Ebook Java: An introduction to problem solving & programming (Sixth edition) – Part 2
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Arrays
7.4 SORTING AND SEARCHING ARRAYS 525
Selection Sort 525
Other Sorting Algorithms 529
Searching an Array 531
7.1 ARRAY BASICS 481
Creating and Accessing Arrays 482
Array Details 485
The Instance Variable length 488
More About Array Indices 491
Initializing Arrays 494
7.5 MULTIDIMENSIONAL ARRAYS 532
Multidimensional-Array Basics 533
Multidimensional-Array Parameters and Returned
Values 536
Java’s Representation of Multidimensional
Arrays 539
Ragged Arrays (Optional ) 540
Programming Example: Employee Time
Records 542
7.2 ARRAYS IN CLASSES
AND METHODS 495
Case Study: Sales Report 495
Indexed Variables as Method Arguments 503
Entire Arrays as Arguments to a Method 505
Arguments for the Method main 507
Array Assignment and Equality 508
Methods That Return Arrays 511
7.3 PROGRAMMING WITH ARRAYS
AND CLASSES 515
Programming Example: A Specialized List Class
Partially Filled Arrays 523
Chapter Summary
556
7
515
Programming Projects
7.6 GRAPHICS SUPPLEMENT 548
Text Areas and Text Fields 548
Programming Example: A Question-and-Answer
Applet 548
The Classes JTextArea and JTextField 551
Drawing Polygons 553
562
Answers to Self-Test Questions
568
They stood at attention in a neat row, all with the same uniform, yet each
with his own values.
8"33&/1&"$&
The Lieutenant’s Array
An array is a special kind of object used to store a collection of data. An array
differs from the other objects you have seen in two ways:
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you might use an array to store a list of values of type double that record
rainfall readings in centimeters. Or you might use an array to store a list
of objects of some class called Species that contain the records for various
endangered species.
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arrays were used by programmers for many years before classes were
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methods.
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in Java.
OBJECTIVES
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PREREQUISITES
This is the first point in this book where you have a significant choice as to
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go on in the book and return to this discussion of arrays at a later time. You
480
7.1 Array Basics
481
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ZPVXJTICFGPSFZPVSFBEUIJTDIBQUFS4FDUJPOTBOEJODMVEFSFGFSFODFT
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be familiar with the material in all of the previous chapters before reading the
remaining sections of this chapter.
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you can—and are encouraged to—read the first part of this chapter’s graphic
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7.1 ARRAY BASICS
And in such indexes, although small pricks
To their subsequent volumes, there is seen
The baby figure of the giant mass
Of things to come.
—WILLIAM SHAKESPEARE, TROILUS AND CRESSIDA
4VQQPTFZPVXBOUUPDPNQVUFUIFBWFSBHFUFNQFSBUVSFGPSUIFTFWFOEBZTJOB
week. You might use the following code:
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter 7 temperatures:");
double sum = 0;
for (int count = 0; count < 7; count++)
{
double next = keyboard.nextDouble();
sum = sum + next;
}
double average = sum / 7;
5IJTXPSLTGJOFJGBMMZPVXBOUUPLOPXJTUIFBWFSBHF#VUMFUTTBZZPVBMTP
want to know which temperatures are above and which are below the average.
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The obvious answer is to use seven variables of type double. This is a bit
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the problem can be even worse. Imagine doing the same thing for each day of
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would be absurd. Arrays provide us with an elegant way to declare a collection
of related variables. An array is a collection of items of the same type. It is Items in an array
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CVUJUIBOEMFTUIFOBNJOHPGUIFWBSJBCMFTJO have the same
data type
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482
CHAPTER 7 / Arrays
Creating and Accessing Arrays
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of a collection of seven variables of type double can be created as follows:
double[] temperature = new double[7];
This is like declaring the following seven strangely named variables to have
the type double:
temperature[0], temperature[1], temperature[2], temperature[3],
temperature[4], temperature[5], temperature[6]
An index is
an integer
expression that
indicates an array
element
Variables like temperature[0] and temperature[1] that have an integer
FYQSFTTJPO JO TRVBSF CSBDLFUT BSF DBMMFE indexed variables, subscripted
variable, array elements, or simply elements.5IFJOUFHFSFYQSFTTJPOXJUIJO
UIFTRVBSFCSBDLFUTJTDBMMFEBOindex or a subscript. Note that the numbering
TUBSUTXJUI
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REMEMBER Array Indices Begin at 0
In Java, the indices of an array always start with 0. They never start with 1
or any number other than 0.
