Wiley software development fundamentals pdf

When a method is invoked on an object, you get a well-defined functionality without the need to worry about the inner complexity of the object or the method itself. Understanding Classes A class is the template from which individual objects are created.

In the real world, objects need a template that defines how they should be built. All objects created from the same template look and behave in a similar way. For example, think about a particular make and model of car. In the software world, a class is the template from which individual objects are created. An object is also known as an instance of a class. Then, perform the following tasks: 1. Add a new Visual C class named Rectangle to the project.

Ensure that there are no errors. A new class is defined by using the Each class is a definition keyword class. Here, the Rectangle class has two data fields, length and width. These fields are of a new data type. An access modifier specifies what region of the Therefore, a class code will have access to a field. For example, the access modifier public will not limit access, definition is sometimes but the access modifier private will limit access within the class in which the field is defined.

This class also defines a method named GetArea. But what, exactly, is a method? In the software world, a method defines the actions or operations supported by a class. A method is defined by specifying the access level, the return type, the name of the method, and an optional list of parameters in parentheses followed by a block of code enclosed in braces. For instance, in the previous example, the class Rectangle defines a single method named GetArea. If a method does not intend to return any parameter list, and the value, its return type is specified by the keyword void.

The method must use a return state- order of data types ment to return a value. The return statement terminates the execution of the method and of the parameters are returns the specified value to the calling code.

A method signature To return to the earlier example, the return type of the method GetArea is double, which must be unique within means that the GetArea method must return a value of the type double. The GetArea a class. If a return statement is not used, as in the InitFields method, the method will stop executing when it reaches the end of the code block.

Constructors are special class methods that are executed when a new instance of a class is created. Constructors are used to initialize the data members of the object.

Constructors must have exactly the same name as the class, and they do not have a return type. Multiple constructors, each with a unique signature, can be defined for a class. A constructor that takes no arguments is called the default constructor. If a class is defined without any constructor, an invisible default constructor that does absolutely nothing is automatically generated.

It is often useful to have additional constructors to provide more ways through which an object is initialized. The Rectangle class, defined earlier, is only one way to create and initialize its object: by calling the constructor that accepts two parameters, both of the default data type.

For this activity, use the console application project Lesson 02 that you created in the previous exercise. Then, perform these steps: 1. GetArea ; Console. A console window will pop up to display the area of the rectangle. The class Rectangle provides only one way to construct an instance of the class: by calling a constructor with two arguments of the double data type.

Here, you create an object by using the new keyword followed by the call to the appropriate class constructor. Later Classes and objects in this block of code, you can use rect to refer to and manipulate the object that was just created. For example, the code calls the for an object but is not method GetArea on the object, and the value returned by the method is stored in the variable an object itself.

On the area. The data fields, length and width, of the object rect are not accessible here because they other hand, an object is are marked as private in the class definition. Properties allow you to access class data in a safe and flexible way. Properties are often used to expose the data fields of a class in a more controlled fields because they can manner. For example, a private field can be exposed by using a public property, but it is not include code for check- necessary to use properties in this way.

The get accessor is used to return the property validity. A property is often defined as public and, by convention, always has a name that begins with a capital letter. In contrast, the convention for naming private data fields is to begin with a lower-case letter. Then, complete the following tasks: 1. Modify the code of class Rectangle as shown below. Properties are often defined with a public access modifier.

In the code for the all the data fields of a property Length, the get accessor simply returns the value of the data field length. However, class should be declared the set accessor checks for the value being assigned using the value keyword to the property private, and that access and modifies the data field length only if the value is positive. The private fields length and to these private fields width are also called backing fields for the properties that respectively expose them.

In this case, the users of the properties that check the class the Main method need to use the default constructor and rely on properties to initialize data values for validity. The Main method uses the properties Length and Width to set the data for the rect object. Trying to set either Length or Width to a negative value will be ignored, and in this case, the data fields will still have their original value of 0.

When defining properties, you can exclude either the get or the set accessor. The auto-implemented properties used with default constructors can also simplify the creation and initialization of objects. You can use the this keyword to refer to any member of the current object. For example, you could define the constructor as follows: are used to add single- line comments to the public Rectangle double length, double width code.

