1.3 OBJECT ORIENTED PROGRAMMING

 

• Inheretance allows us to write one generic class (function) that depends on other functions to deal with special cases (this can be done in other languages with function calls, but this is more consistent)

 

 

• an applet (note a special type of Java program) needs to have four functions,

init() - called when the applet is first loaded

start() - called when an applet becomes visible in the browser

stop() - called when the applet is no longer visible

destroy() - called when leaving a browser page

* init() is always called first, and then start() and stop() will be called while the document using the applet is alive. If the document is exited, then stop() will be called if not already, and destroy() will be called last before exiting.

 

• We can also call out to the parent HTML document. Generally, these calls deal with URL’s and parameters.

URL getDocumentBase() - returns the URL for the HTML document

URL getCodeBase() - returns the URL of the applet

String getParameter(String name) - for the parameter with the ‘name’, the string value is returned

URL A = new URL(“http://www.computer.edu”); - define a URL

URL B = new URL(A, “directory/file.ext”); - define a new URL by extending the existing URL A

 

• Various types of media can also be included using simple functions.

Image getImage(URL) - will set up a link to an image at a remote site to be used when the image is drawn.

Image getImage(URL, filename) - an overloaded version that allows the filename string to be separated from the URL.

AudioClip getAudioClip(URL) - will retrieve (but not play yet) an audio file. Functions available include,

play() - play the clip

loop() - play the clip continuously

stop() - stop playing the current clip.

AudioClip getAudioClip(URL, file_name)

void play(URL) - plays an audio file directly

void play(URL, file_name)

 

• Applets can interact with the browser that has called it,

AppletContext getAppletContext() - gets a structure that is required by other functions.

Applet getApplet(String filename) - retrieves an applet using an HTML parameter.

Enumeration getApplets() - returns applets on the current page.

void show Document(URL) - allows a new document to be loadedin the browser.

void showStatus(String) - the string is printed to the screen.

String get AppletInfo() - Can be used to return information about the applet.

String[][] getParameterInfo() - used to inform the browser what parameters are allowed.

 

• GUIs can be constructed using the ‘.awt’ toolkit. This allows the user to work with a variety of interface tools. The hierarchy is shown below.

 

 

Component - the basic class to represent the position & size of objects

Container - a class that can hold other classes

Panel - container in a container for organizing objects

Window - a rectangular area on the GUI

Frame - a window with a border added

Dialog - will receive input from the user

FileDialog - a file selection box

Button - when clicked on with a mouse will cause an action

Canvas - an area generally for free form I/O like lines and mouse events

Checkbox - will allow toggled or checked inputs

Label - a printed string

List - a scrolling list of strings

Scrollbar - can be attached to canvases to scroll

TextArea - a text editing window

TextField - a single text inport line

 

 

• Components, and all objects in the hierarchy, can use the following functions.

Dimension Size() - returns width and height

Rectangle bounds() - return x, y, width, and height values

void enable() - sets a component to accept user input

void disable() - turns off user input to a component

void show() - makes a component visible

void paint(Graphics g) - a function called to redraw a component

void repaint() - requests that a component be redrawn

void update(Graphics g) - a level above a call to paint, might allow other function to be added

boolean mouseEnter(Event e, int x, int y) - called when the mouse has entered a component

boolean mouseExit(Event e, intx, int y) - called when the mouse leaves a component

boolean mouseMove(Event e, int x, int y) - reports a move inside a component with no buttons pressed

boolean mouseDrag(Event e, int x, int y) - called when a mouse button is held down, and the pointer dragged inside a component

boolean keyDown(Event e, int key) - reports a keyboard event inside the component

boolean handleEvent(Event e) - The normal event handler for component events.

Dimension preferredSize() - returns the idea component size

Dimension minimumSize() - returns the smallest size that a component can be.

 

 

• The container class also has a variety of associated functions (as well as its children)

void add(Component) - add a component

void add(String name, Component) - a component is added, with a name as well.

void setLayout(Layout Manager) - this automatically positions components

 

 

 

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