Each of these seven variables can be used just like any other variable of
type double'PSFYBNQMF
BMMPGUIFGPMMPXJOHTUBUFNFOUTBSFBMMPXFEJO+BWB
temperature[3] = 32;
temperature[6] = temperature[3] + 5;
System.out.println(temperature[6]);
8IFOXFUIJOLPGUIFTFJOEFYFEWBSJBCMFTBTCFJOHHSPVQFEUPHFUIFSJOUPPOF
DPMMFDUJWFJUFN
XFXJMMDBMMUIFNBOBSSBZ4PXFDBOSFGFSUPUIFBSSBZOBNFE
temperatureXJUIPVUVTJOHBOZTRVBSFCSBDLFUT'JHVSFJMMVTUSBUFTUIFBSSBZ
temperature.
#VUUIFTFTFWFOWBSJBCMFTBSFNPSFUIBOKVTUTFWFOQMBJOPMEWBSJBCMFTPG
type double 5IF OVNCFS JO TRVBSF CSBDLFUT JT QBSU PG UIF OBNF PG FBDI PG
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JTBMMPXFE
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter day number (0 - 6):");
int index = keyboard.nextInt();
System.out.println("Enter temperature for day " + index);
temperature[index] = keyboard.nextDouble();
7.1 Array Basics
FIGURE 7.1 A Common Way to Visualize an Array
Indices
0
1
2
3
4
5
6
32
30
25.7
26
34
31.5
29
The array temperature
temperature[5]
4JODFBOJOEFYDBOCFBOFYQSFTTJPO
XFDBOXSJUFBMPPQUPSFBEWBMVFTJOUP
the array temperature
BTGPMMPXT
System.out.println("Enter 7 temperatures:");
for (int index = 0;index < 7; index++)
temperature[index] = keyboard.nextDouble();
5IFVTFSDPVMEUZQFUIFTFWFOWBMVFTPOTFQBSBUFMJOFTPSBMMPOPOFMJOF
TFQBSBUFE
CZTQBDFT"GUFSUIFBSSBZWBMVFTBSFSFBEJO
XFDBOEJTQMBZUIFNBTGPMMPXT
System.out.println("The 7 temperatures are:");
for (int index = 0; index < 7; index++)
System.out.print(temperature[index] + " ");
System.out.println( );
5IFQSPHSBNJO-JTUJOHTIPXTBOFYBNQMFUIBUVTFTPVSTBNQMFBSSBZ
temperature BT TFWFO JOEFYFE WBSJBCMFT
BMM PG UZQF double. Note that the
program uses for loops similar to the ones we just considered.
LISTING 7.1
An Array of Temperatures (part 1 of 2)
/**
Reads 7 temperatures from the user and shows which are above
and which are below the average of the 7 temperatures.
*/
import java.util.Scanner;
public class ArrayOfTemperatures
{
public static void main(String[] args)
{
double[] temperature = new double[7];
// Read temperatures and compute their average:
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter 7 temperatures:");
double sum = 0;
for (int index = 0; index < 7; index++)
(continued)
483
484
CHAPTER 7 / Arrays
LISTING 7.1 An Array of Temperatures (part 2 of 2)
{
temperature[index] = keyboard.nextDouble();
sum = sum + temperature[index];
}
double average = sum / 7;
System.out.println("The average temperature is " +
average);
// Display each temperature and its relation to the average:
System.out.println("The temperatures are");
for (int index = 0; index < 7; index++)
{
if (temperature[index] < average)
System.out.println(temperature[index] +
" below average");
else if (temperature[index] > average)
System.out.println(temperature[index] +
" above average");
else //temperature[index] == average
System.out.println(temperature[index] +
" the average");
}
System.out.println("Have a nice week.");
}
}
Sample Screen Output
Enter 7 temperatures:
32
30
25.7
26
34
31.5
29
The average temperature is 29.7428
The temperatures are
32.0 above average
30.0 above average
25.7 below average
26.0 below average
34.0 above average
31.5 above average
29.0 below average
Have a nice week.
