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Chapter 8 Object-Based Programming 339 new operator pushing into a stack no-argument constructor queue non-publicmethod rapid applications development (RAD) object (or instance) reclaim memory Object Browser reference to a new object Object class resource leak object orientation reusable software component object passed by reference service of a class object speak set accessor of a property object think signature object-based programming (OBP) software reuse object-oriented programming (OOP) stack overloaded constructor standard-time format overloaded method static variable polymorphism static variables have class scope popping off a stack static keyword predicate method structured programming private keyword termination housekeeping privatestatic member this keyword procedural programming language universal-time format program-development process user-defined type programmer-defined type utility method public keyword validity checking publicmethod variable public operations encapsulated in an object WaitForPendingFinalizers method public service of class GC publicstatic member waiting line SELF-REVIEW EXERCISES 8.1 Fill in the blanks in each of the following statements: a) Client code can access a class s members via the operator in conjunction with a reference to an object of the class. b) Members of a class declared are accessible only to methods of the class in which those members are defined. c) A initializes the instance variables of a class. d) A property accessor is used to assign values to private instance variables of a class. e) Methods of a class normally are declared , and instance variables of a class normally are declared . f) A accessor of a property is used to retrieve values of private data of a class. g) The keyword introduces a class definition. h) Members of a class declared are accessible anywhere that an object of the class is in scope. i) The operator allocates memory dynamically for an object of a specified type and returns a to that type. j) A variable represents class-wide information. k) The keyword specifies that an object or variable is not modifiable after it is initialized at execution time. l) A method declared static cannot access class members.
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338 Object-Based Programming Chapter 8 The Visual Studio .NET Class View displays the variables and methods for all classes in a project. The Visual Studio .NET Object Browser lists all classes in the C# library. Developers use the Object Browser to learn about the functionality provided by a specific object. TERMINOLOGY abstract data type (ADT) dynamic link library access method encapsulate action enqueue operation action-oriented .exe aggregation explicit use of this reference assembly extensible language attribute (data) finalizer behavior (method) first-in, first-out (FIFO) data structure block scope garbage collector body of a class definition GC class built-in data types get accessor case sensitivity GUI event handling class helper method class definition hide an instance variable class library hide implementation details class scope hide internal data representation class implements abstract data type implementation class-scope variable hidden by indexer method-scope variable indexer get accessor Class View indexer set accessor class-wide information information hiding client of a class inheritance Collect method of GC initial set of classes compile a class initialize to default values composition initialize a class object consistent state initialize an instance variable constant insert an item into a container object constructor instance of a built-in type create a code library instance of a user-defined type create class from existing class definition instance variable create a namespace instantiate (or create) an object create a reusable class interactions among objects create data types interface data abstraction internal data representation data in support of actions IsEmpty data integrity IsFull data member last-in, first-out (LIFO) data structure data representation of an abstract data type library data structure linked list default constructor local variable of a method destructor member access modifier division by zero is undefined memory leak .dll method overloading dot (.) operator namespace
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Chapter 8 Object-Based Programming 337 Although setand get accessors can provide access to privatedata, the access is restricted by the programmer s implementation of those methods. One form of software reuse is composition, in which a class contains as members references to objects of other classes. The thisreference is used implicitly and explicitly to refer to both the instance variables and the non-static methods of an object. The .NET Framework performs automatic garbage collection. Every class in C# can have a destructor that typically returns resources to the system. The destructor for an object is guaranteed to be called to perform termination housekeeping on the object just before the garbage collector reclaims the memory for the object (called finalization). In certain cases, all objects of a class should share only one copy of a particular variable. Programmers use static variables for this and other reasons. A static variable represents class-wide information all objects of the class share the same piece of data. The declaration of a static member begins with the keyword static. Such variables have class scope. A class s public static members can be accessed via the class name and the dot operator (e.g., Math.PI). A class s privatestatic members can be accessed only through methods or properties of the class. A method declared static cannot access non-static members. C# allows programmers to create constants whose values cannot change during program execution. To create a constant member of a C# class, the programmer must declare that member using either the constor readonly keyword. Members declared const must be initialized in the declaration; those declared with readonly can be initialized in the constructor, but must be initialized before they are used. Neither constnor readonly values can be modified once they are initialized. A class can define indexers to provide subscripted access to the data in an object of that class. Indexers can be defined to use any data type as the subscript. Each indexer can define a getand set accessor. Classes normally hide their implementation details from the clients of the classes. This is called information hiding. C# and the object-oriented style of programming elevate the importance of data. The primary activities of object-oriented programming in C# are the creation of data types (i.e., classes) and the expression of the interactions among objects of those data types. C# programmers concentrate on crafting new classes and reusing existing classes. Software reusability speeds the development of powerful, high-quality software. Rapid applications development (RAD) is of great interest today. Each class and interface in the .NET Framework belongs to a specific namespace (or library) that contains a group of related classes and interfaces. Namespaces provide a mechanism for software reuse. There is a good chance that the names you choose for classes will conflict with the names that other programmers choose for their classes. For this reason, namespaces provide a convention for unique class names.
