A class can implement many interfaces at the same time. In these kind of situations, we should use abstract class. If you create a class that only has pure virtual members, you get a strong visual clue that its an interface and its easy to detect violations. I have covered the abstract class and interface in separate tutorials of oops concepts so i would recommend you to read them first, before going though the differences. Abstract class can contain methods, fields, constants, etc. This is the practice used by the microsoft team which developed the base class library. An objectoriented system might use an abstract base class to provide a common and standardized interface appropriate for all the external applications. An interface contains definitions for a group of related functionalities that a non abstract class or a struct must implement. An abstract class can give complete, default code which should be overridden. While an abstract class can be either partially or fully implemented, an interface. Dec 21, 2016 a simple example that demonstrates the use of both abstract classes and interfaces. Abstract classes are the classes in which at least one method is abstract. Make a method abstract so it must be overridden an interface is like an abstract class whose methods are all abstract and whose. A class is made abstract by declaring at least one of its functions as pure virtual function.
Abstract class is used in situation, when we have partial set of implementation of methods in a class. An abstract class is an incomplete class or special class we cant be instantiated. Abstract class provides a set of rules abstract method, which must be followed override in derived classes. It is used to implement the core identity of class. In java, a class can be made abstract by using abstract keyword. Apr 23, 2020 a class inherits only one abstract class. An interface may define static methods, which must have an implementation. Interface members are by default abstract and public.
You can use an interface as a data type for a variable, as. If many implementations only share methods, then it is superior to use interface. When to use interface and when to use abstract class part two. An abstract class must be declared with an abstract keyword. An interface cannot contain a constructor as it cannot be used to create objects why and when to use interfaces. In contrast, it cannot contain constructor, destructor, data members, and static member functions. This is used to denote that the class is an abstract class. An abstract class can be halfcooked class meaning it has provided few methods with body but few methods are without body etc. A class is made abstract by declaring at least one. Abstract classes act as expressions of general concepts from which more specific classes can be derived. Interfaces cant have a default implementation for any method. An abstract class can have non abstract methods concrete methods while in case of interface all the methods has to be abstract.
For example, both carfactory and icarfactory may have makecar method which returns car, but fordcarfactory may want to have a makecar method return a. To access the interface methods, the interface must be implemented kinda like inherited by another class with the implements keyword instead of extends. Another type of interfaces are such without any declared method and are fully accepted as a marker. Where to use interface and abstract class codeproject. Since a class may implement more than one interface, the names in those interfaces may collide. If many implementations are of the same kind and use common behavior, then it is superior to use abstract class. Abstract class and interface both can have abstract methods which are necessary for abstraction. You can call this function an abstract function as it has no body.
Abstract class 0 to 100% interface 100% abstract class in java. The programmer using a class hierarchy can then do so via a base class that communicates only the interface of classes in the hierarchy. A pure virtual function is marked with a virtual keyword and has 0 after its signature. Difference between abstract class and interface in java. The users of this class must declare a matching member function for the class to compile.
An interface can never implement any member it describes. Most of the time, you will use sunsupplied java interfaces. Sep 01, 2009 use an abstract class to define a common base class for a family of types. The two can be used in many different ways, either exclusively or in combination. When to use abstract classes and when to use interfaces in. To achieve this, java requires some additional kinds of classes. Be sure to use at least one of each of the following in your solution. The compiler however complains, that the class doesnt implement the methods specified in the interfaces.
To solve name collisions, java use a simple mechanism. The trick is to determine if the set of route types and row types should be interfaces or classes inheritance structure. You cant use static and virtual modifiers in abstract method declaration. Jun 16, 2015 when creating a class library which will be widely distributed or reused especially to clients, use an abstract class in preference to an interface. By using interfaces, you can, for example, include behavior from multiple sources in a class.
A class can implement any interface, but if you already have a class with a base type, it will not be capable of inheriting from an abstract class. Make a class abstract so instances of it cannot be created. There are two levels of abstraction in java interfaces, used to define. Abstract class and interface are a core part of the java programming language. An objectoriented system might use an abstract base class to provide a common and. The intent of class account is to provide general functionality, but objects of type account are too general to be useful. How do you choose between an interface and abstract class. An abstract class has no use until unless it is extended by some other class.
Often the best set of classes to use is a pure virtual abstract base class to define a common interface. You have to inherit from an abstract class and implement the abstract methods in the inheriting class i. Jun 06, 2005 an abstract base class is a very similar concept to an interface, and the use of 0 enforces correct use. Abstract class can have normal functions and variables along with a pure virtual function. Join the dzone community and get the full member experience. Abstract classes are similar to interfaces, but interfaces can only have declarations of methods and abstract classes can have non abstract methods and definitions.
Lets take a look at an example using the animal class. The purpose of an abstract class is to provide a blueprint for derived classes and set some rules what the derived classes. A class with pure virtual function is known as abstract class. You cannot use access modifiers for the method, properties, etc. It may or may not include abstract methods which means in abstract class you can have concrete methods methods with body as well along with abstract methods witho. An abstract class is a class that is designed to be specifically used as a base class. This allows multiple inheritance without ambiguity. For example the following function is a pure virtual function. You can use an abstract class which contains access.
Conceptually, an abstract class looks just like an interface, of course, without any implementation, however they have their fair share of differences. While an abstract class is a class, an interface is an interface. If you declare an abstract method in a class then you must declare. If there is some basic method that every implementation should share as a default, then you ne. Apr 11, 2015 a class that is declared using abstract keyword is known as abstract class. When creating a standalone project which can be changed at will, use an interface in preference to an abstract class. You can create an interface as an abstract class with public pure virtual functions that will follow these rules. The following animal class is the same as the last chapter, but. Understanding the use of interface and abstract classes. An interface is abstract so that it cant provide any code. All methods declared in an interface must be public. Another way to achieve abstraction in java, is with interfaces.
