Transcript part 1 - Kirkwood Community College

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 name
width = #
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
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