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Chapter 8 Object-Based Programming 329 The only difference between class Time3in this example and the version in Fig. 8.6 is that we show the namespace, i.e., TimeLibrary, in which Time3is defined. Each class library is defined in a namespacethat contains all the classes in the library. We will demonstrate momentarily how to package class Time3 into TimeLibrary.dll the dynamic link library that we create for reuse in other programs. Programs can load dynamic link libraries at execution time to access common functionality that can be shared among many programs. A dynamic link library represents an assembly. When a project uses a class library, the project must contain a reference to the assembly that defines the class library. 1 // Fig. 8.17: TimeLibrary.cs 2 // Placing class Time3 in an assembly for reuse. 3 4 using System; 5 6 namespace TimeLibrary // specifies namespace for class Time3 7 { 8 // Time3 class definition 9 public class Time3 10 { 11 private int hour; // 0-23 12 private int minute; // 0-59 13 private int second; // 0-59 14 15 // Time3 constructor initializes instance variables to 16 // zero to set default time to midnight 17 public Time3() 18 { 19 SetTime( 0, 0, 0 ); 20 } 21 22 // Time3 constructor: hour supplied, minute and second 23 // defaulted to 0 24 public Time3( int hour ) 25 { 26 SetTime( hour, 0, 0 ); 27 } 28 29 // Time3 constructor: hour and minute supplied, second 30 // defaulted to 0 31 public Time3( int hour, int minute ) 32 { 33 SetTime( hour, minute, 0 ); 34 } 35 36 // Time3 constructor: hour, minute and second supplied 37 public Time3( int hour, int minute, int second ) 38 { 39 SetTime( hour, minute, second ); 40 } 41 Fig. 8.17 Fig. 8.17Fig. 8.FiFi17g. 8.17g. 8.17Assembly TimeLibrarycontains class Time3. (Part 1 of 3.)
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328 Object-Based Programming Chapter 8 The FCL allows C# programmers to achieve software reusability across platforms that support .NET and rapid applications development. C# programmers focus on the high-level programming issues and leave the low-level implementation details to classes in the FCL. For example, a C# programmer who writes a graphics program does not need to know the details of every .NET-platform graphics capability. Instead, C# programmers concentrate on learning and using the FCL s graphics classes. The FCL enables C# developers to build applications faster by reusing preexisting, extensively tested classes. In addition to reducing development time, FCL classes also improve programmers abilities to debug and maintain applications, because proven software compenents are being used. For programmers to take advantage of the FCL s classes, they must familiarize themselves with the FCL s rich set of capabilities. Software reuse is not limited to Windows-application development. The FCL also includes classes for creating Web services, which are applications packaged as services that clients can access via the Internet. Any C# application is a potential Web service, so C# programmers can reuse existing applications as building blocks to form larger more sophisticated Web-enabled applications. Many people believe that Web services represent the next phase in the evolution of software development, in which the Web provides a library of functionality from which developers can build applications in a platform-independent manner. As Microsoft s premier .NET language, C# provides all the features necessary for creating scalable, robust Web services. We formally introduce Web Services in Chapter 21, ASP .NET and Web Services. 8.16 Namespaces and Assemblies As we have seen in almost every example in the text, classes from preexisting libraries, such as the .NET Framework, must be imported into a C# program by adding a reference to the appropriate libraries (a process we demonstrated in Section 3.2). Remember that each class in the Framework Class Library belongs to a specific namespace. The preexisting code in the FCL facilitates software reuse. Programmers should concentrate on making the software components they create reusable. However, doing so often results in naming collisions. For example, two classes defined by different programmers can have the same name. If a program needs both of those classes, the program must have a way to distinguish between the two classes in the code. Common Programming Error 8.12 Attempting to compile code that contains naming collisions will generate compilation errors. Namespaces help minimize this problem by providing a convention for unique class names. No two classes in a given namespace can have the same name, but different namespaces can contain classes of the same name. With hundreds of thousands of people writing C# programs, there is a good chance the names that one programmer chooses to describe classes will conflict with the names that other programmers choose for their classes. We begin our discussion of reusing existing class definitions in Fig. 8.17, which provides the code for class Time3 (originally defined in Fig. 8.6). When reusing class definitions between programs, programmers create class libraries that can be imported for use in a program via a using statement. Only publicclasses can be reused from class libraries. Non-public classes can be used only by other classes in the same assembly.
