JavaHotSpotBlog: Difference between ‘is-a’ relationship and ‘has-a’ relationship in java
This is a great example of something that was "known" but was in fact NOT TRUE . In ActionScript (and Java for that matter), all variables default to a given value HASA. When one class "extends" another class we say that the first class ISA ( is a) In fact, this ISA relationship is so important, that you will see ISA used as a . designed for creating Java class skeleton, listing the class and its members. . HASA relationship is used for composing classes and ISA relationship is used. Association is a relationship between two objects. In other words Example: Relationship between shape and circle is dependency.
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. We can say that a string, an integer or a Person instance is an object instance.
- Inheritance, IS-A and Has-A
- Table of Content
- Types of Relationship in Java
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.
Has-a - Wikipedia
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. 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.Core Java With OCJP/SCJP:OOPs(Object Oriented Programming)Part-3 -- Inheritance--has a relationship
This sometimes means that they depend on other properties which already exist in the class which inherits from them. We could extend this example with more mix-ins which represent the ability to pay fees, the ability to get paid for services, and so on — we could then create a relatively flat hierarchy of classes for different kinds of people which inherit from Person and some number of mix-ins.
Why would we want to do this? Sometimes we want to specify a set of properties that an object needs to have in order to be suitable for some task — for example, we may have written a function which expects one of its parameters to be an object with certain methods that our function will need to use.
We can create a class which serves as a template for suitable objects by defining a list of methods that these objects must implement. This class is not intended to be instantiated because all our method definitions are empty — all the insides of the methods must be implemented in a subclass. The abstract class is thus an interface definition — some languages also have a type of structure called an interface, which is very similar.
We say that a class implements an interface if it inherits from the class which specifies that interface. The formulae for working out area and volume differ depending on what shape we have, and objects for different shapes may have completely different attributes.
Write a simple Item class which has a name attribute and a value attribute — you can assume that all the items you will use will be Item objects. Now write two subclasses of Box which use different underlying collections to store items: ListBox should use a list, and DictBox should use a dict.
There are multiple ways of doing this. We refer association between two objects as Compositionwhen one class owns other class and other class can not meaningfully exist, when it's owner destroyed, for example, Human class is a composition of several body parts including Hand, Leg and Heart. When human object dies, all it's body part ceased to exist meaningfully, this is one example of Composition. Another example of Composition is Car and it's part e.
Individual parts of the car can not function when a car is destroyed. While in the case of Aggregation, including object can exists without being part of the main object e. Another example of Aggregation is Student in School class, when School closed, Student still exist and then can join another School or so. In UML notation, a composition is denoted by a filled diamond, while aggregation is denoted by an empty diamond, which shows their obvious difference in terms of strength of the relationship.
The composition is stronger than Aggregation. In Short, a relationship between two objects is referred as an association, and an association is known as composition when one object owns other while an association is known as aggregation when one object uses another object.
In this OOPS tutorial, we will see a couple of more examples to understand difference between Association, Composition and Aggregation better.
What is the difference between IS -A relationship and HAS-A relationship in Java? - Stack Overflow
By looking at this code, you can gauge differences between these two. By the way, Composition is also very much preferred in object oriented design over inheritance, even Joshua Bloch has stated its importance in the classic book, Effective Java.
Since Engine is-part-of Car, the relationship between them is Composition. Here is how they are implemented between Java classes. Similar way is used to show aggregation and composition between two classes.