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288 Object-Based Programming Chapter 8 34 output += “nnAfter attempting invalid settings: ” + 35 “nUniversal time: ” + time.ToUniversalString() + 36 “nStandard time: ” + time.ToStandardString(); 37 38 MessageBox.Show( output, “Testing Class Time1″ ); 39 40 } // end method Main 41 42 } // end class TimeTest1 Fig. 8.2 Fig. 8.Fig.Fi 8.2g. 8.22Using an abstract data type. (Part 2 of 2.) Fig. 8. Software Engineering Observation 8.6 Note the relationship between operator new and the constructor of a class. When operator new creates an object of a class, that class s constructor is called to initialize the object s instance variables. Note that the TimeTest.cs file does not use keyword using to import the namespace that contains class Time1. If a class is in the same namespace as the class that uses it, the usingstatement is not required. Every class in C# is part of a namespace. If a programmer does not specify a namespace for a class, the class is placed in the default namespace, which includes all compiled classes in the current directory that do not reside in a namespace. In Visual Studio, this current directory is the one in which the current project resides. We must specify usingstatements for classes from the .NET Framework, because they are defined outside the namespace of each new application we create. Note that usingstatements are not required if the program fully qualifies the name of each class by preceding the class name with its namespace name and a dot operator. For example, a program can invoke class MessageBox s Showmethod as follows: System.Windows.Forms.MessageBox.Show( “Your message here” ); However, such lengthy names can be cumbersome. Line 14 declares string reference output to store the string containing the results, which later will be displayed in a MessageBox. Lines 17 20 assign to output the time in universal-time format (by invoking method ToUniversalString of the Time1 object) and standard-time format (by invoking method ToStandardStringof the Time1 object). Note the syntax of the method call in each case the reference time is followed by a the member access operator (.) followed by the method name. The reference name specifies the object that will receive the method call.
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Chapter 8 Object-Based Programming 287 After defining the class, we can use it as a type in declarations such as Time1 sunset; // reference to a Time1 object The class name (Time1) is a type name. A class can yield many objects, just as a primitive data type, such as int, can yield many variables. Programmers can create class types as needed; this is one reason why C# is known as an extensible language. Class TimeTest1 (Fig. 8.2) uses an instance of class Time1. Method Main (lines 11 40) declares and initializes Time1instance time(line 13). When the object is instantiated, operator new allocates the memory in which the Time1object will be stored, then calls the Time1constructor (lines 15 18 of Fig. 8.1) to initialize the instance variables of the Time1 object. As mentioned before, this constructor invokes method SetTime of class Time1 to initialize each privateinstance variable to 0. Operator new(line 13 of Fig. 8.2) then returns a reference to the newly created object; this reference is assigned to time. 1 // Fig. 8.2: TimeTest1.cs 2 // Demonstrating class Time1. 3 4 using System; 5 using System.Windows.Forms; 6 7 // TimeTest1 uses creates and uses a Time1 object 8 class TimeTest1 9 { 10 // main entry point for application 11 static void Main( string[] args ) 12 { 13 Time1 time = new Time1(); // calls Time1 constructor 14 string output; 15 16 // assign string representation of time to output 17 output = “Initial universal time is: ” + 18 time.ToUniversalString() + 19 “nInitial standard time is: ” + 20 time.ToStandardString(); 21 22 // attempt valid time settings 23 time.SetTime( 13, 27, 6 ); 24 25 // append new string representations of time to output 26 output += “nnUniversal time after SetTime is: ” + 27 time.ToUniversalString() + 28 “nStandard time after SetTime is: ” + 29 time.ToStandardString(); 30 31 // attempt invalid time settings 32 time.SetTime( 99, 99, 99 ); 33 Fig. 8.2 Fig. 8.Fig.Fi 8.2g. 8.22Using an abstract data type. (Part 1 of 2.) Fig. 8.
