6.8 Graphics Supplement
■
You can add buttons and icons to applets.
■
In event-driven programming, certain actions—like clicking a button—fire
events. The events are received by listeners that perform actions, the details
of which depend on the event fired.
■
An action listener is a class that has the phrase implements ActionListener at the end of its heading and defines a method actionPerformed. The
method has one parameter of type ActionEvent.
■
You can use the method setVisible to make components, such as labels
and buttons, visible or invisible.
Exer c i ses
1. Create a class that will bundle together several static methods for tax computations. This class should not have a constructor. Its attributes are
basicRate—the basic tax rate as a static double variable that starts at
4 percent
luxuryRate—the luxury tax rate as a static double variable that starts at
10 percent
Its methods are
computeCostBasic(price)—a static method that returns the given price
plus the basic tax, rounded to the nearest penny.
computeCostLuxury(price)—a static method that returns the given
price plus the luxury tax, rounded to the nearest penny.
changeBasicRateTo(newRate)—a static method that changes the basic
tax rate.
changeLuxuryRateTo(newRate)—a static method that changes the luxury tax rate.
roundToNearestPenny(price)—a private static method that returns the
given price rounded to the nearest penny. For example, if the price is
12.567, the method will return 12.57.
2. Consider a class Time that represents a time of day. It has attributes for the
hour and minute. The hour value ranges from 0 to 23, where the range 0 to
11 represents a time before noon. The minute value ranges from 0 to 59.
a. Write a default constructor that initializes the time to 0 hours, 0 minutes.
b. Write a private method isValid(hour, minute) that returns true if the
given hour and minute values are in the appropriate range.
c. Write a method setTime(hour, minute) that sets the time if the given
values are valid.
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d. Write another method setTime(hour, minute, isAM) that sets the
time if the given values are valid. The given hour should be in the range
1 to 12. The parameter isAm is true if the time is an a.m. time and false
otherwise.
3. Write a default constructor and a second constructor for the class RatingScore, as described in Exercise 9 of the previous chapter.
4. Write a constructor for the class ScienceFairProjectRating, as described
in Exercise 10 of the previous chapter. Give this constructor three parameters corresponding to the first three attributes that the exercise describes.
The constructor should give default values to the other attributes.
5. Consider a class Characteristic that will be used in an online dating
service to assess how compatible two people are. Its attributes are
description—a string that identifies the characteristic
rating
this characteristic in another person
a. Write a constructor that sets the description of the characteristic to a
given string and sets the rating to zero to indicate that it has not yet
been determined.
b. Write a private method isValid(aRating) that returns true if the
given rating is valid, that is, is between 1 and 10.
c. Write a method setRating(aRating) that sets the rating to aRating
if it is valid.
d. Write a method setRating that reads a rating from the keyboard,
insisting that the rating supplied by the user be valid.
6. Create a class RoomOccupancy that can be used to record the number of
people in the rooms of a building. The class has the attributes
numberInRoom—the number of people in a room
totalNumber—the total number of people in all rooms as a static variable
The class has the following methods:
addOneToRoom—adds a person to the room and increases the value of
totalNumber
removeOneFromRoom—removes a person from the room, ensuring that
numberInRoom does not go below zero, and decreases the value of
totalNumber as needed
getNumber—returns the number of people in the room
getTotal—a static method that returns the total number of people
7. Write a program that tests the class RoomOccupancy described in the previous exercise.
6.8 Graphics Supplement
8. Sometimes we would like a class that has just a single unique instance. Create
a class Merlin that has one attribute, theWizard, which is static and of type
Merlin. The class has only one constructor and two methods, as follows:
Merlin—a private constructor. Only this class can invoke this constructor; no other class or program can create an instance of Merlin.
summon —a static method that returns theWizard if it is not null ;
if theWizard is null, this method creates an instance of Merlin
using the private constructor and assigns it to theWizard before
returning it.
consult—a non-static method that returns the string "Pull the sword
from the stone".
9. Create a program that tests the class Merlin described in the previous exercise.
toString method to verify that a unique instance has been created.
10. In the previous chapter, Self-Test Question 16 described a class Person to repa string, and an integer age. These variables are name and age, respectively.
a. Write a default constructor for Person that sets name to the string
"No name yet" and age to zero.
b. Write a second constructor for Person that sets name to a given string and
age to a given age.
c. Write a static method createAdult for Person that returns a special instance of this class. The instance represents a generic adult and has the
name "An adult" and the age 21.
