Is possible for a subclass to define a method with the same name and parameters as a method defined by the superclass?

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. It 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.[1] 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.[2] Some languages allow a programmer to prevent a method from being overridden.

Language-specific examples[edit]

Ada[edit]

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.

type T is new Controlled with ......; procedure Op(Obj: in out T; Data: in Integer); type NT is new T with null record; overriding -- overriding indicator procedure Op(Obj: in out NT; Data: in Integer); overriding -- overriding indicator procedure Op(Obj: in out NT; Data: in String); -- ^ compiler issues an error: subprogram "Op" is not overriding

C#[edit]

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

abstract class Animal { public string Name { get; set; } // Methods public void Drink(); public virtual void Eat(); public void Go(); } class Cat : Animal { public new string Name { get; set; } // Methods public void Drink(); // Warning: hides inherited drink(). Use new public override void Eat(); // Overrides inherited eat(). public new void Go(); // 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.[3]

In the above example, hiding causes the following:

Cat cat = new Cat(); cat.Name = …; // accesses Cat.Name cat.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++[edit]

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.[4]

#include <iostream> //--------------------------------------------------------------------------- class Rectangle { public: Rectangle(double l, double w) : length_(l), width_(w) {} virtual void Print() const; private: double length_; double width_; }; //--------------------------------------------------------------------------- void Rectangle::Print() const { // Print method of base class. std::cout << "Length = " << length_ << "; Width = " << width_; } //--------------------------------------------------------------------------- class Box : public Rectangle { public: Box(double l, double w, double h) : Rectangle(l, w), height_(h) {} void Print() const override; private: double height_; }; //--------------------------------------------------------------------------- // Print method of derived class. void Box::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:

int main(int argc, char** argv) { Rectangle rectangle(5.0, 3.0); // Outputs: Length = 5.0; Width = 3.0 rectangle.Print(); Box box(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= 4.0 static_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[edit]

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

type TRectangle = class private FLength: Double; FWidth: Double; public property Length read FLength write FLength; property Width read FWidth write FWidth; procedure Print; virtual; end; TBox = class(TRectangle) public procedure Print; override; end;

Eiffel[edit]

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.[5]

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

class THOUGHT feature message -- Display thought message do print ("I feel like I am diagonally parked in a parallel universe.%N") end end class ADVICE inherit THOUGHT redefine message end feature message -- Precursor do print ("Warning: Dates in calendar are closer than they appear.%N") end end

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:

class APPLICATION create make feature make -- Run application. do (create {THOUGHT}).message; (create {ADVICE}).message end end

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 -- Precursor do print ("Warning: Dates in calendar are closer than they appear.%N") Precursor end

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[edit]

In Java, when a subclass contains a method that overrides a method of the superclass, it can also invoke the superclass method by using the keyword super.[2] Example:

class Thought { public void message() { System.out.println("I feel like I am diagonally parked in a parallel universe."); } } public class Advice extends Thought { @Override // @Override annotation in Java 5 is optional but helpful. public void message() { 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. However, class Advice overrides the method message(), replacing its functionality from Thought.

Thought parking = new Thought(); parking.message(); // Prints "I feel like I am diagonally parked in a parallel universe." Thought dates = new Advice(); // Polymorphism dates.message(); // Prints "Warning: Dates in calendar are closer than they appear."

The super reference can be

public class Advice extends Thought { @Override public void message() { 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.[6]

Kotlin[edit]

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

fun main() { val p = Parent(5) val c = Child(6) p.myFun() c.myFun() } open class Parent(val a : Int) { open fun myFun() = println(a) } class Child(val b : Int) : Parent(b) { override fun myFun() = println("overrided method") }

Python[edit]

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[7] instead of self.method. Example:

class Thought: def __init__(self) -> None: print("I'm a new object of type Thought!") def message(self) -> None: print("I feel like I am diagonally parked in a parallel universe.") class Advice(Thought): def __init__(self) -> None: super(Advice, self).__init__() def message(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) # True isinstance(a, Advice) # True isinstance(a, Thought) # True

Ruby[edit]

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:

class Thought def message puts "I feel like I am diagonally parked in a parallel universe." end end class Advice < Thought alias :super_message :message def message puts "Warning: Dates in calendar are closer than they appear" super end end

Notes[edit]

^ Flanagan 2002, p. 107
^ a b Lewis & Loftus 2006, p.454
^ 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.
^ Malik 2006, p. 676
^ Meyer 2009, page 572-575
^ Deitel & Deitel 2001, p.474
^ super().method in Python 3 - see //docs.python.org/3/library/functions.html#super Archived 2018-10-26 at the Wayback Machine

References[edit]

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 //oreilly.com/catalog/9780596002831/preview#preview
Meyer, Bertrand (2009). Touch of Class: Learning to Program Well with Objects and Contracts. Springer.

External links[edit]

Java Method Overriding by Hemanth Balaji
Introduction to O.O.P. Concepts and More by Nirosh L.w.C.
Overriding and Hiding Methods by Sun Microsystems

Is it possible for a subclass to define a method with the same name and parameters?

A subclass can do more than that; it can define a method that has exactly the same method signature (name and argument types) as a method in its superclass. In that case, the method in the subclass overrides the method in the superclass and effectively replaces its implementation, as shown in Figure 6-3.

Can a subclass have a method with the same name?

If your subclass defines a method with the same name and signature as a method in its superclass, the method in the subclass overrides the one in the superclass. Thus, the subclass does not inherit the method from its superclass.

When method defined in subclass which has the same?

When a method in a subclass has the same name, same parameters or signature, and same return type(or sub-type) as a method in its super-class, then the method in the subclass is said to override the method in the super-class. Method overriding is one of the way by which java achieve Run Time Polymorphism.

What happens if superclass and subclass having same field name?

When declaring a variable in a subclass with the same name as in the superclass, you are hiding the variable, unlike methods which are overwritten.