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286 Object-Based Programming Chapter 8 Lines 15 18 define the constructor of class Time1. A class s constructor initializes objects of that class. When a program creates an object of class Time1 with operator new, the constructor automatically is called to initialize the object. Class Time1 s constructor calls method SetTime (lines 22 32) to initialize instance variables hour, minute and secondto 0(representing midnight). Constructors can take arguments, but cannot return values. As we will see, a class can have overloaded constructors. An important difference between constructors and other methods is that constructors cannot specify a return type. Generally, constructors are declared public. Note that the constructor name must be the same as the class name. Common Programming Error 8.1 Attempting to return a value from a constructor is a syntax error. Method SetTime (lines 22 32) is a public method that receives three int parameters and uses them to set the time. A conditional expression tests each argument to determine whether the value is in a specified range. For example, the hour value must be greater than or equal to 0 and less than 24, because universal-time format represents hours as integers from 0 to 23. Similarly, both minute and second values must be greater than or equal to 0 and less than 60. Any values outside these ranges are invalid values and default to zero. Setting invalid values to zero ensures that a Time1 object always contains valid data (because, in this example, zero is a valid value for hour, minute and second). When users supply invalid data to SetTime, the program might want to indicate that the time was invalid. In Chapter 11, we discuss exception handling, which can be used to indicate invalid initialization values. Software Engineering Observation 8.5 Always define a class so that each of its instance variables always contains valid values. Method ToUniversalString (lines 35 39) takes no arguments and returns a string in universal-time format, consisting of six digits two for the hour, two for the minute and two for the second. For example, if the time were 1:30:07 PM, method ToUniversalString would return 13:30:07. Lines 37 38 use String method Format to configure the universal time string. Line 37 passes to Format the format string “{0:D2}:{1:D2}:{2:D2}”, which contains several format specifications indicating that arguments 0, 1 and 2 (the first three arguments after the format string argument) should each have the format D2 (a two-digit base 10 decimal number format) for display purposes. The D2 format specification causes single-digit values to appear as two digits with a leading 0 (e.g., 8 would be represented as 08). The two colons that separate the curly braces } and { are the colons that separate the hour from the minute and the minute from the second in the resulting string. Method ToStandardString (lines 42 47) takes no arguments and returns a string in standard-time format, consisting of the hour, minute and second values separated by colons and followed by an AM or a PM indicator (e.g., 1:27:06PM). Like method ToUniversalString, method ToStandardString uses String method Format to format the minute and second as two-digit values with leading zeros if necessary. Line 45 determines the value for hour in the string if the hour is 0 or 12 (AM or PM), the hour appears as 12; otherwise, the hour appears as a value from 1 11.
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Chapter 8 Object-Based Programming 285 Good Programming Practice 8.2 Members in a class definition should be grouped by their member access modifiers to enhance clarity and readability. Lines 9 11 declare each of the three int instance variables hour, minute and second with member access modifier private, indicating that these instance variables of the class are accessible only to members of the class this is known as data hiding. When an object of the class encapsulates such instance variables, only methods of that object s class can access the variables. Normally, instance variables are declared private and methods are declared public. However, it is possible to have private methods and public instance variables, as we will see later. Often, private methods are called utility methods, or helper methods, because they can be called only by other methods of that class. The purpose of utility methods is to support the operation of a class s other methods. Using public data in a class is an uncommon and dangerous programming practice. Providing such access to data members is unsafe foreign code (i.e., code in other classes) could set public data members to invalid values, producing potentially disastrous results. Good Programming Practice 8.3 We prefer to list instance variables of a class first, so that, when reading the code, programmers see the name and type of each instance variable before it is used in the methods of the class. Good Programming Practice 8.4 Even though private and public members can be intermixed, list all the private members of a class first in one group, then list all the public members in another group. Software Engineering Observation 8.3 Declare all instance variables of a class as private. The architecture of accessing private data through public properties which first validate the data allows the developer to ensure that an object s data remains in a consistent state. Software Engineering Observation 8.4 Make a class member private if there is no reason for that member to be accessed outside of the class definition. Classes often include access methods that 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. For example, we could design predicate method IsEmpty for a container class a class capable of holding many objects, such as a linked list, a stack or a queue. (These data structures are discussed in detail in Chapter 23, Data Structures.) IsEmpty would return true if the container is empty and false otherwise. A program might test IsEmpty before attempting to read another item from the container object. Similarly, a program might test another predicate method (e.g., IsFull) before attempting to insert an item into a container object. Class Time1 contains constructor Time1(lines 15 18) and methods SetTime (lines 22 32), ToUniversalString (lines 35 39) and ToStandardString (lines 42 47). These are the public methods (also called the public services or the public interface) of the class. Clients of class Time1, such as class TimeTest1 (Fig. 8.2), use a Time1 s public methods to manipulate the data stored in Time1 objects or to cause class Time1 to perform some service.
