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Abstract methods in .NET Implementation Data Matrix ECC200 in .NET Abstract methods




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Abstract methods using barcode maker for visual .net control to generate, create 2d data matrix barcode image in visual .net applications. ASP.NET So far, use VS .NET Data Matrix 2d barcode of the Animal superclass has helped to avoid code duplication in the Rabbit and Fox classes, and made it easier to add new animal types in the future. As we have seen in [ 8], intelligent use of inheritance should also simplify the client class.

We will investigate this now. In the Simulator class, we have used explicit type checks (using the instanceOf operator) and casts to specific animal types to implement a simulation step. Both these techniques are problematic since they hard-code the specific animal types in the simulation, and make it harder to modify or extend the current types of animals.

The relevant code is shown in Figure 117.. for(Iterato r iter = animals.iterator(); iter.hasNext(); ) { Object animal = iter.

next(); if(animal instanceof Rabbit) { Rabbit rabbit = (Rabbit)animal; if(rabbit.isAlive()) { rabbit.run(updatedField, newAnimals); } else { // Remove dead rabbits from collection.

iter.remove(); } } else if(animal instanceof Fox) { Fox fox = (Fox)animal; if(fox.isAlive()) {.

fox.hunt(fi Data Matrix 2d barcode for .NET eld, updatedField, newAnimals); } else { // Remove dead foxes from collection.

iter.remove(); } } else { System.out.

println("found unknown animal"); } } Figure 117: The original solution to make animals act. Now that we have the Animal class, we can improve this. Since all objects in our animals collection are a subtype of Animal , we can cast each element to Animal instead of casting it to Object. If we then ensure that the superclass (Animal) has a method that lets an animal act, and this method is redefined in each subclass, then we can use a polymorphic method call to let each animal act, without the need to cast to the specific animal types.

Let us assume that we create such a method, called act, and investigate the resulting source code. Figure 118 shows the code implementing this solution..

// let all .net framework datamatrix 2d barcode animals act for(Iterator iter = animals.iterator(); iter.

hasNext(); ) { Animal animal = (Animal)iter.next(); if(animal.isAlive()) { animal.

act(field, updatedField, newAnimals); } else { // Remove dead animals from collection. iter.remove(); } } Figure 118: The improved solution to animal action.

Several obs ervations are important at this point: The variable we are using for each collection element (animal ) is of type Animal. This is legal, since all objects in the collection are foxes or rabbit, and thus are all subtypes of Animal. We assume that the specific action methods (run for Rabbit, hunt for Fox ) have been renamed act .

This is more appropriate: Instead of telling each animal exactly what to do, we are just telling it to act , and we leave it up to the animal itself to decide what exactly it wants to do. The run method for Rabbit had only two parameters, updatedField and newAnimals . We have now added a third parameter, field , to make it consistent with the fox s act method.

Now every animal gets all parameters possibly needed to implement flexible action, and each class can choose to ignore any of the parameters. Because the dynamic type of the variable determines which method is actually executed (as discussed in [ 9]), the fox s action method will be executed for foxes, and the rabbit s method for rabbits..

Since stati Data Matrix for .NET c type checking is done using the static type, this code will only compile if class Animal has an act method with the right signature..

The last of these points is the only remaining problem. Since we are using the statement animal.act( Data Matrix ECC200 for .NET field, updatedField, newAnimals);.

and the var iable animal is of type Animal, we saw in [ 9] that this will only compile if Animal defines such a method. However, the situation here is rather different from the situation we encountered with the Item class s print method in [ 9]. There, the superclass version of print had a useful job to do print the fields defined in the superclass.

Here, while each particular animal has a specific set of actions to perform, we cannot describe in any detail the actions for animals in general. The particular actions depend on the specific subtype. Our problem is to decide how we should define Animal s act method.

The problem is a reflection of the fact no instance of class Animal will ever exist. There is no object in our simulation (or in nature) that is just an animal and not also an instance of a more specific subclass. These kinds of classes that are not intended for creating objects but only serve as superclasses, are known as abstract classes.

For animals, for example, we can say that each animal can act, but we cannot describe exactly how it acts without referring to a more specific subclass. This is typical for abstract classes, and it is reflected in Java constructs. For the Animal class, we wish to state that each animal has an act method, but we cannot give a reasonable implementation in class Animal.

