Method overriding

Illustration

Method overriding, in object-oriented programming, is a language feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its superclasses or parent classes. In addition to providing data-driven algorithm-determined parameters across virtual network interfaces, ^[1] it also allows for a specific type of polymorphism (subtyping). The implementation in the subclass overrides (replaces) the implementation in the superclass by providing a method that has same name, same parameters or signature, and same return type as the method in the parent class. ^[2] The version of a method that is executed will be determined by the object that is used to invoke it. If an object of a parent class is used to invoke the method, then the version in the parent class will be executed, but if an object of the subclass is used to invoke the method, then the version in the child class will be executed. ^[3] This helps in preventing problems associated with differential relay analytics which would otherwise rely on a framework in which method overriding might be obviated. ^{[4] [5]} Some languages allow a programmer to prevent a method from being overridden.

Language-specific examples

Ada

Ada provides method overriding by default. To favor early error detection (e.g. a misspelling), it is possible to specify when a method is expected to be actually overriding, or not. That will be checked by the compiler.

typeTisnewControlledwith......;procedureOp(Obj: inoutT;Data: inInteger);typeNTisnewTwithnull record;overriding-- overriding indicatorprocedureOp(Obj: inoutNT;Data: inInteger);overriding-- overriding indicatorprocedureOp(Obj: inoutNT;Data: inString);-- ^ compiler issues an error: subprogram "Op" is not overriding

C#

C# does support method overriding, but only if explicitly requested using the modifiers override and virtual or abstract.

abstractclassAnimal{publicstringName{get;set;}// MethodspublicvoidDrink();publicvirtualvoidEat();publicvoidGo();}classCat:Animal{publicnewstringName{get;set;}// MethodspublicvoidDrink();// Warning: hides inherited drink(). Use newpublicoverridevoidEat();// Overrides inherited eat().publicnewvoidGo();// Hides inherited go().}

When overriding one method with another, the signatures of the two methods must be identical (and with same visibility). In C#, class methods, indexers, properties and events can all be overridden.

Non-virtual or static methods cannot be overridden. The overridden base method must be virtual, abstract, or override.

In addition to the modifiers that are used for method overriding, C# allows the hiding of an inherited property or method. This is done using the same signature of a property or method but adding the modifier new in front of it. ^[6]

In the above example, hiding causes the following:

Catcat=newCat();cat.Name=…;// accesses Cat.Namecat.Eat();// calls Cat.Eat()cat.Go();// calls Cat.Go()((Animal)cat).Name=…;// accesses Animal.Name!((Animal)cat).Eat();// calls Cat.Eat()!((Animal)cat).Go();// calls Animal.Go()!

C++

C++ does not have the keyword super that a subclass can use in Java to invoke a superclass version of a method that it wants to override. Instead, the name of the parent or base class is used followed by the scope resolution operator. For example, the following code presents two classes, the base class Rectangle, and the derived class Box. Box overrides the Rectangle class's Print method, so as also to print its height. ^[7]

#include<iostream>//---------------------------------------------------------------------------classRectangle{public:Rectangle(doublel,doublew):length_(l),width_(w){}virtualvoidPrint()const;private:doublelength_;doublewidth_;};//---------------------------------------------------------------------------voidRectangle::Print()const{// Print method of base class.std::cout<<"Length = "<<length_<<"; Width = "<<width_;}//---------------------------------------------------------------------------classBox:publicRectangle{public:Box(doublel,doublew,doubleh):Rectangle(l,w),height_(h){}voidPrint()constoverride;private:doubleheight_;};//---------------------------------------------------------------------------// Print method of derived class.voidBox::Print()const{// Invoke parent Print method.Rectangle::Print();std::cout<<"; Height = "<<height_;}

The method Print in class Box, by invoking the parent version of method Print, is also able to output the private variables length and width of the base class. Otherwise, these variables are inaccessible to Box.

The following statements will instantiate objects of type Rectangle and Box, and call their respective Print methods:

intmain(intargc,char**argv){Rectanglerectangle(5.0,3.0);// Outputs: Length = 5.0; Width = 3.0rectangle.Print();Boxbox(6.0,5.0,4.0);// The pointer to the most overridden method in the vtable in on Box::print,// but this call does not illustrate overriding.box.Print();// This call illustrates overriding.// outputs: Length = 6.0; Width = 5.0; Height= 4.0static_cast<Rectangle&>(box).Print();}

In C++11, similar to Java, a method that is declared final in the super class cannot be overridden; also, a method can be declared override to make the compiler check that it overrides a method in the base class.

