birth, address and phone number. We can define a subclass of Person called Student that contains the person's grade point average and classes taken, and another subclass of Person called Employee that contains the person's job-title, employer, and salary.
In defining this inheritance hierarchy we have already defined certain restrictions, not all of which are desirable: 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. 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.
Object-oriented programming — Object-Oriented Programming in Python 1 documentation
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. 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.
Chapter 18 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. 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. Print a different error message for each different kind of exception.