ASP.NET PDF Viewer using C#, VB/NET

The previous two sections gave you the basics of putting together a class. Now you ll take a look at really getting the most out of object-oriented programming by adding methods to your class. A method is a function that has access to the fields of an object and can change them if they are mutable. A derived class can define new methods and can override methods inherited from its base class. Methods are defined using four keywords, either member, override, abstract, or default. The simplest way to declare a method is to use the member keyword; this defines a method that cannot be overridden. The override keyword defines a method that overrides an inherited method that has an implementation in a base class. The abstract keyword defines a method that has no implementation and must be overridden in a derived class. The keyword default has a similar meaning to the override keyword, except it is only ever used to override an abstract method. The member, override, and default definitions have the same syntax. The keyword is followed by the parameter that represents the instance of the object whose class you are in the process of defining. You can use this parameter in the method implementation to get access to all the class s fields and properties. After this special parameter comes a dot and then the

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name of the method. Next come the parameters of the method. After that comes an equals sign followed by the implementation of the method. Methods declared using the keyword abstract are a little different, because there is no implementation. Because there is no implementation, you omit the parameter that represents the object itself, so the keyword abstract is followed directly by the method name. Following this is the method s type, which is separated from the method name using a colon, just like any other type annotation. The next example is designed to illustrate the use of all four kinds of methods: #light type Base = class val mutable state: int new() = { state = 0 } member x.JiggleState y = x.state <- y abstract WiggleState: int -> unit default x.WiggleState y = x.state <- y + x.state end type Sub = class inherit Base new() = {} default x.WiggleState y = x.state <- y &&& x.state end let myBase = new Base() let mySub = new Sub() let testBehavior (c : #Base) = c.JiggleState 1 print_int c.state print_newline() c.WiggleState 3 print_int c.state print_newline() print_endline "base class: " testBehavior myBase print_endline "sub class: " testBehavior mySub The results of this example, when compiled and executed, are as follows: base class: 1 4 sub class: 1 1

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<td align="right"> <a class="controls" href="javascript:minimize('weatherContent');"> </a>   <a class="controls" href="javascript:maximize('weatherContent');"> + </a> </td> </tr> </table> </div> <div class="normalText"> Zip Code: <input type="text" name="forecastZipCode" id="forecastZipCode" onkeyup="handleZipCodeChange();" class="normalText" value="<%=ajaxdashboard.Constants.DEFAULT_WEATHER_ZIP_CODE%>"/> </div> <div id="weatherContent"> <%@ include file="weatherTable.jsp" %> </div> </div> You may have noticed that this JSP includes content from two other JSP files: weatherLocation.jsp (see Listing 8-4) and weatherTable.jsp (see Listing 8-5). These two JSPs, respectively, render the weather forecast s location and the actual forecast itself. Before you look at why these two sections are separated into their own JSP files, take a look at their source code. It should look pretty much the way you would expect it to look. The weatherLocation.jsp file simply outputs the name of the current forecast location, and the weatherTable.jsp file renders a one-row-by-seven-column table detailing the weather forecast. Listing 8-4. weatherLocation.jsp <%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%> Weather for ${forecastData.placeName}, ${forecastData.stateCode}

You first implement a method, JiggleState, in class Base. The method cannot be overridden, so all derived classes will inherit this implementation. You then define an abstract method, WiggleState, that can be overridden (and, in fact, must be) by derived classes. To define a new method that can be overridden, you always need to use a combination of the abstract and default keywords. This could mean that abstract is used on the base class while the default is used on the derived class, but often you will use them together in the same class, as shown in the previous example. This requires the programmer to explicitly give types to a method they are providing to be overridden. Although the F# philosophy is generally not to require the programmer to give explicit types and to try to let the compiler work them out, the compiler has no way to infer these types, so you must give them explicitly. As shown in the results when JiggleState is called, the behavior remains the same in both the base class and the derived class, where the behavior of WiggleState changes because it is overridden.