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284 Object-Based Programming Chapter 8 29 second = ( secondValue >= 0 && secondValue < 60 ) ? 30 secondValue : 0; 31 32 } // end method SetTime 33 34 // convert time to universal-time (24 hour) format string 35 public string ToUniversalString() 36 { 37 return String.Format( 38 "{0:D2}:{1:D2}:{2:D2}", hour, minute, second ); 39 } 40 41 // convert time to standard-time (12 hour) format string 42 public string ToStandardString() 43 { 44 return String.Format( "{0}:{1:D2}:{2:D2} {3}", 45 ( ( hour == 12 || hour == 0 ) ? 12 : hour % 12 ), 46 minute, second, ( hour < 12 ? "AM" : "PM" ) ); 47 } 48 49 } // end class Time1 Fig. 8.1 Fig. 8.1Fig. 8.FiFi1g. 8.1g. 8.1Time1abstract data type represents the time in 24-hour format. (Part 2 of 2.) In Fig. 8.1, line 7 begins the Time1 class definition, indicating that class Time1 inherits from class Object (namespace System). C# programmers use inheritance to create classes from existing classes. In fact, every class in C# (except Object) inherits from an existing class definition. On line 7, the : followed by class name Object indicates that class Time1 inherits existing pieces of class Object. If a new class definition does not specify a :and class name to the right of the new class name, the new class implicitly inherits from class Object. It is not necessary to understand inheritance to learn the concepts and programs in this chapter. We explore inheritance and class Objectin detail in Chapter 9. The opening left brace ({) at line 8 and closing right brace (}) at line 49 delineate the body of class Time1. Any information that we place in this body is said to be encapsulated (i.e., wrapped) in the class. For example, lines 9 11 of class Time1declare three intvariables hour, minute and second that represent the time of day in universal-time format (24-hour clock format). Variables declared in a class definition, but not inside a method definition, are called instance variables each instance (object) of the class contains its own separate copy of the class s instance variables. Keywords public and private are member access modifiers. Instance variables or methods with member access modifier public are accessible wherever the program has a reference to a Time1 object. However, instance variables or methods declared with member access modifier private are accessible only in that class definition. A class s publicmembers and privatemembers can be intermixed. Good Programming Practice 8.1 Every instance variable or method definition should be preceded by a member access modifier. The default access modifier for class members is private.
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Chapter 8 Object-Based Programming 283 languages is that client code often is dependent on implementation details of the data used in the code. This dependency might necessitate rewriting the client code if the data implementation changes. ADTs eliminate this problem by providing implementation-independent interfaces to their clients. The creator of a class can change the internal implementation of that class without affecting the clients of that class. Software Engineering Observation 8.2 It is important to write programs that are understandable and easy to maintain. Change is the rule, rather than the exception. Programmers should anticipate that their code will be modified. As we will see, classes facilitate program modifiability. The following example (an susequent examples) will require multiple class definitions in the same project. To add a class to a project, select Add Class from the Project menu. In the Add New Item dialog box that appears, enter the new class name in the Name text box and click the Open button. Note that the file name (ending with the .cs file extension) appears in the Solution Explorer below the project name. Our next example consists of classes Time1 (Fig. 8.1) and TimeTest1 (Fig. 8.2). Class Time1 contains the time of day in 24-hour clock format. Class TimeTest1 contains method Main, which creates an instance of class Time1 and demonstrates the features of that class. 1 // Fig. 8.1: Time1.cs 2 // Class Time1 maintains time in 24-hour format. 3 4 using System; 5 6 // Time1 class definition 7 public class Time1 : Object 8 { 9 private int hour; // 0-23 10 private int minute; // 0-59 11 private int second; // 0-59 12 13 // Time1 constructor initializes instance variables to 14 // zero to set default time to midnight 15 public Time1() 16 { 17 SetTime( 0, 0, 0 ); 18 } 19 20 // Set new time value in 24-hour format. Perform validity 21 // checks on the data. Set invalid values to zero. 22 public void SetTime( 23 int hourValue, int minuteValue, int secondValue ) 24 { 25 hour = ( hourValue >= 0 && hourValue < 24 ) ? 26 hourValue : 0; 27 minute = ( minuteValue >= 0 && minuteValue < 60 ) ? 28 minuteValue : 0; Fig. 8.1 Fig. 8.1Fig. 8.FiFi1g. 8.1g. 8.1Time1abstract data type represents the time in 24-hour format. (Part 1 of 2.)