7.1 Array Basics
485
Array Details
You create an array in the same way that you would create an object of a class
type using the operation new
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creating an array of elements of type Base_Type
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Base_Type[] Array_Name = new Base_Type[Length];
'PSFYBNQMF
UIFGPMMPXJOHDSFBUFTBOBSSBZOBNFEpressureUIBUJTFRVJWBMFOU
UPWBSJBCMFTPGUZQFint:
int[] pressure = new int[100];
"MUFSOBUJWFMZ
UIFQSFDFEJOHDBOCFCSPLFOEPXOJOUPUXPTUFQT
int[] pressure;
pressure = new int[100];
The first step declares as an
array of integers. The second
ASIDE Alternative Syntax for Declaring
step allocates enough memory
an Array
GPSUIFBSSBZUPIPMEVQUP
Although we do not encourage its use, there
integers.
is an alternative syntax for array declarations
The type for the array
that you may encounter. You can write the
square brackets after the variable instead
elements is called the base
of after the base type, as in the following
type of the array. In this
example:
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The number of elements in
char alphabet[];
an array is called the length,
size, or capacity of the array.
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variables pressure[0]through pressure[99]. Note that because the indices
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TVDI BT pressure
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variable pressure[100].
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a class type. The following statement creates an array named entry whose
elements are SpeciesPCKFDUT
XIFSFSpecies is a class:
Species[] entry = new Species[3];
This array is a collection of the three variables entry[0]
entry[1]
BOE
entry[2]
BMMPGUZQFSpecies.
RECAP Declaring and Creating an Array
You declare and create an array in almost the same way that you declare
and create objects of classes. There is only a slight difference in the
syntax.
(continued)
The number of
elements in an
array is its length
The type of the
array elements is
the array’s base
type
486
CHAPTER 7 / Arrays
SYNTAX
Base_Type[] Array_Name
= new Base_Type[Length];
EXAMPLES
char[] symbol = new char[80];
double[] reading = new double[100];
Species[] specimen = new Species[80]; //Species is a class
REMEMBER How to Use Square Brackets with Arrays
There are three different ways to use square brackets [] with an array
name. They can be used
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int[] pressure;
declares—but does not allocate memory for—pressure as an array of inteHFST
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pressure = new int[100];
allocates memory for the array pressurePGJOUFHFST
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pressure[3] in the
following two lines is an indexed variable:
pressure[3] = keyboard.nextInt();
System.out.println("You entered" + pressure[3]);
"TXFNFOUJPOFEQSFWJPVTMZ
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ZPVTIPVMEVTFBDPOTUBOUTVDIBT NUMBER_OF_READINGS instead of
XIFOZPVDSFBUFUIFBSSBZpressure:
Use a named
constant when
defining an array
public static final int NUMBER_OF_READINGS = 100;
int[] pressure = new int[NUMBER_OF_READINGS];
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BT
follows:
7.1 Array Basics
487
FIGURE 7.2 Array Terminology
Array name
temperature[n + 2]
Index (also called a subscript)
Indexed variable (also
called an array element, an
element, or a subscripted
variable)
temperature[n + 2]
temperature[n + 2]
Value of the indexed
variable (also called an
element of the array)
temperature[n + 2] = 32;
System.out.println("How many temperatures do you have?");
int size = keyboard.nextInt();
double[] temperature = new double[size];
:PVDBOBMTPVTFBOZFYQSFTTJPOUIBUFWBMVBUFTUPBOBQQSPQSJBUFJOUFHFS
XIFOOBNJOHBOJOEFYFEWBSJBCMFPGBOBSSBZ
BTJOUIFGPMMPXJOHFYBNQMFT
int point = 2;
temperature[point + 3] = 32;
System.out.println("Temperature is " + temperature[point + 3]);
Note that temperature[point + 3]JOUIFQSFDFEJOHDPEFJTUIFTBNFJOEFYFE
variable as temperature[5]
CFDBVTFpoint + 3FWBMVBUFTUP
'JHVSFJMMVTUSBUFTTPNFPGUIFNPTUDPNNPOUFSNTVTFEXIFOSFGFSSJOH
to arrays. Notice that the word element has two meanings. It can be used to
SFGFSUPBOJOEFYFEWBSJBCMFBTXFMMBTUPUIFWBMVFPGBOJOEFYFEWBSJBCMF
■ PROGRAMMING TIP
In General, Use Singular Names for
Arrays
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Species
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Species[] entries = new Species[20]; //Valid but not nice.