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336 Object-Based Programming Chapter 8 SUMMARY Every class in C# inherits directly or indirectly from class Object. Keywords publicand private are member access modifiers. Instance variables and methods that are declared with member access modifier public are accessible wherever the program has a reference to an object of that class. Instance variables and methods that are declared with member access modifier private are accessible only to non-staticmethods of the class in which the private members are defined. The privatemethods often are called utility methods, or helper methods, because they can be called only by other methods of that class and are used to support the operation of those methods. Access methods can read or display data. Another common use for access methods is to test the truth of conditions such methods often are called predicate methods. A constructor initializes the instance variables of a class object. A class s constructor is called automatically when an object of that class is instantiated. It is common to have overloaded constructors for a class. Normally, constructors are public. Every class in C#, such as the classes from the .NET Framework, belongs to a namespace. If the programmer does not specify the namespace for a class, the class is placed in the default namespace, which includes the compiled classes in the current directory. Instance variables can be initialized by the class constructor, or they can be assigned values by the set accessor of a property. Instance variables that are not initialized explicitly by the programmer are initialized by the compiler (primitive numeric variables are set to 0, bool values are set to false and references are set to null). Classes simplify programming, because the client code need only be concerned with the public operations encapsulated in an object of the class. A class s non-static instance variables and methods belong to that class s scope. Within a class s scope, class members are immediately accessible to all of that class s non-static methods and can be referenced simply by name. Outside a class s scope, class members cannot be referenced directly by name. Variables defined in a method are known only to that method (i.e., they are local to that method). Such variables are said to have block scope. If a method defines a variable that has the same name as a variable with class scope, the class- scope variable is hidden by the block-scope variable in that method. To allow clients to manipulate the value of private data, the class can provide a property definition, which will enable the user to access this privatedata in a safe way. A property definition contains accessor methods that handle the details of modifying and returning data. A property definition can contain a set accessor, a get accessor or both. A get accessor enables the client to read the field s value and the setaccessor enables the client to modify the value. When an object is created, its members can be initialized by a constructor of that object s class. If no constructors are defined for a class, a default constructor will be provided by the compiler. This constructor contains no code and takes no parameters. Methods and constructors of a class can be overloaded. To overload a method of a class, simply provide a separate method definition with the same name for each version of the method. Remember that overloaded methods/constructors must have different parameter lists.
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Chapter 8 Object-Based Programming 335 Fig. 8.21 Fig. 8.21Fig. 8.FiFi21g. 8.21g. 8.21Object Browser when user selects Objectfrom Time1.cs.
