Inheritance (IS-A) vs. Composition (HAS-A) Relationship in java - Stack Overflow
Thanks for the A2A. IS-A Relationship * IS-A relationship also known as ' inheritance'. * By using extends keyword we can implement inheritance. * The main. 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. When we started talking about multiple inheritance, we have said in some . In object-oriented programming, association defines a relationship.
Copy and Paste the Code Below: Abstraction With abstraction, you can hide the internal workings of an object and only show the features the user needs to know about. Java provides two ways to implement abstraction: Abstract classes An abstract class is a superclass parent class that cannot be instantiated.
You need to instantiate one of its child classes if you want to create a new object. Abstract classes can have both abstract and concrete methods.
Inheritance (IS-A) vs. Composition (HAS-A) Relationship
Abstract methods contain only the method signature, while concrete methods declare a method body as well. Abstract classes are defined with the abstract keyword. In the example below, you can see an abstract class called Animal with two abstract and one concrete method.
Both of them set up their own functionality for the move and eat abstract methods. Both call the one concrete label and the two abstract move and eat methods. It can have only static, final, and public fields and abstract methods.
Java interfaces allow us to implement multiple inheritance in our code, as a class can implement any number of interfaces. Classes can access an interface using the implements keyword. In the example, define two interfaces, Animal and Bird. Animal has two abstract methods, while Bird has two static fields and an abstract method. It defines its own functionality for the three abstract methods. The eat and sound methods come from the Animal class, while fly comes from Bird. Has a high-pitched whistling sound.
Flies up to 10, feet. To do so, declare the fields as private and providing access to them with getter and setter methods. The Animal class below is fully encapsulated. It has three private fields and each of them has its own set of getter and setter methods.
In Java, we need to use the extends keyword to create a child class. In the example, the Eagle class extends the Bird parent class. It inherits all of its fields and methods, plus defines two extra fields that belong only to Eagle. Polymorphism Polymorphism makes it possible to use the same entity in different forms.
In Java, this means that you can declare several methods with the same name until they are different in certain characteristics. Java provides us with two ways to implement polymorphism: Static polymorphism Method overloading means that you can have several methods with the same name within a class.
However, the number, names, or types of their parameters need to be different. For example, the Bird class below has three fly methods.
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Firstly, without parameters, secondly, with one integer parameter for height, and thirdly, with two parameters for name and height. The bird is flying feet high. The eagle is flying feet high. Dynamic polymorphism By using the method overriding feature of Java, you can override the methods of a parent class from its child class.
The Bird class extends the Animal class in the example below. Both have an eat method. However, as it also defines its own eat method, Java will override the original method and call eat from the child class. Then, it also creates a Bird object and calls the polymorphic eat method again.
Therefore Java could have differentiated the two eat methods indeed. This bird eats seeds. Do you have errors in your Java code? Add Raygun Error Monitoring in minutes and detect every probelm in your software as they happen. Make, the manufacturer of the automobile; Model, the kind of automobile; and Year, its year of manufacture. Your Automobile class also has a constructor whose arguments are assigned to the property values, and it overrides the Object.
ToString method to produce a string that uniquely identifies the Automobile instance rather than the Automobile class. For example, you don't need to define a Packard type to represent automobiles manufactured by the Packard Motor Car Company. Instead, you can represent them by creating an Automobile object with the appropriate values passed to its class constructor, as the following example does.
Designing the base class and derived classes Let's look at the process of designing a base class and its derived classes. In this section, you'll define a base class, Publication, which represents a publication of any kind, such as a book, a magazine, a newspaper, a journal, an article, etc.
You'll also define a Book class that derives from Publication. You could easily extend the example to define other derived classes, such as Magazine, Journal, Newspaper, and Article. The base Publication class In designing your Publication class, you need to make several design decisions: What members to include in your base Publication class, and whether the Publication members provide method implementations or whether Publication is an abstract base class that serves as a template for its derived classes.