6TJOHBQMVSBM
MJLFentries
TFFNTUPNBLFTFOTF
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UIBOPOFFMFNFOU)PXFWFS
QSPHSBNNFSTGJOEUIBUUIFJSQSPHSBNTPGUFOSFBE
CFUUFSJGUIFZVTFBTJOHVMBSGPSNGPSBOBSSBZOBNF
MJLFUIFGPMMPXJOH
Species[] entry = new Species[20]; //Nicer.
Reading an array
length
488
CHAPTER 7 / Arrays
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statement such as
System.out.println("The entry is " + entry[2]);
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contains the number of hours worked by employee number n
UIF QMVSBM
form hours[n] makes sense. The only sure test of whether to use a singular or
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of your Java code.
■
The Instance Variable length
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namely the variable length
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JGZPVDSFBUFBOBSSBZCZXSJUJOH
The array e has a
length of e.length
Species[] entry = new Species[20];
entry.length IBT B WBMVF PG #Z VTJOH UIF JOTUBODF WBSJBCMF length
instead of a literal like 20
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general. A name like entry.length means more to a reader of your program
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occurrences of entry.length. Note that lengthJTGJOBM
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be changed.
GOTCHA
Assigning a Value to the Instance Variable length
Your program cannot assign a value to the instance variable length
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array is invalid:
entry.length = 10; //Illegal!
■
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the instance variable length. We have also read the size of the array
temperature from the user into the variable size *O UIJT FYBNQMF
XF
could use size instead of temperature.length )PXFWFS
TJODF size is
OPUGJOBM
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temperature.length.
7.1 Array Basics
LISTING 7.2
An Array of Temperatures—Revised (part 1 of 2)
/**
Reads temperatures from the user and shows which are above
and which are below the average of all the temperatures.
*/
import java.util.Scanner;
public class ArrayOfTemperatures2
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner(System.in);
System.out.println("How many temperatures do you have?");
int size = keyboard.nextInt( );
double[] temperature = new double[size];
// Read temperatures and compute their average:
System.out.println("Enter " + temperature.length +
" temperatures:");
double sum = 0;
for (int index = 0; index < temperature.length; index++)
{
temperature[index] = keyboard.nextDouble();
sum = sum + temperature[index];
}
double average = sum / temperature.length;
System.out.println("The average temperature is " +
average);
// Display each temperature and its relation to the
// average:
System.out.println("The temperatures are");
for (int index = 0; index < temperature.length; index++)
{
if (temperature[index] < average)
System.out.println(temperature[index] +
" below average");
else if (temperature[index] > average)
System.out.println(temperature[index] +
" above average");
else //temperature[index] == average
System.out.println(temperature[index] +
" the average");
}
System.out.println("Have a nice week.");
}
}
(continued)
489
490
CHAPTER 7 / Arrays
LISTING 7.2 An Array of Temperatures—Revised (part 2 of 2)
Sample Screen Output
How many temperatures do you have?
3
Enter 3 temperatures:
32
26.5
27
The average temperature is 28.5
The temperatures are
32.0 above average
26.5 below average
27.0 below average
Have a nice week.
■ PROGRAMMING TIP
Use a for Loop to Step Through
an Array
The for statement is the perfect mechanism for stepping through the elements
PGBOBSSBZ'PSFYBNQMF
UIFGPMMPXJOH forMPPQGSPN-JTUJOHJMMVTUSBUFT
one way to step through an array:
for (int index = 0; index
temperature[index] = keyboard.nextDouble();
sum = sum + temperature[index];
}
Another way to step through an entire array—after its elements have been
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'PSFYBNQMF
UIFMBTUMPPQJO-JTUJOHUIBUEJTQMBZTUIFWBMVFTJOUIFBSSBZ
temperature can be revised as follows:
for (int value : temperature)
{
if (value < average)
System.out.println(value + " below average.");
else if (value > average)
System.out.println(value + " above average.");
else //value == average
System.out.println(value + " the average.");
}
■
7.1 Array Basics
491
More About Array Indices
:PVLOPXUIBUUIFJOEFYPGUIFGJSTUFMFNFOUJOBOZ+BWBBSSBZJT5IFMBTU
WBMJEJOEFYPGBOBSSBZXIPTFMFOHUIJTn is n¦*OQBSUJDVMBS
UIFMBTUWBMJE
JOEFYPGUIFBSSBZtemperature is temperature.length - 1.