11. Create a class Android whose objects have unique data. The class has the
following attributes:
tag—a static integer that begins at 1 and changes each time an instance
is created
name—a string that is unique for each instance of this class
Android has the following methods:
Android —a default constructor that sets the name to "Bob"
concatenated with the value of tag . After setting the name, this
constructor changes the value of tag by calling the private method
changeTag .
getName—returns the name portion of the invoking object.
isPrime(n)—a private static method that returns true if n is prime—that
is, if it is not divisible by any number from 2 to n − 1.
changeTag—a private static method that replaces tag with the next
prime number larger than the current value of tag.
12. Create a program that tests the class Android described in the previous exercise.
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PRACTICE PROGRAMS
Practice Programs can generally be solved with a short program that directly applies
the programming principles presented in this chapter.
1. Modify the definition of the class Species in Listing 5.19 of Chapter 5 by
removing the method setSpecies and adding the following methods:
eters for the three instance variables, and a default constructor. Be sure
that each constructor sets all of the instance variables.
set that can reset values: one is the same as the
method setSpecies in Listing 5.16, and the other three each reset one
of the instance variables.
Then write a test program to test all the methods you have added.
Finally, repeat Practice Program 1 in Chapter 5, but be sure to use some
constructor other than the default constructor when you define new
objects of the class Species.
2. Repeat Programming Project 2 in Chapter 5. This time, add the following four constructor methods: one for each instance variable, one with
two parameters for the two instance variables, and a default constructor.
Be sure that each constructor sets all of the instance variables. Write a
driver program to test each of the methods, including each of the four
constructors and at least one true and one false case for each of the test
methods.
Pet from Listing 6.1, write a program to read data for five
pets and display the following data: name of smallest pet, name of largest
pet, name of oldest pet, name of youngest pet, average weight of the five
pets, and average age of the five pets.
to initialize the Trivia object with a question and answer.
5. The following class displays a disclaimer every time an instance is created
using the default constructor. However, we would like the disclaimer to
appear only once when the very first Vehicle object is created. Any future
Vehicle objects that are created should display no disclaimer. Modify the
code to use a static boolean variable that is initialized to false when it
is defined. Once the disclaimer is displayed the variable should be set to
true, and an if statement added around the display so the disclaimer is
only output if the variable has the value false. This should force the program to display the disclaimer only once. Add a main method and test
code to verify that the disclaimer is only displayed once.
6.8 Graphics Supplement
public class Vehicle
{
public Vehicle()
{
System.out.println(“You should not operate this vehicle
under the”);
System.out.println(“influence of alcohol!”);
}
}
the user to initialize the name and alcoholic content of the beer.
7. We can improve the Beer class from the previous Practice Program by using
principles of information hiding. The instance variable percent holds the
percent of alcohol in the beer. If this variable is public then someone might
enter the alcohol percentage the wrong way. For example, if the alcohol
percentage is 5% then someone might set the variable to 5.0 instead of
0.05 that the program expects. As a result, the program would compute
that someone would be legally intoxicated after a miniscule sip of beer.
Address this problem by adding or modifying the mutator method,
setAlcohol. If the variable that is being set is over 1 then set the alcohol
variable to the incoming value divided by 100. Otherwise set the alcohol
variable to the incoming value. You should also do this check for the
percentage set by the constructor. Test your program by setting the percent
alcohol to 5.0 and to 0.05. The results should be unchanged.
PROGRAMMING PROJECTS
Programming Projects require more problem-solving than Practice Programs and
can usually be solved many different ways. Visit www.myprogramminglab.com to
complete many of these Programming Projects online and get instant feedback.
double. Call the class DoubleOut. Include all the methods from the class DollarFormat in Listing 6.14,
all the methods from the class OutputFormat of Self-Test Question 30, and
a method called scienceWrite that displays a value of type double using e
notation, such as 2.13e–12. (This e notation is also called scientific notation,
which explains the method name.) When displayed in e notation, the number
should appear with exactly one nonzero digit before the decimal point—unless the number is exactly zero. The method scienceWrite will not advance
to the next line. Also add a method called scienceWriteln that is the same as
scienceWrite except that it does advance to the next line. All but the last two
method definitions can simply be copied from the text (or more easily from
the source code for this book that is available on the Web.). Note that you will
be overloading the method names write and writeln.