In this case, the Publication class will provide method implementations. The Designing abstract base classes and their derived classes section contains an example that uses an abstract base class to define the methods that derived classes must override. Derived classes are free to provide any implementation that is suitable for the derived type. The ability to reuse code that is, multiple derived classes share the declaration and implementation of base class methods and do not need to override them is an advantage of non-abstract base classes.
Therefore, you should add members to Publication if their code is likely to be shared by some or most specialized Publication types. If you fail to provide base class implementations efficiently, you'll end up having to provide largely identical member implementations in derived classes rather a single implementation in the base class.
The need to maintain duplicated code in multiple locations is a potential source of bugs. Both to maximize code reuse and to create a logical and intuitive inheritance hierarchy, you want to be sure that you include in the Publication class only the data and functionality that is common to all or to most publications.
Derived classes then implement members that are unique to the particular kinds of publication that they represent. How far to extend your class hierarchy. Do you want to develop a hierarchy of three or more classes, rather than simply a base class and one or more derived classes? For example, Publication could be a base class of Periodical, which in turn is a base class of Magazine, Journal and Newspaper.
For your example, you'll use the small hierarchy of a Publication class and a single derived class, Book. You could easily extend the example to create a number of additional classes that derive from Publication, such as Magazine and Article. Whether it makes sense to instantiate the base class. If it does not, you should apply the abstract keyword to the class. Otherwise, your Publication class can be instantiated by calling its class constructor.
If an attempt is made to instantiate a class marked with the abstract keyword by a direct call to its class constructor, the C compiler generates error CS, "Cannot create an instance of the abstract class or interface. By default, a base class can be instantiated by calling its class constructor. You do not have to explicitly define a class constructor. If one is not present in the base class' source code, the C compiler automatically provides a default parameterless constructor.
For your example, you'll mark the Publication class as abstract so that it cannot be instantiated. An abstract class without any abstract methods indicates that this class represents an abstract concept that is shared among several concrete classes like a Book, Journal. Whether derived classes must inherit the base class implementation of particular members, whether they have the option to override the base class implementation, or whether they must provide an implementation.
You use the abstract keyword to force derived classes to provide an implementation. You use the virtual keyword to allow derived classes to override a base class method. By default, methods defined in the base class are not overridable. The Publication class does not have any abstract methods, but the class itself is abstract. Whether a derived class represents the final class in the inheritance hierarchy and cannot itself be used as a base class for additional derived classes.
By default, any class can serve as a base class. You can apply the sealed keyword to indicate that a class cannot serve as a base class for any additional classes. Attempting to derive from a sealed class generated compiler error CS, "cannot derive from sealed type ". For your example, you'll mark your derived class as sealed. The following example shows the source code for the Publication class, as well as a PublicationType enumeration that is returned by the Publication.
In addition to the members that it inherits from Objectthe Publication class defines the following unique members and member overrides: Book ; However, its instance constructor can be called directly from derived class constructors, as the source code for the Book class shows.
Two publication-related properties Title is a read-only String property whose value is supplied by calling the Publication constructor. Pages is a read-write Int32 property that indicates how many total pages the publication has. The value is stored in a private field named totalPages. Publisher-related members Two read-only properties, Publisher and Type.
The values are originally supplied by the call to the Publication class constructor.
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Publishing-related members Two methods, Publish and GetPublicationDate, set and return the publication date. The Publish method sets a private published flag to true when it is called and assigns the date passed to it as an argument to the private datePublished field. The GetPublicationDate method returns the string "NYP" if the published flag is false, and the value of the datePublished field if it is true. Copyright-related members The Copyright method takes the name of the copyright holder and the year of the copyright as arguments and assigns them to the CopyrightName and CopyrightDate properties.
An override of the ToString method If a type does not override the Object. ToString method, it returns the fully qualified name of the type, which is of little use in differentiating one instance from another.
The Publication class overrides Object. ToString to return the value of the Title property.