Learn Java Program | Polymorphism in object-oriented programming

Definition of Polymorphism:-

Polymorphism is the technique of creating object of parent-class through the constructor of child-class. Using polymorphism we can call or execute the child-class overriding method by the parent-class object.

Polymorphism in Object Oriented Programming:-

Subtype polymorphism, almost universally called polymorphism in the context of object-oriented programming, is the ability to create a variable, a function, or an object that has more than one form. In principle, polymorphism can arise in other computing contexts and shares important similarities with the concept of degeneracy in biology.

The purpose of polymorphism is to implement a style of programming called message-passing in the literature[citation needed], in which objects of various types define a common interface of operations for users. In strongly typed languages, polymorphism usually means that type A somehow derives from type B, or type C implements an interface that represents type B. In weakly typed languages types are implicitly polymorphic.

Operator overloading of the numeric operators (+, -, *, and /) allows polymorphic treatment of the various numerical types: integer, unsigned integer, float, decimal, etc.; each of which have different ranges, bit patterns, and representations. Another common example is the use of the "+" operator which allows similar or polymorphic treatment of numbers (addition), strings (concatenation), and lists (attachment). This is a lesser used feature of polymorphism.

The primary usage of polymorphism in industry (object-oriented programming theory) is the ability of objects belonging to different types to respond to method, field, or property calls of the same name, each one according to an appropriate type-specific behavior. The programmer (and the program) does not have to know the exact type of the object in advance, and so the exact behavior is determined at run-time (this is called late binding or dynamic binding).

The different objects involved only need to present a compatible interface to the clients' (calling routines). That is, there must be public or internal methods, fields, events, and properties with the same name and the same parameter sets in all the superclasses, subclasses and interfaces. In principle, the object types may be unrelated, but since they share a common interface, they are often implemented as subclasses of the same superclass. Though it is not required, it is understood that the different methods will also produce similar results (for example, returning values of the same type).

(Subtype) polymorphism is not the same as method overloading or method overriding, (that is known instead as ad-hoc polymorphism. Polymorphism is only concerned with the application of specific implementations to an interface or a more generic base class. Method overloading refers to methods that have the same name but different signatures inside the same class. Method overriding is where a subclass replaces the implementation of one or more of its parent's methods. Neither method overloading nor method overriding are by themselves implementations of polymorphism.

Polymorphism Program:-

 class Man{
    public void fly(){
      System.out.println(”Man can not fly”);
    }
  }

  class SuperMan extends Man{
     public void fly(){
       System.out.println(”Superman can fly”);
     }
  }

  public class TestMan{
     public static void main(String[]args){
       Man m = new SuperMan();    // polymorphism
       m.fly();
     }
  }

Parametric Polymorphism:-

Main article: Parametric polymorphism

Parametric polymorphism allows a function or a data type to be written generically, so that it can handle values identically without depending on their type. Parametric polymorphism is a way to make a language more expressive, while still maintaining full static type-safety.

Parametric polymorphism is ubiquitous in functional programming, where it is often simply referred to as "polymorphism". The following example shows a parametrized list data type and two parametrically polymorphic functions on them:

Subtype Polymorphism (or Inclusion Polymorphism):-

Main Article: Subtype polymorphism

Some languages employ the idea of subtypes to restrict the range of types that can be used in a particular case of parametric polymorphism. In these languages, subtyping polymorphism (sometimes referred to as dynamic polymorphism) allows a function to be written to take an object of a certain type T, but also work correctly if passed an object that belongs to a type S that is a subtype of T (according to the Liskov substitution principle). This type relation is sometimes written S <: T. Conversely, T is said to be a supertype of S—written T :> S.

For example, if Number, Rational, and Integer are types such that Number :> Rational and Number :> Integer, a function written to take a Number will work equally well when passed an Integer or Rational as when passed a Number. The actual type of the object can be hidden from clients into a black box, and accessed via object identity. In fact, if the Number type is abstract, it may not even be possible to get your hands on an object whose most-derived type is Number (see abstract data type, abstract class). This particular kind of type hierarchy is known—especially in the context of the Scheme programming language—as a numerical tower, and usually contains many more types.

Object-oriented programming languages offer subtyping polymorphism using subclassing (also known as inheritance). In typical implementations, each class contains what is called a virtual table—a table of functions that implement the polymorphic part of the class interface—and each object contains a pointer to the "vtable" of its class, which is then consulted whenever a polymorphic method is called. This mechanism is an example of: late binding, because virtual function calls are not bound until the time of invocation, and single dispatch (i.e., single-argument polymorphism), because virtual function calls are bound simply by looking through the vtable provided by the first argument (the this object), so the runtime types of the other arguments are completely irrelevant.

The same goes for most other popular object systems. Some, however, such as CLOS, provide multiple dispatch, under which method calls are polymorphic in all arguments.

In the following example we make cats and dogs subtypes of animals. The procedure write accepts an animal, but will also work correctly if a subtype is passed to it.