Sample Quiz Answers
For Chapter 6
THIS PAGE CONTAINS SAMPLE ANSWERS to the Quiz on
Chapter 6 of this on-line
Java textbook. Note that in many cases, there are lots of correct
answers to a given question.
Question 1:
Programs written for a graphical user interface have to deal with
"events." Explain what is meant by the term event. Give at least
two different examples of events, and discuss how a program might respond
to those events.
Answer:
An event is anything that can occur asynchronously, not under
the control of the program, to which the program might want to respond. GUI programs
are said to be "event-driven" because for the most part, such programs
simply wait for events and respond to them when they occur. In many (but not all)
cases, an event is the result of a user action, such as when the user clicks the
mouse button, types a character, or clicks a button.
The program might respond to a mouse-click on a canvas by
drawing a shape, to a typed character by adding the character to an input box,
or to a click on a button by clearing a drawing. More generally, a programmer
can set up any desired response to an event by writing an event-handling routine
for that event.
Question 2:
What is an event loop?
Answer:
An event-driven program doesn't have a main() routine
that says what will happen from beginning to end, in a step-by-step
fashion. Instead, the program runs in a loop that says:
while the program is still running:
Wait for the next event
Process the event
This is called an event loop. In Java, the event loop is executed
by the system. The system waits for events to happen. When an
event occurs, the system calls a routine that has been designated to handle
events of that type.
Question 3:
Explain carefully what the repaint() method does.
Answer:
The repaint() method is called to notify the system that
the applet (or other component for which it is called) needs to
be redrawn. It does not itself do any drawing (neither directly
nor by calling a paint() or paintComponent() routine). You should
call repaint() when you have made some change to the state of the
applet that requires its appearance to change. Sometime
shortly after you call it, the system will call the
applet's paint() routine or the component's
paintComponent() routine.
Question 4:
What is HTML?
Answer:
HTML, or HyperText Markup Language, is a language that is
used for writing Web pages. A HTML document contains all the
text on a Web page, "marked up" with "tags" that determine how the
text looks -- its size and color and how it is broken into paragraphs, for example.
Other tags can include things like horizontal lines, images, links,
and applets on the page. A Web browser acts as an interpreter for
the HTML language.
Question 5:
Draw the picture that will be produced by the following
paint() method:
public static void paint(Graphics g) {
for (int i=10; i <= 210; i = i + 50)
for (int j = 10; j <= 210; j = j + 50)
g.drawLine(i,10,j,60);
}
Answer:
The outer loop is executed for values of i equal to
10, 60, 110, 160, and 210. For each of these values, the inner loop
is executed for j equal to 10, 60, 110, 160, and 210. The drawLine command
is therefore executed 25 times -- and so, 25 different lines are drawn. These lines connect
the five points (10,10), (60,10), (110,10), (160,10), and (210,10) to the
five points (10,60), (60,60), (110,60), (160,60), and (210,60) in all possible
pairings. Here is the picture:
Question 6:
Suppose you would like an applet that displays a green square
inside a red circle, as illustrated. Write a paint() method that
will draw the image.
Answer:(The size of the square and circle are not specified in the
problem, so any size would be acceptable, as long as the square is in the middle
of the circle. Notice that the drawing commands are fillOval
and fillRect. There are no special routines for drawing circles
or squares.)
public void paint(Graphics g) {
g.setColor(Color.red);
g.fillOval(10,10,80,80);
g.setColor(Color.green);
g.fillRect(30,30,40,40);
}
Question 7:
Suppose that you are writing an applet, and you want the applet to respond in
some way when the user clicks the mouse on the applet. What are the four
things you need to remember to put into the source code of your applet?
Answer:
(1) Since the event and listener classes are defined in the java.awt.event
package, you have to put "import java.awt.event.*;" at the beginning
of the source code, before the class definition.
(2) The applet class must be declared to implement the MouseListener
interface, by adding the words "implements MouseListener"
to the heading. For example: "public class MyApplet extends JApplet
implements MouseListener". (It is also possible for another object
besides the applet to listen for the mouse events.)
(3) The class must include definitions for each of the five methods
specified in the MouseListener interface. Even if a method is not
going to do anything, it has to be defined, with an empty body.
(4) The applet (or other listening object) must be registered to listen
for mouse events by calling addMouseListener(). This is usually
done in the init() method of the applet.
Question 8:
Java has a standard class called MouseEvent. What is the purpose of
this class? What does an object of type MouseEvent do?
Answer:
When an event occurs, the system packages information about the event
into an object. That object is passed as a parameter to the event-handling
routine. Different types of events are represented by different classes
of objects. An object of type MouseEvent represents a mouse
or mouse motion event. It contains information about the location of
the mouse cursor and any modifier keys that the user is holding down.
This information can be obtained by calling the instance methods of
the object. For example, if evt is a MouseEvent
object, then evt.getX() is the x-coordinate of the mouse
cursor, and evt.isShiftDown() is a boolean value that tells you
whether the user was holding down the Shift key.
Question 9:
Explain what is meant by input focus. How is the input focus managed
in a Java GUI program?
Answer:
When the user uses the keyboard, the events that are generated are directed
to some component. At a given time, there is just one component that
can get keyboard events. That component is said to have the input focus.
Usually, the appearance of a component changes if it has the input focus
and wants to receive user input from the keyboard. For example, there
might be a blinking text cursor in the component, or the component might
be hilited with a colored border. In order to change its appearance in
this way, the component needs to be notified when it gains or loses the
focus. In Java, a component gets this notification by listening for
focus events.
Some components, including applets and canvasses, do not get the
input focus unless they request it by calling requestFocus().
If one of these components needs to process keyboard events, it should
also listen for mouse events and call requestFocus() when the
user clicks on the component. (Unfortunately, this rule is not
enforced uniformly on all platforms.)
Question 10:
Java has a standard class called JPanel. Discuss
two ways in which JPanels can be used.
Answer:
A JPanel is a type of component. That is, it is a visible element
of a GUI. By itself, a JPanel is simply a blank rectangular region
on the screen. However, a JPanel is a "container", which means that
other components can be added to it and will then appear on
the screen inside the JPanel.
A JPanel can also be used as a drawing surface. In order to do
this, it is necessary to create a subclass of JPanel and define
a paintComponent() method in that subclass. An
object belonging to that subclass can then be added to an applet
or other component. The paintComponent() method defines
how that object will draw itself on the screen.