An easy mistake to make when programming with arrays is to use an
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consider the following array declaration:
double[] entry = new double[5];
&WFSZJOEFYWBMVFGPSUIFBSSBZ entryNVTUCFPOFPGUIFGJWFJOUFHFST
PS'PSFYBNQMF
JGZPVSQSPHSBNDPOUBJOTUIFJOEFYFEWBSJBCMFentry[n + 2]
UIFWBMVFPGUIFJOEFY n + 2NVTUCFPOFPGUIFTFGJWFJOUFHFST*GBOJOEFY
FYQSFTTJPOFWBMVBUFTUPTPNFJOUFHFSPUIFSUIBOUISPVHIPOFMFTTUIBOUIF
MFOHUIPGUIFBSSBZ
UIFJOEFYJTTBJEUPCFout of bounds or invalid. If your Beware of invalid
DPEFVTFTBOJOEFYFYQSFTTJPOUIBUJTPVUPGCPVOET
ZPVSDPEFXJMMDPNQJMF indices
XJUIPVUBOZFSSPSNFTTBHF
CVUZPVXJMMHFUPOFXIFOZPVSVOZPVSQSPHSBN
REMEMBER Array Indices Must Be Within Bounds to Be Valid
Since the index of the first element in a Java array is always 0, the last
index number is not the length of the array, but is one less than the
length of the array. Be sure that your indices stay within this range.
One common way that array indices go out of bounds is when an arrayQSPDFTTJOHMPPQJTJUFSBUFEPOFUPPNBOZUJNFT'PSFYBNQMF
MFUTDPOTJEFSB
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OVNCFSTGSPNUIFLFZCPBSE
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the end of the data. We might use the following code:
System.out.println("Enter a list of nonnegative integers.");
System.out.println("Place a negative integer at the end.");
int[] list = new int[10];
Scanner keyboard = new Scanner(System.in);
int number = keyboard.nextInt();
int i = 0;
while (number >= 0)
{
list[i] = number;
i++;
number = keyboard.nextInt();
}
*GUIFVTFSFOUFSTNPSFOVNCFSTUIBODBOGJUJOUIFBSSBZ
UIJTDPEFQSPEVDFT
BOBSSBZJOEFYUIBUJTPVUPGCPVOET
492
CHAPTER 7 / Arrays
A better version of the preceding while loop is the following:
while ( (i <list.length) && (number >= 0) )
{
list[i] = number;
i++;
number = keyboard.nextInt();
}
if (number >= 0)
{
System.out.println("Could not read in all the numbers.");
System.out.println("Only able to read" + list.length +
" numbers.");
}
This whileMPPQXJMMFOEJGUIFBSSBZCFDPNFTGVMM
CFDBVTFXFFOTVSFUIBUUIF
JOEFYi is less than list.length.
GOTCHA
Array Index Out of Bounds
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■
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FYBNQMF
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Adjusting code
to deal with 0
indices
public static final int NUMBER_OF_EMPLOYEES = 100;
. . .
int[] hours = new int[NUMBER_OF_EMPLOYEES];
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter hours worked for each employee:");
for (int index = 0; index < hours.length; index++);
{
System.out.println("Enter hours for employee " +
(index + 1));
hours[index] = keyboard.nextInt();
}
8JUIUIJTDPEF
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worked are stored in elements hours[0] through hours[99].
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UIF DPEF XJMM CF FBTJFS UP VOEFSTUBOE JG UIF UXP
numbering schemes match. You can achieve this by rewriting the previous code
UPJODSFBTFUIFTJ[FPGUIFBSSBZCZBOEJHOPSFUIFFMFNFOUBUJOEFY
BTGPMMPXT
7.1 Array Basics
int[] hours = new int[NUMBER_OF_EMPLOYEES + 1];
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter hours worked for each employee:");
for (int index = 1; index
System.out.println("Enter hours for employee" + index);
hours[index] = keyboard.nextInt();
}
8JUIUIJTSFWJTFEDPEF
UIFFNQMPZFFTBSFTUJMMOVNCFSFEUISPVHI
CVU
their hours worked are stored in the elements hours[1] through hours[100].