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282 Object-Based Programming Chapter 8 principle is called information hiding). Although some objects can communicate with one another across well-defined interfaces (just like the driver s interface to a car includes a steering wheel, accelerator pedal, brake pedal and gear shift), objects are unaware of how other objects are implemented (just as the driver is unaware of how the steering, engine, brake and transmission mechanisms are implemented). Normally, implementation details are hidden within the objects themselves. Surely, it is possible to drive a car effectively without knowing the details of how engines, transmissions and exhaust systems work. Later, we will see why information hiding is so crucial to good software engineering. In procedural programming languages (like C), programming tends to be action oriented. C# programming, however, is object oriented. In C, the unit of programming is the function (functions are called methods in C#). In C#, the unit of programming is the class. Objects eventually are instantiated (i.e., created) from these classes and functions are encapsulated within the boundaries of classes as methods. C programmers concentrate on writing functions. They group actions that perform some task into a function and then group functions to form a program. Data are certainly important in C, but they exist primarily to support the actions that functions perform. The verbs in a system-requirements document describing the requirements for a new application help a C programmer determine the set of functions that will work together to implement the system. By contrast, C# programmers concentrate on creating their own user-defined types, called classes. We also refer to classes as programmer-defined types. Each class contains both data and a set of methods that manipulate the data. The data components, or data members, of a class are called member variables, or instance variables (many C# programmers prefer the term fields).1 Just as we call an instance of a built-in type such as int a variable, we call an instance of a user-defined type (i.e., a class) an object. In C#, attention is focused on classes, rather than on methods. The nouns in a system-requirements document help the C# programmer determine an initial set of classes with which to begin the design process. Programmers use these classes to instantiate objects that work together to implement the system. This chapter explains how to create and use classes and objects, a subject known as object-based programming (OBP). Chapters 9 and 10 introduce inheritance and polymorphism key technologies that enable object-oriented programming (OOP). Although we do not discuss inheritance in detail until Chapter 9, it is part of several class definitions in this chapter and has been used in several examples previously. For example, in the program of Section 4.13 (and several subsequent programs), we inherited a class from System.Win- dows.Forms.Form to create an application that executes in its own window. Software Engineering Observation 8.1 All C# objects are passed by reference. 8.2 Implementing a Time Abstract Data Type with a Class Classes in C# facilitate the creation of abstract data types (ADT), which hide their implementation from clients (or users of the class object). A problem in procedural programming 1. We sometimes use industry-standard terminology, such as data members and instance members. rather than C# terms such as fields, For a listing of C#-specific terminology, please see the C# Language Specification, which can be downloaded from msdn.microsoft.com/vstudio/ nextgen/technology/csharpdownload.asp.