The names of the data fields have been shadowed and can be only accessed by using the this keyword. Delegates are special objects that can hold a reference to a method with a specific signature.

A delegate is defined by using the delegate keyword. For instance, you can define a delegate as follows: public delegate void RectangleHandler Rectangle rect ; The delegate definition specifies the signature of the method whose reference can be held by a delegate object. For example, in the above code, you define a RectangleHandler delegate that can hold references to a method that returns void and accepts a single parameter of the Rectangle type.

So, if you have a method with a similar signature, it is an ideal candidate for assignment to a delegate instance. WriteLine rect. This means that you can associate more than one method of compatible signature , thereby creating an invocation list of one or more methods. In this specific example, the handler object refers to only one method DisplayArea, and therefore, the DisplayArea method will be invoked with the rect object as a parameter.

Among many other applications, delegates form the basis for event declarations, as discussed in the next section. The class that sends the notification is called a publisher of the event. The class that receives the notification is called the subscriber of the event. Events are easy to understand in the context of a graphical user interface GUI.

For example, when a user clicks on a button, a Click event occurs. In this type of event communication, the event publishers do not need to know which objects subscribe to the events that are being raised. Events are not just limited to GUI programming. In fact, events play an important role in.

NET Framework class libraries as a way for objects to signal any change in their state. This class is usually derived from the EventArgs class To define an event, you can use a custom delegate.

However, in most cases, if your event holds no event-specific data, using the predefined delegate EventHandler is sufficient. The EventHandler delegate is defined as follows: public delegate void EventHandler Object sender, EventArgs e ; Here, the sender parameter is a reference to the object that raises the event, and the e parameter is a reference to an event data object that contains no event data.

The EventArgs class is used by events that do not pass any event-related information to an event handler when an event is raised. If the event handler requires event-related information, the application must derive a class from the EventArgs class to hold the event-related data. Changed this, EventArgs. A console window will pop up to display later in this lesson, in that the value of the Length property is changed. In the example you just completed, the Rectangle class defines a Changed event that is invoked when the Length property of the Rectangle object is changed.

The delegate of the Changed event is of EventHandler type. In the Rectangle class, the event Changed is invoked when the set accessor of the Length property is called. This technique allows you have multiple event handlers that may respond to an event.

A namespace is a language element that allows you to organize code and create globally unique class names. Chances are that some other company will also ship code that contains a class of the name Widget. In that case, how do you handle the ambiguity in names? The solution is to organize the code within a namespace. A common convention is to use the company name in the namespace.

Widget, whereas the other Widget can be uniquely identified as CompanyB. NET Framework uses namespaces liberally to organize all its classes.

For example, the System namespace groups all the fundamental classes. The System. Data namespace organizes classes for data access. Similarly, the System. Web namespace is used for Web-related classes. Of course, with the use of namespaces, you might end up getting really long fully qualified class names that may result in verbose programs and a lot of typing. C solves this inconvenience via the using directive.

You can use the using directive at the top of the class file like this: using System. The class members discussed so far in this section e. Such members are called as instance members because they can be used only after an instance of a class is created. In contrast, the static keyword is used to declare members that do not belong to individual objects but to a class itself.

Such class members are called as static members. One common example of a static member is the familiar Main method that serves as the entry point for your program. Then, perform the following steps: 1.

A value type directly stores data within its memory. Understanding Structs The keyword struct is used to create user-defined types that consist of small groups of related fields.

Structs are mostly used to create simple types. If you find yourself Understanding Memory Allocation creating a very com- plex struct, you should After you enter a value or text into a cell, you can modify it in a number of ways. In consider using a class particular, you can remove the contents completely, enter a different value to replace instead. A good way to understand how value types differ from reference types is to visualize how each of them is represented in memory.

Figure shows how value types are created in memory. When you create a variable of type int, for instance, a named memory location is created that you can use to store a value of type int. Then, when an assignment is made, the memory address identified by the variable name is updated with the new value 10 in the case of the assignment in Figure When you create a variable of type string, a memory location is created that will be identified by this name.

However, this memory location will not contain the content of the string. Rather, this variable will store the memory address a reference of the location where the string is actually stored.

Modify the code of the Main method as shown below: new operator. A console window will pop up to display a result, after the copy, the values for p1. The value of X for variable p1 remains unaffected. So, after the above initialization, both rect1 and rect2 point to the same memory location and in turn the same Rectangle object.

