Sample Quiz Answers
For Chapter 7
THIS PAGE CONTAINS SAMPLE ANSWERS to the Quiz on
Chapter 7 of this on-line
Java textbook. Note that in many cases, there are lots of correct
answers to a given question.
Question 1:
What is the FontMetrics class used for?
Answer:
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 2:
An off-screen image can be used to do double buffering. Explain this.
(What are off-screen images? How are they used? Why are they important?
What does this have to do with animation?)
Answer:
In Java, an off-screen image is an object belonging to the class Image
and created with the createImage() function.
An off-screen image is a segment of the computer's memory that can be
used as a drawing surface. What is drawn to the off-screen image is
not visible on the screen, but the Image can be quickly copied onto
the screen with a drawImage() command. It is important to use an off-screen
image in a situation where the process of drawing the image should not
be seen by the user. This is true, for example, in animation. Each frame
of the animation can be composed in an off-screen Image and then copied to
the screen when it is complete. The alternative would be to erase the
screen and draw the next frame directly on the screen. This causes unacceptable
flickering of the image. By default, Swing already uses an off-screen image for
double-buffering an applet or frame, so you don't have to program it yourself
just to do simple animation.
Question 3:
One of the main classes in Swing is the JComponent class.
What is meant by a component? What are some examples?
Answer:
A JComponent represents a visual component of the computer's
graphical user interface. A JComponent is not completely independent.
It must be added to a "container," such as an
applet or a frame. Examples of JComponents are JButtons, JTextFields,
and JPanels.
Question 4:
What is the function of a LayoutManager in Java?
Answer:
A LayoutManager implements some policy for laying out
all the visual components that have been added to a container,
such as a JPanel or the content pane of a JApplet.
That is, it sets the sizes and
positions of the components. Different types of layout managers
have different rules about how components are to be arranged.
Some standard layout manager classes are BorderLayout
and GridLayout.
Question 5:
What does it mean to use a null layout manager, and
why would you want to do so?
Answer:
If the layout manager for a container is set to null, then the
programmer takes full responsibility for setting the sizes and positions
of all of the components in that container. This gives the programmer
more control over the layout, but the programmer has to do more work.
For simple layouts in a container that does not change size, the
setBounds() method of each component can be called when
it is added to the container. If the container can change size,
then the sizes and positions should be recomputed when a size
change occurs. This is done automatically by a layout manager,
and this is one good reason to use a layout manager for a container
that can change size.
Question 6:
What is a JCheckBox and how is it used?
Answer:
A JCheckBox is a component that has two possible states, "checked"
and "unchecked". The user can change the state by clicking on the JCheckBox.
If box is a variable of type JCheckBox, then a program can
check the box by calling box.setSelected(true) and can uncheck the
box by calling box.setSelected(false). The current state can be
determined by calling box.isSelected(), which is a boolean-valued
function. A JCheckBox generates an event of type ActionEvent
when it changes state. A program can listen for these events if it wants to
take some action at the time the state changes. Often, however, it's
enough for a program simply to look at the state of the JCheckBox
when it needs it.
Question 7:
What is a thread ?
Answer:
A thread, like a program, executes a sequence of instructions from
beginning to end. Several threads can execute "in parallel" at
the same time. In Java, a thread is represented by an object of type
Thread. A Thread object has a run()
method to execute (usually the run() method of a Runnable
object that is provided when the Thread is constructed).
The thread begins executing the run() routine
when its start() method is called. At the same time,
the rest of the program continues to execute in parallel with
the thread.
Question 8:
Explain how Timers are used to do animation.
Answer:
Displaying an animation requires showing a sequence of frames.
The frames are shown one after the other, with a short delay
between each frame and the next. A Timer can generate
a sequence of ActionEvents. When a timer is used to
do animation, each event triggers the display of another frame.
The ActionListener that processes events from the timer
just needs to be programmed to display a frame when its
actionPerformed() method is called.
Question 9:
Menus can contain sub-menus. What does this mean,
and how are sub-menus handled in Java?
Answer:
Menus can be "hierarchical." A menu can contain other menus,
which are called sub-menus. A sub-menu is represented as a
single item in the menu that contains it. When the user selects
this item, the full sub-menu appears, and the user can select
an item from the sub-menu. In Java, a sub-menu is no different
from any other menu. A menu can be added to a menu bar, or
it can be added to another menu. In the latter case, it
becomes a sub-menu.
Question 10:
What is the purpose of the JFrame class?
Answer:
An object belonging to the class JFrame is an independent
window on the screen. You don't need a Web browser to create a JFrame
like you do for an JApplet. A JFrame can be used
as a user interface for a stand-alone program. It is also possible for
an applet to open a JFrame.