The solution in Java is to declare the method abstract. [concept box: abstract method] Here is an example of an abstract act method:. abstract pu 2d Data Matrix barcode for .NET blic void act(Field currentField, Field updatedField, List newAnimals);. An abstract method is characterized by two details: It is prefixed with the keyword abstract. It does not have a method body. Instead, its header is terminated with a semicolon.

. Since the m .net vs 2010 Data Matrix ethod has no body, it can never be executed. But we have already established that we do not want to execute an Animal s act method so that is not a problem.

Before we investigate in detail the effects of using an abstract method, we will introduce more formally the concept of an abstract class. 10.3.

4 Abstract classes. Not only me thods can be declared abstract, but classes can be declared abstract as well. Figure 119 shows an example of class Animal as an abstract class. [concept box:.

abstract cl VS .NET Data Matrix barcode ass] Classes are declared abstract by inserting the keyword abstract into the class header..

public abst ract class Animal { // fields omitted /** * Make this animal act - that is: make it do whatever * it wants/needs to do. */ abstract public void act(Field currentField, Field updatedField, List newAnimals); // other method omitted } Figure 119: Animal as an abstract class. Classes tha barcode data matrix for .NET t are not abstract (all classes we have seen previously) are called concrete classes. Declaring a class abstract serves several purposes: No instances can be created of abstract classes.

Trying to use the new keyword with an abstract class is an error. This is mirrored in BlueJ: right-clicking on an abstract class in the class diagram will not list any constructors in the popup menu. This serves our intention discussed above: we stated that we did not want instances of class Animal created directly this class serves only as a superclass.

Declaring the class abstract enforces this restriction. Only abstract classes can have abstract methods. This ensures that all methods in concrete classes can always be executed.

If we allowed an abstract method in a concrete class, we could create an instance of a class that lacks an implementation for a method. Abstract classes with abstract methods force subclasses to override and implement those methods declared abstract. If a subclass does not provide an implementation for an inherited abstract method, it is itself abstract, and no instances may be created.

For a subclass to be concrete, it must provide implementations for all inherited abstract methods. [concept box: abstract subclasses]. Now we can start to see the purpose of abstract methods: While they do not provide an implementation, they nonetheless ensure that all subclasses have an implementation of this method. In other words: even though class Animal does not implement the act method, it ensures that all existing animals have an implemented act method. This is done by ensuring that no instance of class Animal can be created directly, and all concrete subclasses must implement the act method.

. Although we gs1 datamatrix barcode for .NET cannot create an instance of an abstract class directly, we can otherwise use an abstract class as a type in the usual ways. For instance, the normal rules of polymorphism allow us to handle foxes and rabbits as instances of the Animal class.

So those parts of the simulation that do not need to know whether they are dealing with a specific subclass can use the superclass type instead. 10-26 Which of the other simulation classes do not need to be aware of whether they are specifically dealing with foxes or rabbits Could they be rewritten to use the Animal class instead Would there be any particular benefits in doing this Review the overriding rules for methods and fields discussed in [ 9]. Why are they particularly significant in our attempts to introduce inheritance into this application The changes made in this section have removed the dependencies (couplings) of the simulateOneStep method to the Fox and Rabbit class.

The Simulator class, however, is still coupled to Fox and Rabbit, because these classes are referenced in the populate method. There is no way to avoid this: when we create animal instances, we have to specify exactly what kind of animal to create. This could be improved by splitting the Simulator into two classes: one class, Simulator, which runs the simulation and is completely decoupled from the concrete animal classes, and one class, PopulationGenerator (created and called by the simulator), which creates the population.

Only this class is coupled to the concrete animal classes, making it easier for a maintenance programmer to find places where change is necessary when the application is extended. Try implementing this refactoring step. The PopulationGenerator class should also define the colors for each type of animal.

10-29 Challenge exercise: The canBreed methods of Fox and Rabbit are textually identical, yet we chose not to move them to the Animal class. Why is this Try moving the methods from Fox and Rabbit to Animal and making them protected. Is there any way to make the resulting classes compile and, even if there is, does the resulting simulation work as it should How can you tell .

10-27. 10-28. The project foxes-and-rabbits-v2 provides an implementation of our simulation with the improvements discussed here..
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