In other words, there is only one rectangle object in memory, and both rect1 and rect2 are referring to it. The next statement modifies the Length of that rectangle object: rect2.

The heap is the memory available to a program at runtime for dynamic memory allocation. In contrast, some data items can be created on the execution stack or the call stack. Items created on the stack are the method parameters and the local variables declared within a method. The stack memory is reclaimed when the stack unwinds when a method returns, for example.

The memory allocated in the heap is automatically reclaimed by the garbage collector when the objects are not in use any more i. Encapsulation is a mechanism to restrict access to a class or class members in order to hide design decisions that are likely to change.

Encapsulation gives class designers the flexibility to change a section of code when needed without changing all the other code that makes use of that code. Also, when you hide information, you hide the complexity associated with it. As a result, with the help of encapsulation, you can write code that is easier to understand and maintain. In the previous exercises, you saw encapsulation at work when you declared the data members as private and enforced data-field initialization via a constructor.

Because the data members are hidden from the users of the class, the developer of the Rectangle class can change the names of the data fields without requiring any changes in the calling code. Properties offer a great way to encapsulate data fields along with any accompanying logic. Also, access modifiers such as private and public allow you to control the level of access for a class member or for the class itself. You should use the most restrictive access level All types and type members have an access level that specifies where that class or its members that makes sense for a can be used in your code.

The access level can be set using one of the access modifiers speci- type member. When C code is compiled, the output executable code contained within a.

An assembly is a unit of executable code that can be independently versioned and installed. Access modifiers are not allowed in namespace declarations; public access is implied for namespaces. The internal access modifier is the default for a class if no access modifier is speci- Do you understand fied. For instance, the class Rectangle defined in the previous exercise defaults to having an encapsulation? The accessibility of a nested class may not be less restrictive than the acces- 2. The class that inherits the functionality is called a derived class, and the class whose functionality is inherited is called a base class.

A derived class inherits all the functionality of the base class and can also define additional features that make it different from the base class. These classes will have some common properties, such as width and length. For this case, Unlike classes, the you can create a base class Polygon with the Width and Length properties, and the derived structs do not support classes Rectangle and Triangle will inherit these properties while providing their own func- inheritance.

The following exercise explains this concept in more detail. Width, rect. Length, rect. A console window will pop up to display the width, length, and the area of the rectangle. To define a derived class, you put a colon after the derived class name, followed by the name of the base class.

Here, the Polygon class is the base class for the Rectangle class. This means that access to the set accessor is available only inside the Polygon class and its derived classes. The Rectangle class inherits all the non-private data and behavior of the Polygon class.

In addition, the Rectangle class defines additional functionality GetArea method that is not available in the base class. Understanding Abstract and Sealed Classes The abstract classes provide a common definition of a base class that can be shared by multiple derived classes.

The sealed classes, on the other hand, provide complete func- tionality but cannot be used as base classes. In the previous exercise, you defined a GetArea method on the Rectangle class. Suppose you want to create another class, Triangle, that is of the Polygon type.

Often, base classes act as the repository of common functionality. But in general, we can expect all classes of the Polygon type to be able to calculate their area.

Such expectations can be rolled over to the base class with the help of an abstract keyword. Note that no modification to the Main method is needed. The main reason for including this method in the base class is that now the base class can provide a common template of functionality for the derived classes.

This situation can be handled by You cannot create marking the method as abstract. An abstract method provides a definition but does not offer instances of an abstract any implementation the method body. If any of the members of a class are abstract, the class. An abstract class cannot be instantiated. Derived classes can provide an implementation of an abstract class to create a concrete class a non-abstract class.

The derived classes can offer an implementation of an abstract method by overriding it in a derived class. For example, the Rectangle class overrides the abstract GetArea method of the base class and provides a full implementation. As a result, the Rectangle class is no longer an abstract class and can be instantiated directly.

Sealed classes, on the other hand, are defined when your implementation is complete and you do not want a class to be inherited. It is also possible to mark selected class members as sealed to avoid them being overridden in a derived class.