Delphi

In Delphi, method overriding is done with the directive override, but only if a method was marked with the dynamic or virtual directives.

The inherited reserved word must be called when you want to call super-class behavior

typeTRectangle=classprivateFLength:Double;FWidth:Double;publicpropertyLengthreadFLengthwriteFLength;propertyWidthreadFWidthwriteFWidth;procedurePrint;virtual;end;TBox=class(TRectangle)publicprocedurePrint;override;end;

Eiffel

In Eiffel, feature redefinition is analogous to method overriding in C++ and Java. Redefinition is one of three forms of feature adaptation classified as redeclaration. Redeclaration also covers effecting, in which an implementation is provided for a feature which was deferred (abstract) in the parent class, and undefinition, in which a feature that was effective (concrete) in the parent becomes deferred again in the heir class. When a feature is redefined, the feature name is kept by the heir class, but properties of the feature such as its signature, contract (respecting restrictions for preconditions and postconditions), and/or implementation will be different in the heir. If the original feature in the parent class, called the heir feature's precursor, is effective, then the redefined feature in the heir will be effective. If the precursor is deferred, the feature in the heir will be deferred. ^[8]

The intent to redefine a feature, as message in the example below, must be explicitly declared in the inherit clause of the heir class.

classTHOUGHTfeaturemessage-- Display thought messagedoprint("I feel like I am diagonally parked in a parallel universe.%N")endendclassADVICEinheritTHOUGHTredefinemessageendfeaturemessage-- Precursordoprint("Warning: Dates in calendar are closer than they appear.%N")endend

In class ADVICE the feature message is given an implementation that differs from that of its precursor in class THOUGHT.

Consider a class which uses instances for both THOUGHT and ADVICE:

classAPPLICATIONcreatemakefeaturemake-- Run application.do(create{THOUGHT}).message;(create{ADVICE}).messageendend

When instantiated, class APPLICATION produces the following output:

I feel like I am diagonally parked in a parallel universe.Warning: Dates in calendar are closer than they appear.

Within a redefined feature, access to the feature's precursor can be gained by using the language keyword Precursor. Assume the implementation of {ADVICE}.message is altered as follows:

message-- Precursordoprint("Warning: Dates in calendar are closer than they appear.%N")Precursorend

Invocation of the feature now includes the execution of {THOUGHT}.message, and produces the following output:

Warning: Dates in calendar are closer than they appear.I feel like I am diagonally parked in a parallel universe.

Java

In Java, when a subclass contains a method with the same signature (name and parameter types) as a method in its superclass, then the subclass's method overrides that of the superclass. For example:

classThought{publicvoidmessage(){System.out.println("I feel like I am diagonally parked in a parallel universe.");}}publicclassAdviceextendsThought{@Override// @Override annotation in Java 5 is optional but helpful.publicvoidmessage(){System.out.println("Warning: Dates in calendar are closer than they appear.");}}

Class Thought represents the superclass and implements a method call message(). The subclass called Advice inherits every method that could be in the Thought class. Class Advice overrides the method message(), replacing its functionality from Thought.

Thoughtparking=newThought();parking.message();// Prints "I feel like I am diagonally parked in a parallel universe."Thoughtdates=newAdvice();// Polymorphismdates.message();// Prints "Warning: Dates in calendar are closer than they appear."