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334 Object-Based Programming Chapter 8 Fig. 8.20 Fig. 8.20Fig. 8.FiFi20g. 8.20g. 8.20Class View of class Time1(Fig. 8.1) and class TimeTest(Fig. 8.2). boxes). The lock icons, placed to the left of the blue-box icons for the instance variables, specify that the variables are private. Class TimeTest1contains method Main. Note that both class Time1and class TimeTest1contain the Bases and Interfaces node. If you expand this node, you will see class Object in each case, because each class inherits from class System.Object(discussed in Chapter 9). Visual Studio s Object Browser lists all classes in the C# library. Developers use the Object Browser to learn about the functionality provided by a specific class. To open the Object Browser, right click any built-in C# class or method in the code editor and select Go To Definition. Figure 8.21 depicts the Object Browser when the user right clicks the class name Objectin the code editor. Note that the Object Browser lists all methods provided by class Object in the Members of ‘Object’ window this window offers developers instant access to information regarding the functionality of various objects. Note also that the Object Browser lists in the Objects window all classes in the FCL. The Object Browser can be a quick mechanism to learn about a class or method of a class. Remember that you can also view the complete description of a class or method in the online documentation available through the Help menu in Visual Studio .NET. This chapter is the first in a series of three chapters that cover the fundamentals of object-based and object-oriented programming. In this chapter, we discussed how to create proper class definitions, how to control access to class members and several features commonly used to craft valuable classes for reuse by other programmers. Chapter 9, focusses on inheritance. In that chapter, you will learn how to build classes that inherit data and functionality from existing class definitions. You also will learn other C# features that are specific to the inheritance relationship between classes. These features serve as the basis for the object-oriented programming concept called polymorphism that we present in Chapter 10.
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Chapter 8 Object-Based Programming 333 1 // Fig. 8.19: AssemblyTest.cs 2 // Using class Time3 from assembly TimeLibrary. 3 4 using System; 5 using TimeLibrary; 6 7 // AssemblyTest class definition 8 class AssemblyTest 9 { 10 // main entry point for application 11 static void Main( string[] args ) 12 { 13 Time3 time = new Time3( 13, 27, 6 ); 14 15 Console.WriteLine( 16 “Standard time: {0}nUniversal time: {1}n”, 17 time.ToStandardString(), time.ToUniversalString() ); 18 } 19 } Standard time: 1:27:06 PM Universal time: 13:27:06 Fig. 8.19 Fig. 8.19Fig. 8.FiFi19g. 8.19g. 8.19Assembly TimeLibraryused from class AssemblyTest. Before class AssemblyTest can use class Time3, the project containing class AssemblyTestmust have a reference to the TimeLibrary assembly. To add the reference, select Add Reference from the Project menu. Using the Browse button, select TimeLibrary.dll (located in the binDebug directory of the TimeLibrary project), then click OK to add the resource to the project. After adding the reference, use keyword using to inform the compiler that we will use classes from namespace Time- Library(line 5 in Fig. 8.19). 8.17 Class View and Object Browser Now that we have introduced key concepts of object-based programming, we present two features that Visual Studio provides to facilitate the design of object-oriented applica- tions Class View and Object Browser. The Class View displays the variables and methods for all classes in a project. To access this feature, select Class View from the View menu. Figure 8.20 shows the Class View for the TimeTest1 project of Fig. 8.1 and Fig. 8.2 (class Time1 and class TimeTest1). The view follows a hierarchical structure, positioning the project name (TimeTest1) as the root and including a series of nodes (e.g., classes, variables, methods etc.). If a plus sign (+) appears to the left of a node, that node can be expanded to show other nodes. By contrast, if a minus sign (-) appears to the left of a node, that node has been expanded (and can be collapsed). According to the Class View, project TimeTestcontains class Time1 and class TimeTest1 as children. Class Time1 contains methods SetTime, Time1, ToStandardString and ToUniversalString (indicated by purple boxes) and instance variables hour, minute and second (indicated by blue
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332 Object-Based Programming Chapter 8 there is no entry point into the program. In fact, if you try running the program by selecting the Debug menu and choosing Start, Visual Studio .NET displays an error message. Compiling the project creates an assembly (a dynamic link library) that represents the new class library. This assembly can be found in the binDebugdirectory of the project. By default, the assembly name will include the namespace name. (In this case, the name will be TimeLibrary.dll.) The assembly file contains class Time3, which other projects can use. Assembly files, which have file extensions .dlland .exe, are integral to C#. The Windows operating system uses executable files (.exe) to run applications, whereas it uses library files (.dll, or dynamic link library) to represent code libraries that can be loaded dynamically by many applications and shared among those applications. Next, we define a console application project containing class AssemblyTest (Fig. 8.19), which uses class Time3in assembly TimeLibrary.dllto create a Time3 object and display its standard and universal string formats. Fig. 8.18 Fig. 8.18Fig. 8.FiFi18g. 8.18g.8.18Simple Class Library.