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CHAPTER 6 / More About Objects and Methods
Write a driver program to test your method scienceWriteln. This driver
program should use a stub for the method scienceWrite. (Note that
this means you can write and test scienceWriteln before you even
write scienceWrite.) Then write a driver program to test the method
scienceWrite. Finally, write a program that is a sort of super driver
program that takes a double value as input and then displays it using the
two writeln methods and the scienceWriteln
5 for the number of digits after the decimal point when you need to
specify such a number. This super driver program should allow the user to
repeat this testing with additional numbers of type double until the user
is ready to end the program.
2. Write a new class TruncatedDollarFormat that is the same as the class
DollarFormat from Listing 6.14, except that it truncates rather than rounds
to obtain two digits after the decimal point. When truncating, all digits
after the first two are discarded, so 1.229 becomes 1.22, not 1.23. Repeat
Programming Project 3 in Chapter 4 using this new class.
3. Complete and fully test the class Time that Exercise 2 describes. Add two
more constructors that are analogous to the setTime methods described in
Parts c and d of Exercise 2. Also include the following methods:
getTime24 returns a string that gives the time in 24-hour notation
hhmm. For example, if the hour value is 7 and the minute value is 25,
return "0725". If the hour value is 0 and the minute value is 5, return
"0005" . If the hour value is 15 and the minute value is 30, return
"1530" .
getTime12 returns a string that gives the time in 12-hour notation h:mm
xx. For example, if the hour value is 7 and the minute value is 25, return "7:25 am". If the hour value is 0 and the minute value is 5, return
"12:05 am". If the hour value is 15 and the minute value is 30, return
"3:30 pm".
4. Complete and fully test the class Characteristic that Exercise 5 describes.
Include the following methods:
getDescription—returns the description of this characteristic.
getRating—returns the rating of this characteristic.
getCompatability(Characteristic otherRating)—returns the com-
patibility measure of two matching characteristics, or zero if the descriptions do not match.
getCompatibilityMeasure(Characteristic otherRating)—a private
method that returns a compatibility measure as a double value using the
formula
[
m = 1–
(r1 – r2)2
81
]
6.8 Graphics Supplement
479
when both ratings are nonzero; m is zero if either rating is zero. (Recall
from Exercise 5 that the constructor sets the rating to zero, indicating that
it has not yet been determined.)
isMatch(Characteristic otherRating)—a private method that re-
turns true if the descriptions match.
5. Write a Java enumeration LetterGrade that represents letter grades A
variable to hold a boolean value that is true if the grade is passing. Also,
define a constructor that initializes this instance variable, an accessor
method isPassing to return its value, and a method toString that returns the grade as a string. Finally, write a program to demonstrate the
enumeration.
6. Complete and fully test the class Person that Exercise 10 describes. Include
the following additional methods:
getName—returns the name of the person as a string.
getAge—returns the age of the person.
setName(first, last)—sets the name of the person, given a first and
last name as strings.
setName(name)—sets the name of the person, given the entire name as
one string.
setAge(age)—sets the age of the person.
createToddler—a static method that returns a special instance of the
class to represent a toddler. The instance has the name "A toddler" and
the age 2.
createPreschooler—a static method that returns a special instance of
the class to represent a preschooler. The instance has the name "A preschooler" and the age 5.
createAdolescent—a static method that returns a special instance of
the class to represent an adolescent. The instance has the name "An
adolescent" and the age 9.
createTeenager—a static method that returns a special instance of the
class to represent a teenager. The instance has the name "A teenager"
and the age 15.
7. Write a Temperature class that represents temperatures in degrees in both
and a character for the scale: either 'C' for Celsius or 'F' for Fahrenheit.
The class should have
for both the degrees and the scale, and a default constructor. For each
of these constructors, assume zero degrees if no value is specified and
Celsius if no scale is given.
VideoNote
Solving a similar problem
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CHAPTER 6 / More About Objects and Methods
from Practice Program 5 of Chapter 3 and round to the nearest tenth
of a degree.
and one to set both.
equal, one to test whether one temperature is greater than another, and
one to test whether one temperature is less than another.
Write a driver program that tests all the methods. Be sure to invoke each
of the constructors, to include at least one true and one false case for each
comparison method, and to test at least the following three temperature
pairs for equality: 0.0 degrees C and 32.0 degrees F, −40.0 degrees C and
−40.0 degrees F, and 100.0 degrees C and 212.0 degrees F.
