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Answers for Quiz on Chapter 12

This page contains sample answers to the quiz on Chapter 12 of Introduction to Programming Using Java. Note that generally, there are lots of correct answers to a given question.

Question 1:

Describe the object that is created by the following statement, and give an example of how it might be used in a program:

BufferedImage OSC = new BufferedImage(32,32,BufferedImage.TYPE_INT_RGB);

A BufferedImage is a region in memory that can be used as a drawing surface. In this statement, the image that is created is 32 pixels wide and 32 pixels high, and the color of each pixel is an RGB color that has red, green, and blue components in the range 0 to 255. The picture in a BufferedImage can easily be copied into a graphics context g by calling one of the g.drawImage methods. However, since the image is so small in this case, it seems more likely that is going to be used to define an ImageIcon.

Question 2:

Many programs depend on resource files. What is meant by a resource in this sense? Give an example.


In addition to the program code itself, programs often use other types of data such as images and sounds. These are said to be resources for the program. In order to run, the program must be able to locate and load the resources. An example of a resource is a small image file that will be used by the program to define a custom cursor.

Question 3:

What is the FontMetrics class used for?


An object that belongs to the class FontMetrics can be used to obtain information about the sizes of characters and strings that are drawn in a specific font. The font is specified when the FontMetrics object is created. If fm is a variable of type FontMetrics, then, for example, fm.stringWidth(str) gives the width of the string str and fm.getHeight() is the usual amount of vertical space allowed for one line of text. This information could be used, for example, for positioning the string is a component.

Question 4:

If a Color, c, is created as c = new Color(0,0,255,125), what is effect of drawing with this color?


When a color is constructed from four integers, the fourth parameter is the alpha component of the color. When drawing, the alpha component is interpreted as the degree of opaqueness of the color. If the alpha component is less than its maximum possible value, 255, then the color is partially transparent. In this case, c represents a blue color that is about 50% transparent. When a pixel is colored with c, the pixel does not become entirely blue. Instead, the new color of the pixel is obtained by blending c with the previous color of the pixel. The effect is like adding a blue tint to the pixel, or like looking at the previous color of the pixel through blue-colored glass.

Question 5:

What is antialiasing?


Antialiasing is used to make shapes that are drawn look less "jagged." The jaggedness is called "aliasing," and it arises because shapes have to be drawn by coloring individual pixels. If antialiasing is off, then the only decision is either to color a pixel or not to color it. As a result, a boundary that should be a smooth curve will look like a jagged staircase. When antialiasing is on, the amount of color that is applied to a pixel depends on how much of that pixel is covered by the ideal geometric shape. For pixels along the boundary of the shape, the color of the shape will be blended with the previous color of the pixel. Although antialiasing does not produce a perfect result, it does tend to make pictures look better.

Question 6:

How is the ButtonGroup class used?


A ButtonGroup object is used with a set of radio buttons (or radio button menu items), to make sure that at most one of the radio buttons in the group can be selected at any given time. To use the ButtonGroup class, you have to create a ButtonGroup object, grp. Then each radio button, rb, that is supposed to be part of the group is added to the group by calling grp.add(rb). Nothing further needs to be done with the ButtonGroup object.

Question 7:

What does the acronym MVC stand for, and how does it apply to the JTable class?


MVC stands for "Model-View-Controller." In a JTable, the view is the actual visible component on the screen. The model is the collection of data that specifies, among other things, what appears in each cell of the table and in the column headings. The model is represented by a separate object from the object that represents the view. The controller is responsible for interaction with the user. It consists mostly of a bunch of listener objects that listen for events generated when the user interacts with the table. The listeners respond by making changes in the model, which will in turn cause a change in the view.

Question 8:

Describe the picture that is produced by the following paintComponent() method:

public void paintComponent(Graphics g) {
   Graphics2D g2 = (Graphics2D)g;
   g2.translate( getWidth()/2, getHeight()/2 );
   g2.rotate( 30 * Math.PI / 180 );

This shows a filled black square that is 100-by-100 pixels in size. The corner of the square is at the center of the component that is being painted, and the top side of the square descends at a 30 degree angle from that point. (The translate command moves the origin, (0,0) to the point (getWidth()/2,getHeight()/2), so that when the fillRect command places the corner of the square at (0,0), the corner actually appears at the center of the component. Furthermore, the rotate command rotates the picture by 30 degrees in a clockwise direction about the origin. This means that the top of the square is rotated from the horizontal position onto a line that is 30 degrees clockwise of the horizontal. That line descends at a 30 degree angle.

Question 9:

What is meant by Internationalization of a program?


Internationalization refers to writing the program in a way that will make it easy to adapt the program for use in a variety of "locales." For example, it should be easy to translate all the strings that are used in the program into other languages. To make this possible the strings should not be hard coded into the program itself. Instead, they are placed in a separate resource file, so that the program can be translated into another language simply by writing a resource file for that language. Internationalization also applies to the format that is used for dates and numbers.)

Question 10:

Suppose that the class that you are writing has an instance method doOpen() (with no parameters) that is meant to be used to open a file selected by the user. Write a code segment that creates an Action that represents the action of opening a file. Then show how to create a button and a menu item from that action.


Here is the code for the three parts of the problem:

Action openAction = new AbstractAction( "Open..." ) {
   public void actionPerformed( ActionEvent e ) {

JButton openButton = new JButton( openAction );
JMenuItem openCommand = new JMenuItem( openAction );

(Since Action is only an interface, the class AbstractAction has to be used to create the action object. The most natural way to write the code is to create an anonymous inner class that as a subclass of AbstractAction. This subclass must define the actionPerformed() method -- which in this case only has to call the doOpen() method. As an alternative to createing the JMenuItem, the action could have simply been added directly to a JMenu. By the way, the "..." in the name of the action is there, by convention, to tell the user that selecting this command will cause a dialog box to pop up.)

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