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Chapter 8 Object-Based Programming 281 Outline 8.1 Introduction 8.2 Implementing a Time Abstract Data Type with a Class 8.3 Class Scope 8.4 Controlling Access to Members 8.5 Initializing Class Objects: Constructors 8.6 Using Overloaded Constructors 8.7 Properties 8.8 Composition: Objects References as Instance Variables of Other Classes 8.9 Using the this Reference 8.10 Garbage Collection 8.11 static Class Members 8.12 const and readonly Members 8.13 Indexers 8.14 Data Abstraction and Information Hiding 8.15 Software Reusability 8.16 Namespaces and Assemblies 8.17 Class View and Object Browser Summary Terminology Self-Review Exercises Answers to Self-Review Exercises Exercises 8.1 Introduction In this chapter, we investigate object orientation in C#. Some readers might ask, why have we deferred this topic until now? There are several reasons. First, the objects we build in this chapter partially are composed of structured program pieces. To explain the organization of objects, we needed to establish a basis in structured programming with control structures. We also wanted to study methods in detail before introducing object orientation. Finally, we wanted to familiarize readers with arrays, which are C# objects. In our discussions of object-oriented programs in Chapters 1 7, we introduced many basic concepts (i.e., object think ) and terminology (i.e., object speak ) that relate to C# object-oriented programming. We also discussed our program-development methodology: We analyzed typical problems that required programs to be built and determined what classes from the .NET Framework Class Library were needed to implement each program. We then selected appropriate instance variables and methods for each program and specified the manner in which an object of our class collaborated with objects from the .NET Framework classes to accomplish the program s overall goals. Let us briefly review some key concepts and terminology of object orientation. Object orientation uses classes to encapsulate (i.e., wrap together) data (attributes) and methods (behaviors). Objects have the ability to hide their implementation from other objects (this
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8 Object-Based Programming Objectives To understand encapsulation and data hiding. To understand the concepts of data abstraction and abstract data types (ADTs). To be able to create, use and destroy objects. To be able to control access to object instance variables and methods. To be able to use properties to keep objects in consistent states. To understand the use of the this reference. To understand namespaces and assemblies. To be able to use the Class View and Object Browser. My object all sublime I shall achieve in time. W. S. Gilbert Is it a world to hide virtues in? William Shakespeare, Twelfth Night Your public servants serve you right. Adlai Stevenson Classes struggle, some classes triumph, others are eliminated. Mao Zedong This above all: to thine own self be true. William Shakespeare, Hamlet
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Chapter 7 Arrays 279 c) Starting from the right, but beginning with the element before 89, compare each element to 37 until an element less than 37 is found, then swap 37 and that element. The first element less than 37 is 10, so 37 and 10 are swapped. The new array is 12 2 6 4 10 8 37 89 68 45 d) Starting from the left, but beginning with the element after 10, compare each element to 37 until an element greater than 37 is found, then swap 37 and that element. There are no more elements greater than 37, so when we compare 37 to itself, we know that 37 has been placed in its final location of the sorted array. Once the partition has been applied to the previous array, there are two unsorted subarrays. The sub- array with values less than 37 contains 12, 2, 6, 4, 10 and 8. The subarray with values greater than 37 contains 89, 68 and 45. The sort continues with both subarrays being partitioned in the same manner as the original array. Using the preceding discussion, write recursive method QuickSort to sort a single-subscripted integer array. The method should receive as arguments an integer array, a starting subscript and an ending subscript. Method Partition should be called by QuickSort to perform the partitioning step. 7.11 (Maze Traversal) The following grid of #s and dots (.) is a double-subscripted array representation of a maze: # # # # # # # # # # # # # . . . # . . . . . . # . . # . # . # # # # . # # # # . # . . . . # . # # . . . . # # # . # . . # # # # . # . # . # . # # . . # . # . # . # . # # # . # . # . # . # . # # . . . . . . . . # . # # # # # # # . # # # . # # . . . . . . # . . . # # # # # # # # # # # # # The #s represent the walls of the maze, and the dots represent squares in the possible paths through the maze. Moves can be made only to a location in the array that contains a dot. There is a simple algorithm for walking through a maze that guarantees finding the exit (assuming there is an exit). If there is not an exit, you will arrive at the starting location again. Place your right hand on the wall to your right and begin walking forward. Never remove your hand from the wall. If the maze turns to the right, you follow the wall to the right. As long as you do not remove your hand from the wall, eventually you will arrive at the exit of the maze. There may be a shorter path than the one you have taken, but you are guaranteed to get out of the maze if you follow the algorithm. Write recursive method MazeTraverse to walk through the maze. The method should receive as arguments a 12-by-12 character array representing the maze and the starting location of the maze. As MazeTraverse attempts to locate the exit from the maze, it should place the character X in each square in the path. The method should display the maze after each move so the user can watch as the maze is solved.
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