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Fundamentals of
Web Programming
Lecture 16: Java Applets &
AWT
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
1
Outline
• Why use Java Applets?
• What Java Applets can and can’t
do (example)?
• Compiling and adding an Applet to
a web page.
• Applet structure.
• Abstract Window Toolkit (AWT).
• Events.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
2
Why use Java Applets?
• Easily added to html.
• Executed within the browser
(client-side).
• Extensive computation at the client
possible, a full-blown programming
language (Java vs. JavaScript).
• Good support of graphics.
• Platform independence.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
3
Structure of an Applet
• All applets extend the Applet
class.
• They inherit a bunch of methods
that need to be redefined.
• They usually do not have the main
method (unless standalone).
• States in the browser: loaded,
running, stopped, unloaded.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
4
Applet states and methods
• Loaded: the code of the applet is
loaded with the web page;
• The browser calls init();
• Destroyed: when the applet is
unloaded (new page);
• The browser calls destroy()just
before unloading;
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
5
Applet states and methods
• Running: the applet is visible to
the user.
• When it becomes visible the
browser runs start().
• Stopped: when the applet
becomes temporarily invisible.
• The browser runs stop().
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
6
Applet states and methods
• init()typically allocates needed
resources, gets images/sounds
from the server (saves time), etc.
• start() is usually used to start
an animation.
• stop() stops an animation.
• destroy()deallocates system
resources other than memory.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
7
Applet states and methods
• Applets that provide graphical
contents usually redefine the
paint() method.
• The browser will call this method
whenever the applet window needs
to be updated.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
8
Restrictions on Applets
• Applets contain foreign code.
• It can be used to attack the client.
• Certain restrictions on what
applets can do.
• E.g., Applets have very restricted
access to the file system.
• More in the next lecture.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
9
Abstract Window Toolkit
• HTML provides ways to encode
standard elements of a web page
(checkboxes, labels, text fields,
etc).
• The user can assume that his web
page will look similar in different
browsers.
• AWT also platform independent.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
10
AWT
• AWT is a Java package, a collection
of reusable objects and methods.
• AWT provides graphical
components that look similar on all
platforms running Java.
• Provides labels, checkboxes, radio
buttons, buttons, choices, text
fields and areas (same as HTML).
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
11
AWT
• But also menus, containers and
various ways to align elements in
the applet window.
• Basic graphics supported: you can
draw lines, rectangles, ovals, etc
(not possible in HTML, unless you
load an image).
• You can have different buttons, not
only SUBMIT and CLEAR...
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
12
Layout Managers
• HTML doesn’t support many layout
choices.
• Typical HTML elements are laid out
in sequence as they are described
in the HTML code according to the
available space.
• This corresponds to the flow
layout.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
13
Layout Managers
• In HTML you can use a table to
align components.
• You can use the grid layout in an
Applet.
• Border layout is very handy;
divides the window in a five areas:
“North”, “South”, “East”, “West”
and “Center”.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
14
Layout Managers
• Components can be placed in
these areas.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
15
Layout Managers
• Grid Bag layout: like the grid
layout but you don’t have to
specify how may rows and
columns you need.
• Other layouts: card layout a stack
of containers with one visible at a
time.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
16
Containers
• In addition to Layout Managers,
AWT provides containers.
• Containers are used to organize
components into manageable
groups.
• Some containers (Panel) are
invisible and they only hold a
bunch of components together.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
17
Basic Graphics in AWT
• Basic methods for displaying
graphics in the Graphics object.
• An applet has a Graphics object
associated with it.
• It provided the context which all
graphical elements are drawn.
• It’s the argument of the paint
method.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
18
Graphics in AWT
• Graphics provides a coordinate
system in the applet window.
• The origin is in the upper left
corner.
• public void paint(Graphics g)
{
g.drawLine(0,0,100,100);
}
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
19
Interaction with the user
• Interaction with the user is mediated by
events.
• An event is an object that AWT creates
when a user performs an action such as
moving the mouse.
• The event has to be handled by the
Applet code if you wish to react to the
action the user performed.
• You can also ignore the event.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
20
Examples of events
• Events are associated with a
component.
• Different events can be generated
for different components.
• Examples: button pressed, mouse
down, mouse dragged over a
component, changed the contents
of a text field.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
21
Handling events
• Unfortunately, Java 1.1 redefined
the event handling model.
• Will look only at the old one, since
it’s easier.
• You should use the new model in
your code.
• Yet another function to redefine:
action(Event e, Object arg)
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
22
Handling events
• public boolean
action(Event e, Object arg) {
if (e.target == button) {
buttonClicked();
return true;
}
return false;
}
• arg contains the parameter of e.
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
23
Other window toolkits
• AWT is very simple.
• It’s possible to use your own,
customized window toolkit.
• Sun provided the Swing
components with Java 1.2.
• New look, customizable, but not all
browsers support Java 1.2 yet...
20-753: Fundamentals of
Web Programming
Lecture 16: Java Applets & AWT
Copyright © 1999, Carnegie Mellon. All Rights Reserved.
24