Quick Guide To Polymorphism In Java
To quote the polymorphism chapter of Bruce Eckel's Thinking in Java: " polymorphism (also called dynamic binding or late binding or run-time. What is polymorphism? It is used to one object behaving as multiple forms. Or polymorphism means one name many forms. Polymorphism In C# With Real Time. Deleting items · Managing links . eiffel:: I2E: Polymorphism and Dynamic Binding . In other words, the language reconciles dynamic binding with static typing.
Assume for example a class POLYGON, describing polygons, whose features include an array of points representing the vertices and a function perimeter which computes a polygon's perimeter by summing the successive distances between adjacent vertices. Here it is appropriate to redefine perimeter for rectangles as there is a simpler and more efficient algorithm. Note the explicit redefine sub clause which would come after the rename if present.
The version to use in any call is determined by the run-time form of the target. Consider the following class fragment: This is known as dynamic binding. Dynamic binding provides a high degree of flexibility. The advantage for clients is the ability to request an operation such as perimeter computation without explicitly selecting one of its variants; the choice only occurs at run-time. This is essential in large systems, where many variants may be available; dynamic binding protects each component against changes in other components.
This technique is particularly attractive when compared to its closest equivalent in traditional approaches, where you would need records with variant components, or union types Ctogether with case switch instructions to discriminate between variants. This means that every client must know about every possible case, and that any extension may invalidate a large body of existing software.
The combination of inheritance, feature redefinition, polymorphism and dynamic binding supports a development mode in which every module is open and incremental. When you want to reuse an existing class but need to adapt it to a new context, you can define a new descendant of that class with new features, redefined ones, or both without any change to the original. This facility is of great importance in software development, an activity that -- by design or circumstance -- is invariably incremental.
The power of these techniques demands adequate controls. First, feature redefinition, as seen above, is explicit. Second, because the language is typed, a compiler can check statically whether a feature application a. In contrast, dynamically typed object-oriented languages defer checks until run-time and hope for the best: Such errors will not happen during the execution of a type-checked Eiffel system. In the following, we carry out this plan.
Abstract Classes If we distill the essence of the summation algorithm, we will notice that the general algorithm looks like the following: A data source contains multiple data entries. The entries can be accessed sequentially. In particular, a data source offers the following facilities: Every data source provides a means to test if all data entries have been extracted.Polymorphism in C++ (HINDI)
Every data source provides a means to extract the next data entry, and subsequently exclude the extracted entry from further consideration. We capture the abstract notion of a data source into an abstract class called DataSource. Check if the data source has been exhausted.
Extract the next available entry in the data source.
I2E: Polymorphism and Dynamic Binding
The data source has not been exhausted yet. The next available entry in the data source. It defines nothing but an abstract interface on which sumDataSource may depend.
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Specifically, a member function exhausted is provided so that client code may check if the data source has been exhausted, and a member function next is provided so that client code may extract the next data entry from the data source. These member functions are purely virtual function: Because of this, the class is an abstract class.
Abstract classes can not be instantiated. The only thing one can do with an abstract class is to derive subclasses from it.
What is special about this implementation is that it can be applied to instances of any subclass of DataSource. Consequently, the function sumDataSource is said to be polymorphic. This is made possible by declaring the parameter ds to be a reference to DataSource.
More details about this point will be given in the following discussion. For now, what is important is to recognize the polymorphic nature of sumDataSource. Interface Inheritance An array data source can be declared as a concrete subclass of the abstract class DataSource. ArrayDataSource const int A, unsigned N ; bool exhausted const; int next ; private: Not only that, it is a concrete subclass in the sense that it is not a mere abstract interface, but rather, it provides implementation for all its member functions, and it can actually be instantiated into concrete objects.
ArrayDataSource overrides the purely virtual functions it inherits from DataSource. Specifically, it provides implementations for the two methods extract and next.
oop - Late Binding vs. Polymorphism - what is the difference? - Stack Overflow
What we have achieved is that we have successfully created a data source that encapsulates an array. By subclassing from DataSource, ArrayDataSource promises that it will conform to the abstract interface specified by the superclass, and offer concrete implementations for the abstract interface. Specifically, the implementation of ArrayDataSource is given below.
Previously, we used inheritance to reuse code that resides in the superclass. We called that style of inheritance implementation interitance. We were saying that such kind of inheritance is not a particularly fruitful way of reuse. Here, with ArrayDataSource, we are not inheriting any code per se. Instead, we inherit an abstract interface, namely, the interface of a DataSource. As we shall see in the next example, this allows us to reuse client code that depends on the DataSource interface.
Dynamic Binding The sumDataSource function can now be applied to array data sources as follows. This is essential for providing polymorphism.
Consider what happens inside the sumDataSource function. The exhausted member function of the ds parameter will be invoked. But ds is a reference of type DataSource, which defines no implementation for exhausted.
This is where the virtual keyword takes effect. At run time, if the exhausted member function is invoked via a DataSource reference, then, rather than invoking the exhausted member function declared in DataSource which does not exist in this caseinvoke instead the exhausted member function of the concrete object to which the DataSource reference is bound.
Difference between Polymorphism and Dynamic Method lookup (Beginning Java forum at Coderanch)
In the case of our example, the exhausted member function of ArrayDataSource is invoked, producing the exact effect we wanted to achieving. The same applies to the next member function.
This is what we call static binding: Yet, when we look at the body of sumDataSource, we do not know which exhausted will actually be invoked. Such a mechanism is called dynamic binding: Dynamic binding is necessary for object-oriented programming languages to implement polymorphism.