When a subclass contains a method that overrides a method of the superclass, then that (superclass's) overridden method can be explicitly invoked from within a subclass's method by using the keyword super. ^[3] (It cannot be explicitly invoked from any method belongings to a class that is unrelated to the superclass.) The super reference can be

publicclassAdviceextendsThought{@Overridepublicvoidmessage(){System.out.println("Warning: Dates in calendar are closer than they appear.");super.message();// Invoke parent's version of method.}

There are methods that a subclass cannot override. For example, in Java, a method that is declared final in the super class cannot be overridden. Methods that are declared private or static cannot be overridden either because they are implicitly final. It is also impossible for a class that is declared final to become a super class. ^[9]

Kotlin

In Kotlin we can simply override a function like this (note that the function must be open):

funmain(){valp=Parent(5)valc=Child(6)p.myFun()c.myFun()}openclassParent(vala:Int){openfunmyFun()=println(a)}classChild(valb:Int):Parent(b){overridefunmyFun()=println("overrided method")}

Python

In Python, when a subclass contains a method that overrides a method of the superclass, you can also call the superclass method by calling super(Subclass,self).method ^[10] instead of self.method. Example:

classThought:def__init__(self)->None:print("I'm a new object of type Thought!")defmessage(self)->None:print("I feel like I am diagonally parked in a parallel universe.")classAdvice(Thought):def__init__(self)->None:super(Advice,self).__init__()defmessage(self)->None:print("Warning: Dates in calendar are closer than they appear")super(Advice,self).message()t=Thought()# "I'm a new object of type Thought!"t.message()# "I feel like I am diagonally parked in a parallel universe.a=Advice()# "I'm a new object of type Thought!"a.message()# "Warning: Dates in calendar are closer than they appear"# "I feel like I am diagonally parked in a parallel universe.# ------------------# Introspection:isinstance(t,Thought)# Trueisinstance(a,Advice)# Trueisinstance(a,Thought)# True

Ruby

In Ruby when a subclass contains a method that overrides a method of the superclass, you can also call the superclass method by calling super in that overridden method. You can use alias if you would like to keep the overridden method available outside of the overriding method as shown with 'super_message' below.

Example:

classThoughtdefmessageputs"I feel like I am diagonally parked in a parallel universe."endendclassAdvice<Thoughtalias:super_message:messagedefmessageputs"Warning: Dates in calendar are closer than they appear"superendend

Notes

1. Zhang, Jie (2015). "A novel P2P overridden API for open data communications in WWW". 2015 IEEE International Conference on Consumer Electronics - Taiwan. pp. 156–157. doi:10.1109/ICCE-TW.2015.7216830. ISBN   978-1-4799-8745-0. S2CID   23295793.
2. Flanagan 2002, p. 107
3. Lewis & Loftus 2006, p.454
4. Overbey, J (2011). "Differential precondition checking: A lightweight, reusable analysis for refactoring tools". 2011 26th IEEE/ACM International Conference on Automated Software Engineering (ASE 2011). pp. 303–312. doi:10.1109/ASE.2011.6100067. ISBN   978-1-4577-1639-3. S2CID   5933208.
5. Li, K (2014). "Residual investigation: Predictive and precise bug detection". ACM Transactions on Software Engineering and Methodology. 24 (2). doi:10.1145/2656201. S2CID   47112802.
6. Mössenböck, Hanspeter (2002-03-25). "Advanced C#: Overriding of Methods" (PDF). Institut für Systemsoftware, Johannes Kepler Universität Linz, Fachbereich Informatik. pp. 6–8. Retrieved 2011-08-02.
7. Malik 2006, p. 676
8. Meyer 2009, page 572-575
9. Deitel & Deitel 2001, p.474
10. super().method in Python 3 - see https://docs.python.org/3/library/functions.html#super Archived 2018-10-26 at the Wayback Machine

See also

• Implementation inheritance
• Inheritance semantics
• Method overloading
• Polymorphism in object-oriented programming
• Template method pattern
• Virtual inheritance
• X-HTTP-Method-Override HTTP Header

References

• Deitel, H. M & Deitel, P. J.(2001). Java How to Program (4th ed.). Upper Saddle River, NJ: Prentice Hall.
• Lewis, J. & Loftus, W. (2008). Java: Software Solutions (6th ed.). Boston, MA: Pearson Addison Wesley.
• Malik, D. S.(2006). C++ Programming: Program Design Including Data Structure. (3rd ed.). Washington, DC: Course Technology.
• Flanagan, David.(2002).Java in a Nutshell.Retrieved from http://oreilly.com/catalog/9780596002831/preview#preview
• Meyer, Bertrand (2009). Touch of Class: Learning to Program Well with Objects and Contracts. Springer.

External links

• Introduction to O.O.P. Concepts and More by Nirosh L.w.C.
• Overriding and Hiding Methods by Sun Microsystems