We do not use hours[0]
BOETJODFUIFTJ[FPGUIFBSSBZJTJOTUFBEPG
hours[100] is valid.
Note that the last valid value of index is hours.length − 1
BTJUXBTJO
PVSGJSTUWFSTJPOPGUIJTDPEF)PXFWFS
hours.length here is one larger than
it was earlier. Replacing
index < hours.length
in the for statement with
index <= NUMBER_OF_EMPLOYEES
JT MJLFMZ UP CF DMFBSFS UP NPTU QFPQMF )PXFWFS
JG PUIFS QSPHSBNNFST BSF
HPJOHUPCFMPPLJOHBUZPVSDPEF
UIFZNBZCFFYQFDUJOHBWBMVFUPCFTUPSFE
BUJOEFY5IFCPUUPNMJOFJTUIBUBMMQSPHSBNNFSTDPMMBCPSBUJOHPOBQSPKFDU
should use the same coding practices or confusion and errors may result.
■ PROGRAMMING TIP
Don’t Be Afraid to Waste an Element
in an Array
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XF OFWFS VTF hours[0] :FT
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FMFNFOU
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PUIFSQSPHSBNNFSTXIP
may be working with your code should be aware of your coding practice. In
+BWB
UIFBNPVOUPGNFNPSZXFXBTUFEJTOPUTJHOJGJDBOU&WFOJGXFDSFBUFBO
BSSBZPGMBSHFPCKFDUT
+BWBTUPSFTPOMZUIFBEESFTTFTPGUIFPCKFDUTJOUIFBSSBZ
5IJTNJHIUOPUCFUIFDBTFGPSPUIFSQSPHSBNNJOHMBOHVBHFT'PSFYBNQMF
$
XPVMETUPSFUIFPCKFDUJUTFMG
OPUJUTBEESFTT
JOUIFBSSBZ8BTUJOHBMPDBUJPOJOB
$
BSSBZNJHIUCFNPSFTJHOJGJDBOUUIBOEPJOHTPJOB+BWBQSPHSBN
■
■ PROGRAMMING TIP
Get Used to Index Values That Are 0
5IFJOEFYPGUIFGJSTUFMFNFOUJOBOBSSBZJT+BWBQSPHSBNNFSTLOPXUIJT6OMFTT
your application gives you a good reason to adjust your code so that you can ignore
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EPOU 5IF QSFWJPVT FYBNQMF TIPXFE BO BEKVTUNFOU UIBU BMJHOT UXP
OVNCFSJOH TZTUFNT
SFEVDJOH DPOGVTJPO BOE NBLJOH FSSPST MFTT MJLFMZ )PXFWFS
BEKVTUJOHZPVSDPEFUPBWPJEVTJOHBJOEFYTIPVMEnot be a routine practice.