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Chapter 8 Object-Based Programming 327 It might, for example, quietly produce an incorrect result. Mathematical integers do not have this problem. Therefore, the notion of a computer int is only an approximation of the notion of a real-world integer. The same is true of float and other built-in types. We have taken the notion of int for granted until this point, but we now consider it from a new perspective. Types like int, float, char and others are all examples of abstract data types. These types are representations of real-world notions to some satisfactory level of precision within a computer system. An ADT actually captures two notions: A data representation and the operations that can be performed on that data. For example, in C#, an int contains an integer value (data) and provides addition, subtraction, multiplication, division and modulus operations; however, division by zero is undefined. C# programmers use classes to implement abstract data types. Software Engineering Observation 8.17 Programmers can create types through the use of the class mechanism. These new types can be designed so that they are as convenient to use as the built-in types. This marks C# as an extensible language. Although the language is easy to extend via new types, the programmer cannot alter the base language itself. Another abstract data type we discuss is a queue, which is similar to a waiting line. Computer systems use many queues internally. A queue offers well-understood behavior to its clients: Clients place items in a queue one at a time via an enqueue operation, then get those items back one at a time via a dequeue operation. A queue returns items in first-in, first-out (FIFO) order, which means that the first item inserted in a queue is the first item removed. Conceptually, a queue can become infinitely long, but real queues are finite. The queue hides an internal data representation that keeps track of the items currently waiting in line, and it offers a set of operations to its clients (enqueue and dequeue). The clients are not concerned about the implementation of the queue clients simply depend upon the queue to operate as advertised. When a client enqueues an item, the queue should accept that item and place it in some kind of internal FIFO data structure. Similarly, when the client wants the next item from the front of the queue, the queue should remove the item from its internal representation and deliver the item in FIFO order (i.e., the item that has been in the queue the longest should be the next one returned by the next dequeue operation). The queue ADT guarantees the integrity of its internal data structure. Clients cannot manipulate this data structure directly only the queue ADT has access to its internal data. Clients are able to perform only allowable operations on the data representation; the ADT rejects operations that its public interface does not provide. 8.15 Software Reusability C# programmers concentrate both on crafting new classes and on reusing classes from the Framework Class Library (FCL), which contains thousands of predefined classes. Developers construct software by combining programmer-defined classes with well-defined, carefully tested, well-documented, portable and widely available FCL classes. This kind of software reusability speeds the development of powerful, high-quality software. Rapid applications development (RAD) is of great interest today.
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326 Object-Based Programming Chapter 8 value in valueTextBox to the location specified in indexTextBox. The event handler for button Get Value by Name (159 164) invokes the overloaded method ShowValue- AtIndex (lines 125 129) to retrieve the value with the name specified in valueTextBox. The event handler for button Set Value by Name (lines 167 175) assigns the value in valueTextBoxto the location with the name specified in indexTextBox. 8.14 Data Abstraction and Information Hiding As we pointed out at the beginning of this chapter, classes normally hide the details of their implementation from their clients. This is called information hiding. As an example of information hiding, let us consider a data structure called a stack. Students can think of a stack as analogous to a pile of dishes. When a dish is placed on the pile, it is always placed at the top (referred to as pushing the dish onto the stack). Similarly, when a dish is removed from the pile, it is always removed from the top (referred to as popping the dish off the stack). Stacks are known as last-in, first-out (LIFO) data structures the last item pushed (inserted) on the stack is the first item popped (removed) from the stack. Stacks can be implemented with arrays and with other data structures, such as linked lists. (We discuss stacks and linked lists in Chapter 23, Data Structures.) A client of a stack class need not be concerned with the stack s implementation. The client knows only that when data items are placed in the stack, these items will be recalled in last-in, first-out order. The client cares about what functionality a stack offers, but not about how that functionality is implemented. This concept is referred to as data abstraction. Although programmers might know the details of a class s implementation, they should not write code that depends on these details. This enables a particular class (such as one that implements a stack and its operations, push and pop) to be replaced with another version without affecting the rest of the system. As long as the publicservices of the class do not change (i.e., every method still has the same name, return type and parameter list in the new class definition), the rest of the system is not affected. Most programming languages emphasize actions. In these languages, data exist to support the actions that programs must take. Data are less interesting than actions. Data are crude. Only a few built-in data types exist, and it is difficult for programmers to create their own data types. C# and the object-oriented style of programming elevate the importance of data. The primary activities of object-oriented programming in C# is the creation of data types (i.e., classes) and the expression of the interactions among objects of those data types. To create languages that emphasize data, the programming-languages community needed to formalize some notions about data. The formalization we consider here is the notion of abstract data types (ADTs). ADTs receive as much attention today as structured programming did decades earlier. ADTs, however, do not replace structured programming. Rather, they provide an additional formalization to improve the program- development process. Consider built-in type int, which most people would associate with an integer in mathematics. Rather, an int is an abstract representation of an integer. Unlike mathematical integers, computer ints are fixed in size. For example, type int in .NET is limited approximately to the range 2 billion to +2 billion. If the result of a calculation falls outside this range, an error occurs, and the computer responds in some machine-dependent manner.
