Framework. Set of cooperating classes/interfaces. Example: Swing package is framework for problem domain of GUI programming

Transcription

1 Frameworks 1

2 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 2

3 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 3

4 Application Frameworks Implement services common to any type of application Supply sets of classes for programmer to augment to build applications (often by subclassing framework classes) 4

5 Inversion of Control Framework classes, not application-specific classes, control flow of execution Framework determines which methods to call and when 5

6 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 6

7 Applet characteristics No main() method Programmer overrides some or all of the following methods: init() start() stop() destroy() paint() 7

8 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 8

9 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() 9

10 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 10

11 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 11

12 HTML for applet inclusion <APPLET codebase = URL for code location code = class name width = # height = #> <param name = somename value = value> Alternate text to display in lieu of applet </APPLET> 12

13 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() 13

14 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 14

15 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 15

16 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 16

17 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 17

18 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 18

19 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 19

20 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) 20

21 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 21

22 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() 22

23 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 23

24 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 24