8. Repeat Programming Project 8 of the previous chapter, but include constructors.
VideoNote
Solution to Project 9
9. Write and fully test a class that represents rational numbers. A rational
number can be represented as the ratio of two integer values, a and b,
where b is not zero. The class has attributes for the numerator and denominator of this ratio. The ratio should always be stored in its simplest form.
That is, any common factor of a and b should be removed. For example,
the rational number 40/12 should be stored as 10/3.
The class has the following constructors and methods:
and converts the resulting ratio to simplified form.
simplify—a private method that converts the rational number to sim-
plified form.
getGCD(x, y)—a private static method that returns the largest common
factor of the two positive integers x and y, that is, their greatest common
divisor. For example, the greatest common divisor of 40 and 12 is 4.
getValue—returns the rational number as a double value.
toString—returns the rational number as a string in the form a/b.
10. Write a program that will record the votes for one of two candidates by
using the class VoteRecorder, which you will design and create. Vote
Recorder will have static variables to keep track of the total votes for candidates and instance variables to keep track of the votes made by a single
person. It will have the following attributes:
nameCandidatePresident1—a static string that holds the name of the
first candidate for president
6.8 Graphics Supplement
nameCandidatePresident2—a static string that holds the name of the
second candidate for president
nameCandidateVicePresident1—a static string that holds the name of
the first candidate for vice president
nameCandidateVicePresident2—a static string that holds the name of
the second candidate for vice president
votesCandidatePresident1—a static integer that holds the number of
votes for the first candidate for president
votesCandidatePresident2—a static integer that holds the number of
votes for the second candidate for president
votesCandidateVicePresident1—a static integer that holds the number of votes for the first candidate for vice president
votesCandidateVicePresident2—a static integer that holds the number of votes for the second candidate for vice president
myVoteForPresident—an integer that holds the vote of a single individual for president (0 for no choice, 1 for the first candidate, and 2 for
the second candidate)
myVoteForVicePresident—an integer that holds the vote of a single
individual for vice president (0 for no choice, 1 for the first candidate,
and 2 for the second candidate)
In addition to appropriate constructors, VoteRecorder has the following
methods:
setCandidatesPresident(String name1, String name2)—a static
method that sets the names of the two candidates for president
setCandidatesVicePresident(String name1, String name2)—a
static method that sets the names of the two candidates for vice president
resetVotes—a static method that resets the vote counts to zero
getCurrentVotePresident—a static method that returns a string with
the current total number of votes for both presidential candidates
getCurrentVoteVicePresident—a static method that returns a string
with the current total number of votes for both vice presidential candidates
getAndConfirmVotes
votes, confirms them, and then records them
getAVote(String name1, String name2)—a private method that returns a vote choice for a single race from an individual (0 for no choice,
1 for the first candidate, and 2 for the second candidate)
getVotes—a private method that returns a vote choice for president and
vice president from an individual
confirmVotes
president and vice president, asks whether the voter is happy with
these choices, and returns true or false according to a yes-or-no
response
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recordVotes
the appropriate static variables
Create a program that will conduct an election. The candidates for president are Annie and Bob. The candidates for vice president are John and
Vote
Recorder object for each voter. After all the voters are done, present the results.
11. Repeat Programming Project 10 from Chapter 5, but include constructors.
Graphics
12. Change the applet in Listing 6.24 so that after the button is clicked, the
button disappears. The label and the icon should remain visible just as in
Listing 6.24. (Hint: This is not a big change.)
Graphics
labeled Red, White, and Blue. When a button is clicked, the background of
the applet changes to the color named on the button.
Graphics
Light that simulates a simple light.
Create one button labeled On/Off. As you click this button, the background
color will change between DARK_GRAY and YELLOW.
Graphics
Give it two buttons labeled Odd and Even. The user must guess whether a
secret number is odd or even by clicking one of these buttons. After a guess
is made, the applet should display either Congratulations, you are correct! or Sorry, you are wrong. In either case, also display The secret
number was
sages.
The program will have a private instance variable secretNumber of type
long that holds the secret number. You will need to set it in the init
method using the following line of code:
secretNumber =
java.util.Calendar.getInstance().getTimeInMillis() % 100;
In the actionPerformed method, check which button was pressed and
make the appropriate response label visible. After that, make the two buttons invisible (only one guess is allowed) and the label containing the
secret number visible.
A n s wers to Self - Test Q ues t io n s
1. If a class is named Student, every constructor for this class must also be
named Student.
