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In object-oriented programming , inheritance is when an object or class is based on another object prototypal inheritance or class class-based inheritance , using the same implementation. Inheritance in most class-based object-oriented languages is a mechanism in which one object acquires all the properties and behaviours of the parent object. Inheritance allows programmers to create classes that are built upon existing classes, [1] to specify a new implementation to maintain the same behaviour realizing an interface , to reuse code and to independently extend original software via public classes and interfaces.

The relationships of objects or classes through inheritance give rise to a directed graph. An inherited class is called a subclass of its parent class or super class. The term "inheritance" is loosely used for both class-based and prototype-based programming, but in narrow use is reserved for class-based programming one class inherits from another , with the corresponding technique in prototype-based programming being instead called delegation one object delegates to another.

Inheritance should not be confused with subtyping. To distinguish these concepts, subtyping is also known as interface inheritance , whereas inheritance as defined here is known as implementation inheritance or code inheritance.

Inheritance is contrasted with object composition , where one object contains another object or objects of one class contain objects of another class ; see composition over inheritance.

Composition implements a has-a relationship, in contrast to the is-a relationship of subtyping. Inheritance was invented in for Simula. Thus, multiple inheritance seemed more of a challenge. Since I had considered multiple inheritance as early as and found a simple and efficient implementation technique in I couldn't resist the challenge.

I suspect this to be the only case in which fashion affected the sequence of events. A Subclass , "derived class", heir class , or child class is a modular , derivative class that inherits one or more language entities from one or more other classes called superclass , base classes , or parent classes.

The semantics of class inheritance vary from language to language, but commonly the subclass automatically inherits the instance variables and member functions of its superclasses. The general form of defining a derived class is:.

Some languages support the inheritance of other constructs also. For example, in Eiffel , contracts that define the specification of a class are also inherited by heirs. The superclass establishes a common interface and foundational functionality, which specialized subclasses can inherit, modify, and supplement. The software inherited by a subclass is considered reused in the subclass. A reference to an instance of a class may actually be referring to one of its subclasses. The actual class of the object being referenced is impossible to predict at compile-time.

A uniform interface is used to invoke the member functions of objects of a number of different classes. Subclass may replace superclass functions with entirely new functions that must share the same method signature. In some languages a class may be declared as uninheritable by adding certain class modifiers to the class declaration. Such modifiers are added to the class declaration before the "class" keyword and the class identifier declaration.

Such sealed classes restrict reusability , particularly when developers only have access to precompiled binaries and not source code. The sealed class has no subclasses, so it can be easily deduced at compile time that references or pointers to objects of that class are actually referencing instances of that class and not instances of subclasses they don't exist or instances of superclasses upcasting a reference type violates the type system.

Because the exact type of the object being referenced is known before execution, early binding or " static dispatch " can be used instead of late binding also called " dynamic dispatch " or " dynamic binding " , which requires one or more virtual method table lookups depending on whether multiple inheritance or only single inheritance are supported in the programming language that is being used.

A "final" method in Java , a "sealed" method in C or a frozen feature in Eiffel cannot be overridden. If the superclass method is a virtual method , then invocations of the superclass method will be dynamically dispatched. Some languages require methods to be specifically declared as virtual e. An invocation of a non-virtual method will always be statically dispatched i. Static dispatch is faster than dynamic dispatch and allows optimisations such as inline expansion. Many object-oriented programming languages permit a class or object to replace the implementation of an aspect—typically a behavior—that it has inherited.

This process is usually called overriding. Overriding introduces a complication: The answer varies between programming languages, and some languages provide the ability to indicate that a particular behavior is not to be overridden and should behave as defined by the base class. For instance, in C , the base method or property can only be overridden in a subclass if it is marked with the virtual, abstract, or override modifier.

Implementation inheritance is the mechanism whereby a subclass re-uses code in a base class. 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. In most quarters, class inheritance for the sole purpose of code reuse has fallen out of favor. An alternative technique, explicit delegation , requires 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. Another frequent use of inheritance is to guarantee that classes maintain a certain common interface; that is, they implement the same methods.

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. 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.

In programming languages that do not support inheritance as a subtyping mechanism , the relationship between a base class and a derived class is only a relationship between implementations a mechanism for code reuse , as compared to a relationship between types. Inheritance, even in programming languages that support inheritance as a subtyping mechanism, does not necessarily entail behavioral subtyping. 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. 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:. The composite reuse principle is an alternative to inheritance. This technique supports polymorphism and code reuse by separating behaviors from the primary class hierarchy and including specific behavior classes as required in any business domain class.

This approach avoids the static nature of a class hierarchy by allowing behavior modifications at run time and allows one class to implement behaviors buffet-style, instead of being restricted to the behaviors of its ancestor classes. Implementation inheritance is controversial among programmers and theoreticians of object-oriented programming since at least the s. Among them are the authors of Design Patterns , who advocate interface inheritance instead, and favor composition over inheritance.

For example, the decorator pattern as mentioned above has been proposed to overcome the static nature of inheritance between classes. As a more fundamental solution to the same problem, role-oriented programming introduces a distinct relationship, played-by , combining properties of inheritance and composition into a new concept.

According to Allen Holub , the main problem with implementation inheritance is that it introduces unnecessary coupling in the form of the "fragile base class problem": Using interfaces avoids this problem because no implementation is shared, only the API. Reportedly, Java inventor James Gosling has spoken against implementation inheritance, stating that he would not include it if he were to redesign Java.

Complex inheritance, or inheritance used within an insufficiently mature design, may lead to the yo-yo problem. Another issue with inheritance is that subclasses must be defined in code, which means that program users cannot add new subclasses. Other design patterns such as Entity—component—system allow program users to define variations of an entity at runtime.

From Wikipedia, the free encyclopedia. Not to be confused with object composition or subtyping. This article has multiple issues. Please help improve it or discuss these issues on the talk page. Learn how and when to remove these template messages. This article needs attention from an expert in Computer science. Please add a reason or a talk parameter to this template to explain the issue with the article. WikiProject Computer science may be able to help recruit an expert.

This article may require cleanup to meet Wikipedia's quality standards. The specific problem is: Cluttered Please help improve this article if you can. April Learn how and when to remove this template message. Inheritance is not subtyping. Typeful Programming Technical report. SRC Research Report A study of the fragile base class problem PDF. Lecture Notes in Computer Science. What programmers do with inheritance in Java PDF. United States of America: Retrieved 28 June Tata McGrawhill Education Pvt.

Exception-Safe Class Design, Part 2: Venugopal, Rajkumar Buyya Concepts in programming language. Retrieved 10 March Designing an object-oriented programming language with behavioural subtyping PDF.