make barcode with vb.net Tester myTester = new Tester( ); // instantiate an object of type Tester in Visual C#.NET

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Tester myTester = new Tester( ); // instantiate an object of type Tester
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As you ll see later in this chapter, creating an instance of the Tester class allows you to call other methods on the object you ve created (myTester).
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Classes Versus Objects
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One way to understand the difference between a class and an instance (object) is to consider the distinction between the type int and a variable of type int. You can t assign a value to a type:
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int = 5; // error
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Instead, you assign a value to an object of that type (in this case, a variable of type int):
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int myInteger; myInteger = 5; // ok
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Similarly, you can t assign values to fields in a class; you must assign values to fields in an object. Thus, you can t write:
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Dog.weight = 5;
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This is not meaningful. It isn t true that every dog s weight is 5 pounds. You must instead write:
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milo.weight = 5;
This says that a particular dog s weight (Milo s weight) is 5 pounds.
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7: Classes and Objects
Creating a Box Class
We ll start out with a very simple class a three-dimensional box. The internal state of the box should keep track of the length, width, and height of the box. You probably also want some way to show the box to the user. In a graphical environment, you would probably draw the box, but because we re working with console applications, we ll compromise and just output the dimensions. You might implement such a class by defining a single method and three variables, as shown in Example 7-1.
using using using using System; System.Collections.Generic; System.Linq; System.Text;
namespace Example_7_1_ _ _ _The_Box_Class { public class Box { // private variables private int length; private int width; private int height; // public methods public void DisplayBox( ) { Console.WriteLine("Stub for displaying the box."); } } public class Tester { static void Main( ) { Box boxObject = new Box( ); boxObject.DisplayBox( ); } } }
This code creates a new user-defined type: Box. The Box class definition begins with the declaration of a number of member variables: Length, Width, and Height. The keyword private indicates that these values can be accessed only by methods of this class. The private keyword is an access modifier, explained later in this chapter.
Defining Classes |
Many C# programmers prefer to put all of the member fields together, either at the very top or at the very bottom of the class declaration, though that is not required by the language.
The only method declared within the Box class is the method DisplayBox( ). The DisplayBox( ) method is defined to return void; that is, it will not return a value to the method that invokes it. For now, the body of this method has been stubbed out. Stubbing out a method is a temporary measure you might use when you first write a program to allow you to think about the overall structure without filling in every detail when you create a class. When you stub out a method body, you leave out the internal logic and just mark the method, perhaps with a message to the console:
public void DisplayBox( ) { Console.WriteLine("Stub for displaying the box."); }
After the closing brace, a second class, Tester, is defined. Tester contains our now familiar Main( ) method. In Main( ), an instance of Box is created, named boxObject:
Box boxObject = new Box( );
Technically, an unnamed instance of Box is created in an area of memory called the heap, and a reference to that object is returned and used to initialize the Box reference named boxObject. Because that is cumbersome, we ll simply say that a Box instance named boxObject was created.
Because boxObject is an instance of Box, Main( ) can make use of the DisplayBox( ) method defined for objects of that type and can call it to display the dimensions of the box:
boxObject.DisplayBox( );
You invoke a method on an object by writing the name of the object (boxObject) followed by the dot operator (.), the method name (DisplayBox), and the parameter list in parentheses (in this case, the list is empty). You ll see how to pass in values to initialize the member variables in Constructors later in this chapter.
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