6.8 Graphics Supplement
2. You specify no return type for a constructor, not even void.
3. A default constructor is a constructor without parameters.
4. Classes do not always have a default constructor. If you give no constructor
definition for a class, Java will automatically provide a default constructor.
However, if you provide one or more constructors of any sort, Java does
not provide any constructors for you. So if none of your constructors is a
default constructor, the class has no default constructor.
5. yourPet.setPet(correctName, correctAge, correctWeight);
6. The declaration of an instance variable within a class definition has no
keyword static, but the declaration of a static variable does use static.
Every object of the class has its own instance variables. A class has only one
of each static variable, and all objects share that static variable.
7. Yes.
8. Yes. An example is the static variable interestRate in the method setInterestRate in Listing 6.7.
9. No, you cannot reference an instance variable within the definition of a
static method, because a static method can be invoked without an object,
and so there are no instance variables.
10. Yes.
11. Yes. Note that this means you can reference both static variables and instance variables within a non-static method.
12. It is valid, but a more normal way of doing the same thing is
double inches = imensionConverter.convertFeetToInches(2.5);
13. Yes.
14. Yes, you can invoke a non-static method within the definition of a static
method, but only if you have an object of the class and use that object in
the invocation of the non-static method.
15. Yes. Invoking a static method within the definition of a non-static method
requires nothing special.
16. No, because addInterest is not a static method.
17. a. 2; b. 3; c. 2.0; d. 2.0; e. 3.0; f. 3.0; g. A random integer ≥10 and ≤ 19. Note
that the first two values are of type long, while the last four values are of
type double.
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18. approxSpeed = (int)Math.round(speed);
Because Math.round returns a value of type long, and you want a value of
type int, you must use a type cast.
19. longSpeed = Math.round(speed);
Because Math.round returns a value of type long, no type cast is necessary.
20. long. Since one argument is of type long, the result is of type long.
21. The following class and a demonstration program are included in the code
that is on the Web.
public class CircleCalculator
{
public static double getArea(double radius)
{
return Math.PI * radius * radius;
}
public static double getCircumference(double radius)
{
return Math.PI * (radius + radius);
}
}
22. They are all legal.
23. Double.toString(x)
24. Integer.parseInt(s)
25. Integer.parseInt(s.trim())
26. System.out.println("Largest double is " + Double.MAX_VALUE);
System.out.println("Smallest double is " + Double.MIN_VALUE);
27. $7.00
long, rather than variables of type int. Note that,
because the method Math.round returns a value of type long, you will not
need a type cast in the method writePositive. For example,
int allCents = (int)(Math.round(amount * 100));
would become
long allCents = Math.round(amount * 100);
The variables dollars and cents should also be changed to type long.
6.8 Graphics Supplement
29. This will produce a “Null Pointer Exception” error message. The variables
s1 and s2 do not name any objects. The lines
Species s1 = null;
Species s2 = null;
should be changed to
Species s1 = new Species();
Species s2 = new Species();
30. The class OutputFormat is in the file OutputFormat.java included with the
source code provided on the Web. We also list it here:
public class OutputFormat
{
/**
Displays a number with digitsAfterPoint digits after the
decimal point. Rounds any extra digits.
Does not advance to the next line after output.
*/
public static void write(double number, int digitsAfterPoint)
{
if (number >= 0)
writePositive(number, digitsAfterPoint);
else
{
double positiveNumber = -number;
System.out.print('-');
writePositive(positiveNumber, digitsAfterPoint);
}
}
//Precondition: number >= 0
//Displays a number with digitsAfterPoint digits after
//the decimal point. Rounds any extra digits.
private static void writePositive(double number,
int digitsAfterPoint)
{
int mover = (int)(Math.pow(10, digitsAfterPoint));
//1 followed by digitsAfterPoint zeros
int allWhole; //number with the decimal point
//moved digitsAfterPoint places
allWhole = (int)(Math.round(number * mover));
int beforePoint = allWhole / mover;
int afterPoint = allWhole % mover;
System.out.print(beforePoint);
System.out.print('.');
writeFraction(afterPoint, digitsAfterPoint);
}
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//Displays the integer afterPoint with enough zeros
//in front to make it digitsAfterPoint digits long.
private static void writeFraction(int afterPoint,
int digitsAfterPoint)
{
int n = 1;
while (n < digitsAfterPoint)
{
if (afterPoint < Math.pow(10, n))
System.out.print('0');
n = n + 1;
}
System.out.print(afterPoint);
}
/**
Displays a number with digitsAfterPoint digits after the decimal point. Rounds any extra digits.