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Chapter 8 Object-Based Programming 325 After setting value by dimension name Before getting value by index number After getting value by index number Fig. 8.16 Fig. 8.1FiFig. 8.16g. 8.16Indexers provide subscripted access to an object s members. (Part 6 of 6.) Fig. 8.16 The privatedata members of class Boxare stringarray names(line 16), which contains the names (i.e., “length”, “width”and “height”) for the dimensions of a Box, and doublearray dimensions(line 17), which contains the size of each dimension. Each element in array names corresponds to an element in array dimensions (e.g., dimensions[2]contains the height of the Box). Boxdefines two indexers (lines 28 42 and lines 45 72) that each returna double value representing the size of the dimension specified by the indexer s parameter. Indexers can be overloaded like methods. The first indexer uses an intsubscript to manipulate an element in the dimensions array. The second indexer uses a string subscript representing the name of the dimension to manipulate an element in the dimensions array. Each indexer returns -1if its getaccessor encounters an invalid subscript. Each indexer s setaccessor assigns valueto the appropriate element of dimensionsonly if the index is valid. Normally, the programmer would have an indexer throw an exception if an indexer received an invalid index. We discuss how to throw exceptions in Chapter 11, Exception Handling. Notice that the string indexer uses a while structure to search for a matching string in the names array (lines 64 66). If a match is found, the indexer manipulates the corresponding element in array dimensions(line 69). Class IndexerTest is a System.Windows.Forms.Form that manipulates the private data members of class Box through Box s indexers. Instance variable box is declared at line 96 and initialized in the constructor at line 105 with dimensions of 0.0. The event handler for button Get Value by Index (lines 139 145) invokes method ShowValueAtIndex( lines 118 122) to retrieve the value at the index number specified in index- TextBox. The event handler for button Set Value by Index (lines 148 156) assigns the
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324 Object-Based Programming Chapter 8 170 box[ indexTextBox.Text ] = 171 Double.Parse( valueTextBox.Text ); 172 173 ShowValueAtIndex( “set: “, indexTextBox.Text ); 174 ClearTextBoxes(); 175 } 176 177 } // end class IndexerTest Before setting value by index number After setting value by index number Before getting value by dimension name After getting value by dimension name Before setting value by dimension name Fig. 8.16 Fig. 8.1FiFig. 8.16g. 8.16Indexers provide subscripted access to an object s members. (Part 5 of 6.) Fig. 8.16
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Chapter 8 Object-Based Programming 323 117 // display value at specified index number 118 private void ShowValueAtIndex( string prefix, int index ) 119 { 120 resultTextBox.Text = 121 prefix + “box[ ” + index + ” ] = ” + box[ index ]; 122 } 123 124 // display value with specified name 125 private void ShowValueAtIndex( string prefix, string name ) 126 { 127 resultTextBox.Text = 128 prefix + “box[ ” + name + ” ] = ” + box[ name ]; 129 } 130 131 // clear indexTextBox and valueTextBox 132 private void ClearTextBoxes() 133 { 134 indexTextBox.Text = “”; 135 valueTextBox.Text = “”; 136 } 137 138 // get value at specified index 139 private void intGetButton_Click( 140 object sender, System.EventArgs e ) 141 { 142 ShowValueAtIndex( 143 “get: “, Int32.Parse( indexTextBox.Text ) ); 144 ClearTextBoxes(); 145 } 146 147 // set value at specified index 148 private void intSetButton_Click( 149 object sender, System.EventArgs e ) 150 { 151 int index = Int32.Parse( indexTextBox.Text ); 152 box[ index ] = Double.Parse( valueTextBox.Text ); 153 154 ShowValueAtIndex( “set: “, index ); 155 ClearTextBoxes(); 156 } 157 158 // get value with specified name 159 private void nameGetButton_Click( 160 object sender, System.EventArgs e ) 161 { 162 ShowValueAtIndex( “get: “, indexTextBox.Text ); 163 ClearTextBoxes(); 164 } 165 166 // set value with specified name 167 private void nameSetButton_Click( 168 object sender, System.EventArgs e ) 169 { Fig. 8.16 Fig. 8.1FiFig. 8.16g. 8.16Indexers provide subscripted access to an object s members. (Part 4 of 6.) Fig. 8.16