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Chapter 8 Object-Based Programming 331 95 96 set 97 { 98 second = ( ( value >=0 && value < 60 ) ?value : 0 ); 99 } 100 101 } // end property Second 102 103 // convert time to universal-time (24 hour) format string 104 public string ToUniversalString() 105 { 106 return String.Format( 107 "{0:D2}:{1:D2}:{2:D2}", Hour, Minute, Second ); 108 } 109 110 // convert time to standard-time (12 hour) format string 111 public string ToStandardString() 112 { 113 return String.Format( "{0}:{1:D2}:{2:D2} {3}", 114 ( ( Hour == 12 || Hour == 0 ) ? 12 : Hour % 12 ), 115 Minute, Second, ( Hour < 12 ? "AM" : "PM" ) ); 116 } 117 118 } // end class Time3 119 } Fig. 8.17 Fig. 8.17Fig. 8.FiFi17g. 8.17g. 8.17Assembly TimeLibrarycontains class Time3. (Part 3 of 3.) We now describe, step-by-step, how to create the class library TimeLibrary containing class Time3: 1. Create a class library project. From the File menu, choose option New, followed by Project . In the New Project dialog, ensure that C# Projects is selected in the Project Types section and click Class Library. Name the project TimeLibrary and choose the directory in which you would like to store the project. A simple class library will be created, as shown in Fig. 8.18. There are two important points to note about the generated code. The first is that the class does not contain a Mainmethod. This indicates that the class in the class library cannot be used to begin the execution of an application. This class is designed to be used by other programs. Also notice that Class1 is created as a public class. If another project uses this library, only the library s publicclasses are accessible. We created class Time3 as public for this purpose (line 9 of Fig. 8.17) by renaming the class Class1 (created by Visual Studio as part of the project) to Time3. In the Solution Explorer, we also renamed the Class1.csfile as Time3.cs. 2. Add the code for class Time3. Delete the code for the Class1constructor. Then, copy the remainder of the Time3 code (lines 11 116) from Fig. 8.17 (you can find this file in the examples on the CD that accompanies this book) and paste the code in the body of the class definition shown in Fig. 8.18. 3. Compile the code. From the Build menu, choose option Build Solution. The code should compile successfully. Remember that this code cannot be executed
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330 Object-Based Programming Chapter 8 42 // Time3 constructor: initialize using another Time3 object 43 public Time3( Time3 time ) 44 { 45 SetTime( time.Hour, time.Minute, time.Second ); 46 } 47 48 // Set new time value in 24-hour format. Perform validity 49 // checks on the data. Set invalid values to zero. 50 public void SetTime( 51 int hourValue, int minuteValue, int secondValue ) 52 { 53 Hour = hourValue; 54 Minute = minuteValue; 55 Second = secondValue; 56 } 57 58 // property Hour 59 public int Hour 60 { 61 get 62 { 63 return hour; 64 } 65 66 set 67 { 68 hour = ( ( value >= 0 && value < 24 ) ? value : 0 ); 69 } 70 71 } // end property Hour 72 73 // property Minute 74 public int Minute 75 { 76 get 77 { 78 return minute; 79 } 80 81 set 82 { 83 minute = ( ( value >=0 && value < 60 ) ?value : 0 ); 84 } 85 86 } // end property Minute 87 88 // property Second 89 public int Second 90 { 91 get 92 { 93 return second; 94 } Fig. 8.17 Fig. 8.17Fig. 8.FiFi17g. 8.17g. 8.17Assembly TimeLibrarycontains class Time3. (Part 2 of 3.)
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