Advances to the next line after output.
*/
public static void writeln(double number,
int digitsAfterPoint)
{
write(number, digitsAfterPoint);
System.out.println();
}
}
31. Yes, you could use the names print and println, rather than write and
writeln, in the class OutputFormat. Java would have no name confusion
with System.out.println, because when you invoke a method in OutputFormat, you specify the class name before the dot. (If you invoke the
method with an object instead of the class name, Java still knows the class
name because it knows the type of the object.) However, the methods in
OutputFormat behave a little differently from the method System.out.
println, so using a different name would be clearer to people.
32. Yes, the 7 would be converted to 7.0 by Java so that the types would match
the heading of the constructor having three parameters.
33. No, you cannot overload a method name on the basis of return type. These
two methods have the same signatures.
34. Yes, they differ in the type of their parameter, so this is a valid overloading of the
method name convertValue. The fact that the methods return values of different types does not affect whether or not both definitions can be used. Only the
types of the parameters matter in making overloading valid in this case.
35. Yes, it would be valid because no other method named setSpecies has the
same number and types of parameters. The definition follows:
6.8 Graphics Supplement
public void setSpecies(String newName)
{
name = newName;
population = 0;
growthRate = 0;
}
36. Yes, it would be valid because no other method named setSpecies has the
same number and types of parameters. The definition follows:
public void setSpecies(String newName)
{
name = newName;
}
37. No; if you add both of these new methods setSpecies, the class will have
two definitions of this method that have the same number and types of
parameters. That is, both methods have the same signature.
38. Simply delete all occurrences of this from the definition.
39. Add an invocation of the method trim. The rewritten version follows
public void set(String amountString)
{
String dollarsString;
String centsString;
amountString = amountString.trim();
<The rest of the method definition is the same as in Listing
6.16.>
40. public String getNameOfFirst()
{
return first.getName();
}
public int getAgeOfFirst()
{
return first.getAge();
}
public double getWeightOrFirst()
{
return first.getWeight();
}
41. a.
b.
c.
d.
e.
f.
g.
3
false
A positive integer
Suit.CLUBS
Suit.SPADES
"black"
"spades"
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42. import mypackages.library1.*;
43. You must make the following the first instruction line in the file, ignoring
blank lines and comments:
package mypackages.library1;
44. A package name must be a path name for the directory that contains the
classes in the package, but the package name uses dots in place of slashes.
When naming the package, you use a relative path name that starts from any
directory named in the setting of the class path (environment) variable.
45. The way to do this depends a little on your operating system and on your
personal preferences. Here is an outline of what you should do: Choose a
package name and insert the following at the start of the file Pet.java:
package Package_Name;
Compile the modified file Pet.java. Place both of the files Pet.java and
Pet.class into the directory corresponding to Package_Name.
46. You may name the content pane anything you wish, other than a keyword.
So the following is perfectly legal:
Container insideOfApplet = getContentPane();
Of course, if you make this change, you must replace contentPane with
insideOfApplet wherever else it occurs in the method actionPerformed.
47. When we say that an event is “sent” to a listener object, what we really mean
is that some method in the listener object is invoked and given the event
object as the argument. This invocation happens automatically. Your applet
class definition will not normally contain an invocation of this method.
48. It would have no effect on the program, as long as you also replace e with
buttonEvent wherever else e occurs in the method actionPerformed. The
e is a parameter for the method, and we can use any non-keyword identifier as the parameter.
49. Change the code
JButton cloudyButton = new JButton("Cloudy");
contentPane.add(cloudyButton);
cloudyButton.addActionListener(this);
to the following:
JButton cloudyButton = new JButton("Cloudy");
ImageIcon nastyFaceIcon = new ImageIcon("nasty.gif");
6.8 Graphics Supplement
cloudyButton.setIcon(nastyFaceIcon);
contentPane.add(cloudyButton);
cloudyButton.addActionListener(this);
The complete applet code is in the file ButtonIconDemo2.java included
with the source code for this book that is available on the Web.
e must be included in the method heading but
need not be used in the method body.
51. The listener, which is the applet itself, is listening to only one button, so it
need not decide which button was clicked. There is only one button and
hence only one action.
52. Take the following three steps:
aButton.setVisible(false);
as the last one in the definition of the method actionPerformed.
aButton a private instance variable by adding the statement
private JButton aButton;
JButton from the statement that creates aButton.
489