This declaration ensures that the method GetArea cannot be overridden in a derived class. Inheriting from the Object Class The Object class is the ultimate base class of all the classes in the. All classes in the. NET Framework inherit either directly or indirectly from the Object class. As part of this inheritance, a derived class can override the methods of the Object class. By default, it returns the full name of the class.

It is often useful to override this method so that it returns a string representation of the current state of the object. For example, we can say that the Rectangle is a Polygon.

Thus, an object of the Rectangle class has effectively two data types in this case: the object is a Rectangle, and the object is also a Polygon.

In C , the runtime allows you to cast an object to its class or to any of its base classes. When the latter assignment happens, an explicit cast is required because you are converting a more general object to a less general object.

The runtime checks whether the value of the variable o is compatible with the Rectangle class. If, at execution time, the value of o is not compatible with the Rectangle class, the runtime throws a System. Then, the cast statement is only executed if o contains a Rectangle object.

The as operator is similar to the cast operation type, the is operator but, in the case of as, if the type conversion is not possible, null is returned instead of raising check is not necessary. If, at runtime, it is not possible to cast the value of variable o to a rectangle, a value of null is 2. No exceptions will be raised. You are developing an application that allows users to work with different kind of polygons. You have a collection that contains several types of polygons, such as a rectangle, a triangle, and a square.

Each polygon provides you with its own implementation of the Draw method. Polymorphism enables you to do exactly this. Polymorphism allows the objects of a derived class to be treated at runtime as objects of the base class. When a method is invoked at runtime, its exact type is identified, and the appro- priate method is invoked from the derived class. Add new Polygon ; polygons.

Add new Rectangle ; polygons. A console window will pop up to display the drawing message for each polygon. In this exercise, the definitions of the Polygon and the Rectangle class are simplified to emphasize the concept of polymorphism. The base class provides a single Draw method. The important thing to note here is the keyword virtual.

This keyword allows the derived classes to override the method. Both the Rectangle and Triangle classes override the base class Draw method with their own definition by using the override keyword. The foreach loop is iterating over a collection of Polygon objects.

The underlying type of the first object is Polygon, but the second and third objects in the collection are actually Rectangle and Triangle objects that just happen to be cast as Polygons. The runtime will look at the actual underlying type and invoke the overridden method from the derived class.

Understanding the Override and New Keywords The override keyword replaces a base class member in a derived class. The new keyword creates a new member of the same name in the derived class and hides the base class implementation.

When a base class defines a virtual member, the derived class has two options for handling it—specifically, the derived class can use either the override keyword or the new keyword. The override keyword takes priority over the base-class definition of the member.

Here, the object of the derived class will call the overridden member instead of the base-class member. In comparison, if the new keyword is used, a new definition of the member is created and the base-class member is hidden.

However, if the derived class is cast to an instance of the base class, the hidden members of the class can still be called. However, if the method is executed when the derived class is 2. Object class provides a ToString method. By convention, you should use this method to return the human-readable representation for a class. Interfaces are defined by using the interface keyword. An interface definition By convention, all cannot consist of any data fields or any implementation details such as method bodies.

This is begin with a capital I. The return value of this method indicates the result of comparing the given parameter with the current object. How does IComparable decide how to compare two Rectangle objects or two Employee objects?

The classes that are interested in such comparisons must implement the IComparable interface by providing a method body for the CompareTo method. Each class that implements IComparable is free to provide its own custom comparison logic inside the CompareTo method. GetArea - target. WriteLine rect1. A console window will pop up and display the value —1 because the area of rect1 is less than the area of rect2.

Here, the class Rectangle both derives from the Polygon class and implements the IComparable interface. A class that implements an interface must implement all the methods declared in that interface. An interface is similar to an abstract class, but there are some noticeable differences.

For one, an abstract class provides incomplete implementation, whereas an interface provides no implementation at all. A class can also implement multiple interfaces but is limited to inheriting from only a single base class. So, how do you decide whether to use an abstract class or an interface? On the other hand, encapsulation?

Therefore, the 2. The class that sends a notification is called the publisher of the event, and the class that receives the notification is called the subscriber of the event. Structs are value types, whereas classes are reference types. Encapsulation provides class designers with the flexibility to change a section of code as needed without changing all other code that makes use of that code. For example, a public access modifier does not limit access, but a private access modifier limits access within the class in which the field is defined.

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