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Frameworks

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Framework

• Set of cooperating classes/interfaces– Structure essential mechanisms of a problem

domain– Programmer can extend framework classes,

creating new functionality

• Example: Swing package is framework for problem domain of GUI programming

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Frameworks Vs. Patterns

• Pattern: general design rule for solving a problem

• Framework: set of classes providing functionality in a particular domain; often uses multiple patterns

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Application Frameworks

• Implement services common to an type of application

• Supply sets of classes for programmer to augment to build applications (often by subclassing framework classes)

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Inversion of Control

• Framework classes, not application-specific classes, control flow of execution

• Framework determines which methods to call and when

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Framework example: Applets

• Applet: Java program that can be included in a web document to add interactions and dynamism

• Java source is referenced by an HTML document• Package java.applet is a simple application

framework– consists of superclasses to make applets

– programmer adds classes, overrides methods to create actual applets

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Applet characteristics

• No main() method

• Programmer overrides some or all of the following methods:– init()– start()– stop()– destroy()– paint()

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How applets work

• Since an applet has no main() method, initialization of object takes place in method init(), which takes the place of both main() and the constructor

• Method init() is typically used to perform initializations of short duration; longer running initializations, e.g. loading a file across a network, should be done in separate threads

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Methods of Applet class

• Method init() is empty, and must be overridden by the child class

• Method start() is called to set up the applet to run– may create several threads– called when applet is loaded & initialized the

first time, resized, or when user returns to page from elsewhere

– paint() automatically called after start()

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Methods of Applet class

• Method stop() is called to stop the applet - usually means halting threads started by start() when, for example, the user leaves the page

• Method destroy() performs final cleanup before the applet is unloaded

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Methods of Applet class

• Control sequence is:– init()– start()– paint()– then stop() or destroy(), as appropriate

• Applet can transition many times between start() and stop() states

• May define all, some, or none of these methods when extending an Applet; should only define those necessary for specific applet

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HTML for applet inclusion

<APPLET codebase = “URL for code location”code = class namewidth = #height = #>

<param name = somename value = value>Alternate text to display in lieu of applet</APPLET>

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Applet HTML dissected

• Codebase: URL web address where code can be found (can be just a file name)

• code: name of class (name of file in an application)

• height and width tell browser how much space to allocate for applet

• param tags: provide parameters, analogous to command-line parameters - values can be accessed within applet via call to getParameter()

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More fun facts about applets

• Applet class is actually subclass of Panel - so inherits graphical component attributes

• Events, in particular, are handled same way as other graphical components, and window repainting same as application

• Default layout for applet is flow layout, not border layout

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Other methods provided by Applet class

• getImage(URL): takes URL and retrieves image from that location (must be gif or jpg)

• getAudioClip(URL): returns audio object from given location; can subsequently be asked to play - shorthand version play(URL) combines get and play methods

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Other methods provided by Applet class

• getCodeBase() returns URL for codebase specified for applet

• getParameter() takes String argument and returns associated String value if <param> tag was provided - returns null if none was provided

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Java class URL

• URL class is provided by java.net.*

• URL object supplies methods to parse URL, open network connections, and retrieve information

• Example:URL newURL = new URL(String s);

creates URL object newURL with specification s

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Java class URL

• Can create URLs using Java’s URL class - address is formed from a String, or previous URL plus a String - latter from could be used, for example, to retrieve several files from the same directory

• URL class provides method openStream, which returns an InputStream value - once an URL object is created, can use this method to read from the URL using normal InputStream methods

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Using applet to open a new page

• Can use showDocument() method of class appletContext to instruct browser to load a new web page:

appletContext.showDocument(URL)– URL refers to a Java URL object (not a string)– browser will display the page corresponding to

the URL

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Applets vs. Applications

• An applet has no main method, as it is structured to run inside another application

• The Java Virtual Machine runs applications, but not applets

• Can use appletviewer program to test and debug applet code

• Most applets are event-driven (but don’t have to be)

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Applets vs. Applications

• Applets do I/O through the browser GUI, and displays error messages (only) to stdout

• An applet is a subclass of Applet from java.applet

• Applets have security restrictions: can’t read or write files, run local executable programs, communicate with hosts other than the server, or learn many facts about the client computer

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Creating Applet/Application Hybrids

• Applet class pays no attention to static methods contained in class definition

• Key idea is that Applet is a Panel– can nest inner class within Applet to create Frame necessary for

application

– only component created for this Frame will be Panel constructed by Applet

– Method main(), which is ignored when executing as applet, creates instance of applet when run as an application

– Applet then creates instance of Frame, placing itself in center

– Constructor for Frame executes requisite init() and start()

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Applets as Framework

• Programmer for individual applet uses inheritance to extend the applet framework class to specific program

• Applet class deals with behavior common to all applets; programmer only fills in custom behavior for specific application

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Applets as Framework

• Inversion of control means applet programmer not concerned with overall control flow - fills in handlers for initialization, starting, stopping and painting, but has no control over when these methods are called