■
493
Ignoring the
element at index 0
494
CHAPTER 7 / Arrays
Initializing Arrays
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ZPV
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UIFNBGUFSUIFBTTJHONFOUPQFSBUPS
BTJOUIFGPMMPXJOHFYBNQMF
double[] reading = {3.3, 15.8, 9.7};
5IFTJ[FPGUIFBSSBZUIBUJT
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UIFHJWFOWBMVFT4PUIJTJOJUJBMJ[JOHEFDMBSBUJPOJTFRVJWBMFOUUPUIFGPMMPXJOH
statements:
double[] reading = new double[3];
reading[0] = 3.3;
reading[1] = 15.8;
reading[2] = 9.7;
Explicit
initialization is
safer than default
initialization
*GZPVEPOPUJOJUJBMJ[FUIFFMFNFOUTPGBOBSSBZ
UIFZNJHIUBVUPNBUJDBMMZ
CFJOJUJBMJ[FEUPBEFGBVMUWBMVFGPSUIFCBTFUZQF'PSFYBNQMF
JGZPVEPOPU
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following for loop:
int[] count = new int[100];
for (int i = 0; i < 100; i++)
count[i] = 0;
S E L F -TE S T QU ESTI ONS
8IBUPVUQVUXJMMCFQSPEVDFECZUIFGPMMPXJOHDPEF
int[] anArray = new int[10];
for (int i = 0; i
anArray[i] = 2 * i;
for (int element : anArray)
System.out.print(element + " ");
System.out.println();
8IBUPVUQVUXJMMCFQSPEVDFECZUIFGPMMPXJOHDPEF
char[] vowel = {'a', 'e', 'i', 'o', 'u'};
for (int index = 0; index
7.2 Arrays in Classes and Methods
8IBUPVUQVUXJMMCFQSPEVDFECZUIFGPMMPXJOHDPEF
double tide[] = {12.2, −7.3, 14.2, 11.3};
System.out.println("Tide 1 is " + tide[1]);
System.out.println("Tide 2 is " + tide[2]);
$POTJEFSUIFGPMMPXJOHBSSBZ
int[] a = new int[10];
8IBUJTUIFMBTUJOEFYPGa 8IBUJTUIFWBMVFPGa.length
8IBUJTXSPOHXJUIUIFGPMMPXJOHDPEFUPJOJUJBMJ[FBOBSSBZb
int[] b = new int[10];
for (int i = 1; i <= b.length; i++)
b[i] = 5 * i;
8SJUFBDPNQMFUF+BWBQSPHSBNUIBUSFBETWBMVFTPGUZQFdouble from the
LFZCPBSEJOUPBOBSSBZ%JTQMBZUIFMBTUUI
OVNCFSSFBE5IFOEJTQMBZ
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OVNCFS SFBE 'PS FYBNQMF
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UIF
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JT
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POFQFS
MJOF
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7.2 ARRAYS IN CLASSES AND METHODS
A little more than kin, and less than kind.
—WILLIAM SHAKESPEARE, HAMLET
Arrays can be used as instance variables in classes. Methods can have an
JOEFYFEWBSJBCMFPSBOFOUJSFBSSBZBTBOBSHVNFOUBOEDBOSFUVSOBOBSSBZ*O
TIPSU
BSSBZTDBOCFVTFEXJUIDMBTTFTBOENFUIPETKVTUBTPUIFSPCKFDUTDBO
We begin with a case study that uses an array as an instance variable in a class.
CASE STUDY Sales Report
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company’s team of sales associates. The company wants to easily see which The task’s
specification
associate or associates have the highest sales and to know how the sales of
each associate compare to the average.
4JODFXFOFFEUPSFDPSEBOBNFBOEUIFTBMFTGJHVSFTGPSFBDIBTTPDJBUF
XF
can design a class for a single sales associate that holds these two data items.
Our class can perform input and output and have a reasonable complement
of accessor and mutator methods. This class definition is rather routine and is
TIPXOJO-JTUJOH
495
496
CHAPTER 7 / Arrays
LISTING 7.3 Sales Associate Class
import java.util.Scanner;
/**
Class for sales associate records.
*/
public class SalesAssociate
{
private String name;
private double sales;
public SalesAssociate()
{
name = "No record";
sales = 0;
}
public SalesAssociate(String initialName, double initialSales)
{
set(initialName, initialSales);
}
public void set(String newName, double newSales)
{
name = newName;
sales = newSales;
}
public void readInput()
{
System.out.print("Enter name of sales associate: ");
Scanner keyboard = new Scanner(System.in);
name = keyboard.nextLine();
System.out.print("Enter associate's sales: $");
sales = keyboard.nextDouble();
}
public void writeOutput()
{
System.out.println("Name: " + name);
System.out.println("Sales: $" + sales);
}
public String getName()
{
return name;
}
public double getSales()
{
return sales;
}
}
7.2 Arrays in Classes and Methods
497
Our program will need an array to keep track of the data for all sales
associates. It will also need to record the highest sales and the average sales.