Abstract classes are mainly used for upcasting, so that its derived classes can use its interface. You declare a pure virtual function by using a pure specifier 0 in the declaration of a virtual member function in the class declaration. A class can implement multiple interfaces use an abstract class if you want to define default behaviour in non abstract methods so that the endusers can reuse it without rewriting it again and again. This means that the method cannot be changed by the child class. In this tutorial we will give an introduction to abstraction in java and define a simple. An interface can inherit functions from one or more base interface. Then, through inheritance from that abstract base class, derived classes are formed that all operate similarly. Interfaces help to define the peripheral abilities of a class. This article is meant to be a theoretical and practical overview of interfaces and abstract classes. The interface in java is a mechanism to achieve abstraction. An abstract class looks a lot like an interface, but the concept is a bit confusing for the beginners of oop. In the article, i have explain the differences between an abstract class and an interface. An interface is a completely abstract class that is used to group related methods with empty bodies.
Its a way of forcing a contract between the class designer and the users of that class. Similarly a function can be made pure virtual or abstract by using abstract keyword. A class which is declared as abstract is known as an abstract class. How to use an interface, and when do i use an abstract class. Whether to choose an interface or abstract class is a design decision that every architect faces. An abstract class doesnt provide full abstraction but an interface does provide full abstraction. Interface a special type of class a pure abstract class. Difference between abstract class and interface in java multiple inheritance of abstract class and interface in java despite the fact that both abstract class and interface are primarily used for abstraction, they are quite different from each other. It allows us to have the flexibility of an interface and. In this article, we will discuss the difference between abstract class and interface in java with examples. This tutorial will try to explain the difference between abstract class. It is used to achieve abstraction and multiple inheritance in java.
You can use an abstract class which contains access modifiers. Thats because the lion class can derive from the abstract class in the same way it can derive from a regular public class. All abstract methods must be override in the derived classes. When should i use an interface over an abstract class. I will try to incorporate all the requirements and especially when and where to use abstract class and interface. Use the form implements x, y, z to have your class implement several interfaces. Where to use interface and where to use abstract class part 1 i got a lot of feedback on my previous article and some users are desperately waiting for this one. A class that contains at least one pure virtual function is considered an abstract class. Notice that the lion class derives from its parent animal class, but nothing has changed from the previous chapters class.
An interface class is a class that specifies the polymorphic interface i. There can be only abstract methods in the java interface, not method body. Both abstract class and interfaces are abstractions in java that cannot be instantiated. In general, a pure abstract class is used to define an interface and is intended to be inherited by concrete classes. You can also derive from an abstract class and then use the child class as its own abstract class. An interface is a contract, a class is a concrete representation of something, and an abstract class is a unecessary and limiting combination of the twoan additional language construct with a very specific use that can be accomplished naturally other ways. To examples of usage could be template pattern and strategy pattern. Abstract classes and interfaces information and computer. Abstract class takes part in hierarchy of classes based on inheritance.
Explicit interface implementation can also be good if its possible that a derived class override of a member may wish to return a type which is derived from the base class implementation. Creating an abstract class that implements multiple. When creating a class library which will be widely distributed or reusedespecially to clients, use an abstract class in preference to an interface. An interface class contains only a virtual destructor and pure virtual functions. An abstract class contains at least one pure virtual function.
The abstract modifier indicates that the thing being modified has a missing or incomplete implementation. Difference between abstract class and interface is one of the popular interview questions. Next, we are defining our method which does nothing. Classes derived from the abstract class must implement the pure virtual function or they, too, are abstract classes. This means if a class, implementing an interface, does not implement all members described by that interface then such class must be abstract class. Consider the following example where parent class provides an interface to the. Both an abstract class and an interface can implement static methods. Then use an abstract class to further refine an implementation for a set of concrete classes and lastly define the set of concrete classes. The first thing to note is that an interface does not have any code to share. Classes inheriting an abstract class must implement all pure virtual functions, or else they will become abstract too. An abstract class is a class, that has atleast one abstract method or you can also make all your methods as abstract. Use of abstract classes are that all base classes implementing this class should give implementation of abstract methods declared in parent class.
It is used to implement peripheral abilities of class. An abstract class can declare or use any variables while an interface is not allowed to do so. Combining new interface with an abstract base class is a common pattern. If a subclass of an abstract superclass does not implement all the abstract methods, the subclass. An interface does not have implementation of any of its methods, it can be considered as a collection of method. The abstract modifier can be used with classes, methods, properties, indexers, and events.
It needs to be extended and its method implemented. Im looking at some concrete examples right now, so i think i can help. A class which is not abstract is referred as concrete class. Consider the example presented in virtual functions. The performance of interface is slow because it requires time to search actual method in the corresponding class. Use the abstract modifier in a class declaration to indicate that a class is intended only to be a base class of other classes, not instantiated on. Out of four methods, we have an implementation of two methods and we need derived class to implement other two methods. We know that an interface is defined by the interface keyword and all methods are abstract. You cannot create an object of an abstract class type. Learn when it would be more helpful to use an abstract class in java rather than the interface. When we have to use interface and abstract practically. An abstract class can declare or use any variables while an interface is. When to use abstract class and interface dzone java. Subclass only a base class in a hierarchy to which the class logically belongs.