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322 Object-Based Programming Chapter 8 64 while ( i < names.Length && 65 name.ToLower() != names[ i ] ) 66 i++; 67 68 if ( i != names.Length ) 69 dimensions[ i ] = value; 70 } 71 72 } // end indexer 73 74 } // end class Box 75 76 // Class IndexerTest 77 public class IndexerTest : System.Windows.Forms.Form 78 { 79 private System.Windows.Forms.Label indexLabel; 80 private System.Windows.Forms.Label nameLabel; 81 82 private System.Windows.Forms.TextBox indexTextBox; 83 private System.Windows.Forms.TextBox valueTextBox; 84 85 private System.Windows.Forms.Button nameSetButton; 86 private System.Windows.Forms.Button nameGetButton; 87 88 private System.Windows.Forms.Button intSetButton; 89 private System.Windows.Forms.Button intGetButton; 90 91 private System.Windows.Forms.TextBox resultTextBox; 92 93 // required designer variable 94 private System.ComponentModel.Container components = null; 95 96 private Box box; 97 98 // constructor 99 public IndexerTest() 100 { 101 // required for Windows Form Designer support 102 InitializeComponent(); 103 104 // create block 105 box = new Box( 0.0, 0.0, 0.0 ); 106 } 107 108 // Visual Studio .NET generated code 109 110 // main entry point for application 111 [STAThread] 112 static void Main() 113 { 114 Application.Run( new IndexerTest() ); 115 } 116 Fig. 8.16 Fig. 8.1FiFig. 8.16g. 8.16Indexers provide subscripted access to an object s members. (Part 3 of 6.) Fig. 8.16
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Chapter 8 Object-Based Programming 321 12 // Box class definition represents a box with length, 13 // width and height dimensions 14 public class Box 15 { 16 private string[] names = { “length”, “width”, “height” }; 17 private double[] dimensions = new double[ 3 ]; 18 19 // constructor 20 public Box( double length, double width, double height ) 21 { 22 dimensions[ 0 ] = length; 23 dimensions[ 1 ] = width; 24 dimensions[ 2 ] = height; 25 } 26 27 // access dimensions by integer index number 28 public double this[ int index ] 29 { 30 get 31 { 32 return ( index < 0 || index >= dimensions.Length ) ? 33 -1 : dimensions[ index ]; 34 } 35 36 set 37 { 38 if ( index >= 0 && index < dimensions.Length ) 39 dimensions[ index ] = value; 40 } 41 42 } // end numeric indexer 43 44 // access dimensions by their string names 45 public double this[ string name ] 46 { 47 get 48 { 49 // locate element to get 50 int i =0; 51 52 while ( i < names.Length && 53 name.ToLower() != names[ i ] ) 54 i++; 55 56 return ( i == names.Length ) ? -1 : dimensions[ i ]; 57 } 58 59 set 60 { 61 // locate element to set 62 int i =0; 63 Fig. 8.16 Fig. 8.1FiFig. 8.16g. 8.16Indexers provide subscripted access to an object s members. (Part 2 of 6.) Fig. 8.16
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320 Object-Based Programming Chapter 8 integer value. A benefit of indexers is that the programmer can define both integer subscripts and non-integer subscripts. For example, a programmer could allow client code to manipulate data using strings as subscripts that represent the data items names or descriptions. When manipulating conventional C# array elements, the array subscript operator always returns the same data type i.e., the type of the array. Indexers are more flexible they can return any data type, even one that is different from the type of the data in the list of elements. Although an indexer s subscript operator is used like an array-subscript operator, indexers are defined as properties in a class. Unlike normal properties, for which the programmer can choose an appropriate property name, indexers must be defined with keyword this. Indexers have the general form: accessModifier returnType this[ IndexType1 name1,IndexType2 name2, ] { get { // use name1, name2, … here to get data } set { // use name1, name2, … here to set data } } The IndexType parameters specified in the brackets ([]) are accessible to the getand set accessors. These accessors define how to use the index (or indices) to select or modify the appropriate data member. As with properties, get must return a value of type return- Type and set can use the value keyword to reference the value that should be assigned to the data member. Common Programming Error 8.11 Declaring indexers as static is a syntax error. The program of Fig. 8.16 contains two classes class Box (lines 14 74) represents a box with a length, a width and a height, and class IndexerTest(lines 77 177) demonstrates class Box s indexers. 1 // Fig. 8.16: IndexerTest.cs 2 // Indexers provide access to an object’s members via a 3 // subscript operator. 4 5 using System; 6 using System.Drawing; 7 using System.Collections; 8 using System.ComponentModel; 9 using System.Windows.Forms; 10 using System.Data; 11 Fig. 8.16 Fig. 8.1FiFig. 8.16g. 8.16Indexers provide subscripted access to an object s members. (Part 1 of 6.) Fig. 8.16
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