5IFSFGPSF
XFXJMMOFFEUPEFTJHOBOPUIFSDMBTT8FDBOHJWFPVSOFXDMBTTUIF
following instance variables to record the desired data:
private double highestSales;
private double averageSales;
private SalesAssociate[] team;
The instance
variables
We need to know the number of associates. This number will be the
same as team.length
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UIF OVNCFS PG BTTPDJBUFT JT B HPPE JEFB 4P MFUT JODMVEF UIF GPMMPXJOH
instance variable:
private int numberOfAssociates; //Same as team.length
The job of our program breaks down into these main subtasks:
1. Get ready.
2. Obtain the data.
$PNQVUFTPNFTUBUJTUJDTVQEBUFUIFJOTUBODFWBSJBCMFT
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will look like this:
public class SalesReporter
{
private double highestSales;
private double averageSales;
private SalesAssociate[] team;
private int numberOfAssociates; //Same as team.length
public static void main(String[ ] args)
{
SalesReporter clerk = new SalesReporter();
clerk.getData();
clerk.computeStats();
clerk.displayResults();
}
<More stuff needs to be added here.>
}
"MMUIBUSFNBJOTJTUPXSJUFUIFUISFFNFUIPET
getData, computeStats, and
displayResults and to test and debug the program. We will tackle the three
methods in order.
The program’s
subtasks
498
CHAPTER 7 / Arrays
FIGURE 7.3 Class Diagram for the Class Sales Reporter
SalesReporter
-
highestSales: double
averageSales: double
team: SalesAssociate[]
numberOfAssociates: int
+ getData(): void
+ computeStats(): void
+ displayResults(): void
The loop in
getData
The input method getData JT SFMBUJWFMZ TUSBJHIUGPSXBSE
FTQFDJBMMZ
since we have an input method for objects of the class SalesAssociate.
"GUFS XF SFBE UIF OVNCFS PG BTTPDJBUFT
XF DBO XSJUF UIF GPMMPXJOH CBTJD
input loop:
for (int i = 1; i <= numberOfAssociates; i++)
{
System.out.println("Enter data for associate number" + i);
team[i].readInput();
}
Associate i is in
team[i]
"MUIPVHI UIF BSSBZ JOEJDFT CFHJO BU
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XFIBWFVTFE team[i] for associate i5IBUJT
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ignore team[0]5IVT
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follows:
team = new SalesAssociate[numberOfAssociates + 1];
4JODFXFSFBE numberOfAssociates in getData
XFXJMMQMBDFUIJTTUBUFNFOU
in that method as well.
#VUBOPUIFSQSPCMFNSFNBJOT8IFOXFUFTUUIFQSFWJPVTMPPQ
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because the base type of the array team JT B DMBTT UZQF 5P TFF UIF QSPCMFN
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SalesAssociate s;
s.readInput();
5IJTDPEFXPVMEQSPEVDFUIFTBNFFSSPSNFTTBHFSFHBSEJOHBiOVMMQPJOUFSw
The problem is that the variable s is just a name; it does not yet reference any
7.2 Arrays in Classes and Methods
object of the class SalesAssociate. The preceding code omitted the usual use
of new. The code should be
SalesAssociate s = new SalesAssociate();
s.readInput();
5IFJOEFYFEWBSJBCMF team[i]JTBMTPBWBSJBCMFPGBDMBTTUZQF
BOETPJU
is also just a name. You need to assign a SalesAssociate object to team[i]
CFGPSFFYFDVUJOH
team[i].readInput();
5IVT
XFOFFEUPBEEUIFGPMMPXJOHTUBUFNFOUUPPVSMPPQ
team[i] = new SalesAssociate();
The complete definition of the method getData with this line inserted is
TIPXOJO-JTUJOH
LISTING 7.4
A Sales Report Program (part 1 of 3)
import java.util.Scanner;
/**
Program to generate sales report.
*/
public class SalesReporter
{
private double highestSales;
private double averageSales;
private SalesAssociate[] team;
The main method is at
the end of the class.
//The array object is
//created in getData.
private int numberOfAssociates; //Same as team.length
/**
Reads the number of sales associates and data for each one.
*/
public void getData()
{
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter number of sales associates:");
numberOfAssociates = keyboard.nextInt();
team = new SalesAssociate[numberOfAssociates + 1];
for (int i = 1; i <= numberOfAssociates; i++)
Array object
{
created here.
team[i] = new SalesAssociate();
System.out.println("Enter data for associate " + i);
team[i].readInput();
System.out.println();
SalesAssociate
}
objects created here.
}
(continued)
499
