Abstract * Foil Index for this file
Given by Nancy J. McCracken at ECS400 Senior Undergraduate Course on Spring Semester 1996. Foils prepared 27 February 1996
Abstract * Foil Index for this file
| Java is an object-oriented language based on C++ suitable for general distributed applications programming. In this course, we will concentrate on Java applets to program application interfaces on the World Wide Web. |
| These lecture slides on programming in Java will show a series of small programming examples, designed to illustrate the main features of the language. |
| They accompany the Java Course Module, by Geoffrey Fox, which covers more details about the language. |
| Text: "Teach yourself Java in 21 days", by Laura Lemay and Charles L. Perkins, February 1996, Sams.net Publishing. |
This table of Contents
Abstract
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| Nancy McCracken |
| NPAC |
| Syracuse University |
| 111 College Place |
| Syracuse NY 13244-4100 |
| March 6, 1996 |
| Click here for body text |
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| Java is an object-oriented language based on C++ suitable for general distributed applications programming. In this course, we will concentrate on Java applets to program application interfaces on the World Wide Web. |
| These lecture slides on programming in Java will show a series of small programming examples, designed to illustrate the main features of the language. |
| They accompany the Java Course Module, by Geoffrey Fox, which covers more details about the language. |
| Text: "Teach yourself Java in 21 days", by Laura Lemay and Charles L. Perkins, February 1996, Sams.net Publishing. |
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| Since Java is object-oriented, programs are organized into modules called classes, which may have data in variables and functions called methods. |
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| Java applets are part of the class hierarchy that can call methods to display on a screen (within the browser window). This example defines the public method paint in this class and calls a method drawString defined in the class Graphics. |
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| You should name the file with your applet name, HelloWorldApplet.java, run the compiler (javac), getting a bytecode file HelloWorldApplet.class, which you put in a web directory. |
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| An object of the graphics class represent a rectangular drawing grid, with a coordinate system in pixels. |
| When you draw objects, there is a current "state" consisting of a font and a color. |
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| Fonts and colors are objects (sometimes called instances) of the font and color class in the awt package. |
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| Within the applet tags, any number of param tags may occur. Attributes can control the alignment of the applet window on the html page. |
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| A standard method, init(), is executed when your applet is loaded; it can call the method getParameter. |
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| Methods are available for drawing lines, rectangles, rounded rectangles, ovals, arcs, and polygons. This example draws a rectangle filled with green with a black border, and a circle filled with magenta. |
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| The method applet.size() returns the width and height as a dimension object, which has width and height variables. FontMetrics has variables for various attributes of the font, including width and height. |
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| This example fills the applet's drawing area with square boxes, each of which has a randomly chosen color. |
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| This demonstrates 2-dimensional arrays by first filling arrays with random colors, and then drawing, uses a fixed size drawing window for simplicity. |
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Each applet's flow of control goes through a sequence of standard methods:
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| See ColorBoxes examples with init and start. |
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| A thread is a single sequential flow of control within a process. |
| If a process has more than one thread, then each thread executes concurrently. |
| Any Java applet which has extensive execution or loops to repaint the window must run as a concurrent thread with the browser window. |
To make an applet with a thread, which is almost always recommended:
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| These start and stop methods can always be used |
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| The body of the applet is in the run method, in this case a loop to keep showing the date. |
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| We define a parent class for movable point objects. Each instance of this class is represented by an x,y location, by a dx,dy offset for the object to move, and a color for the object. |
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| We include methods to change the default values for the offsets and color, a method to move the object, and one to paint a point. |
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| We define a class for movable rectangle objects. It uses x,y from the parent class mPoint for the location of the rectangle, but adds h,w to specify height and width of the rectangle. |
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| This applet creates 2 mRectangles and loops to move them. |
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| The repaint method calls update, which is overridden here to move the objects, and paint, which is overridden here to redraw whole graphics area. |
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| You are allowed to have more than one constructor in a class, if they constructors have different types or numbers of arguments. For example, for people who are using the java.awt.Point class, you can add a constructor to mRectangle which creates a rectangle by giving the top left and bottom right corner points. |
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| Once we have structured the concepts of our data into classes, it is easy to add new shapes of movable objects as child classes of mRectangle. These classes only have to redefine paint to draw a differently shaped object. |
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| The Applet class provides a method getImage, which retrieves an image from a web server and creates an instance of the Image class. |
| Image img = getImage(new URL("http://www.tc.com/image.gif")); |
| Another form of getImage retrieves the image file relative to the directory of the HTML or the directory of the java code. |
| Image img = getImage(getDocumentBase(), "images/image.gif"); |
| Image img = getImage(getCodeBase(), "images/image.gif"); |
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| The Graphics class provides a method drawImage to actually display the image on the browser screen. |
| You can also scale the image to a particular width and height. |
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| This example shows how to get the actual height and width of the image to use in scaling the image under java program control. |
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| This technique is most effective in eliminating flicker, but does take longer and use more memory. |
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| We set up a test program that creates 3 movable objects, a rectangle, circle and triangle, as in the earlier example. In this program, we start with them all red. Whenever the mouse is detected to be over one of the objects, it's color is changed to cyan. If the mouse button is used to drag the object, we move the object to the mouse location. |
| Note that it is not necessary to introduce a thread for this applet since it is not running continuously - it is mostly waiting for mouse events. |
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| Whenever the user moves the mouse, a message is sent to from the client workstation to the Java system. It generates a form of interrupt called an event. Your Java applet can choose to provide a method which does appropriate response for any event. This is called an event handler - you must return true to show the general event handler that you have intervened. |
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| These handle the sequence of the user clicks the mouse (mouse down), moves the mouse (mouse drag) and releases the mouse button (mouse up). |
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| We add methods to detect when the mouse is inside an object, and a more general method moveTo for large moves under dragging. |
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| You may also choose to respond to individual keys being pressed from the keyboard. (Later we will learn of a more general way to handle text input.) |
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| A simple example of using the Abstract Windowing Toolkit divides the applet space into two components. One is a Canvas for drawing graphics or images; one is a Panel which has three buttons. We choose to draw the Canvas within the Applet and to create the Panel in another class. |
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| The initialization of the Canvas was done in the applet init method; the other method for painting the Canvas simply changes the background color. About the only thing a Canvas can do is explicit drawing and graphics. |
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| This class does not need to be a subclass of Applet - like all components except Canvas, you don't need to paint, just add components and handle events and actions. |
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| The Button component generates an action whenever the button is clicked on. In this example, we call the method that changes the color of the Canvas in another part of the layout. |
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| We design a Java applet to provide a more interactive interface to our Pizza Order CGI script. We start by laying out the web page in which people either select a special or choose toppings. |
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| We design a set of classes to hold the components of the graphical user interface. The top level page consists of three panels, where the second is the first page of a card layout, with a specials page and a toppings page. |
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| The constructor of this component reads the specials file on the server and displays the contents here. |
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| Users may drag toppings onto the pizza area to select them. Unlike our HTML form, we allow users to select multiple toppings (and yes, we'll have to change our pizza2.pl CGI script a little bit to allow that). |
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| The main function of the pizza order applet is to fill in values for the variables in the order component: address, phoneno. and pizzatype. The instance of the order component class is constructed by the top page, so it is passed to the other constructors so that they have access to the variables and methods of ordercomp. |