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Programming with Java
1
Chapter 9
Arrays
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Objectives
• Establish an array of variables and refer to individual elements in
the array with variable subscripts.
• Use a for loop to step through an array.
• Use a loop to look up a matching value in an array.
• Accumulate totals using array elements.
• Distinguish between direct access and indirect access of a table.
• Combine the advantages of list objects with arrays.
• Store data in multidimensional arrays.
• Create and use an array of objects.
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Arrays
• An array is a series (or list) of values that all have the same data
type and all have the same name.
• An array is used when you need to keep a series of values for
later use such as reordering, calculating, or printing.
• An array can be a list of variables or a list of objects.
• Imagine trying to enter data for 500 products and each time the
data is entered it has to be saved somewhere to enter the data for
the next product.
• That somewhere is an array.
• To store multiple values use an array.
• An array is a series of individual variables, all referenced by the
same name.
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Arrays
• Arrays are sometimes called lists, tables, or subscripted variables.
• The position in the array is represented by a subscript.
• For example strName[0], strName[1], … the numbers are
subscripts.
• The subscripts (also known as index) are always in square
brackets.
• The individual variable is called an element in the array.
• The first element in the array always starts with a subscript of
zero.
• If you have 10 elements, the subscripts will go from 0 to 9.
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Subscripts
• Subscripts may be constants, variables, or numeric expressions.
• Although, subscripts have to integers, the non-integers are
rounded off.
• To know how many elements in the array, you must specify the
number of elements in the array at the time of initialization.
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Declaring an Array
• You can choose several techniques for a declaring an array.
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Using the new Keyword to declare
an Array—General Format
DataType VariableName[] = new DataType [size];
DataType[] VariableName = new DataType[size];
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Using the new Keyword to
Declare an Array—Examples
int intEmployeeCount[] = new int[50];
float[] fltTotal = new float[10];
String strName[] = new String[25];
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Declaring an Array
• The declaration statement allocates storage for a specified
number of elements and initializes each element to the default
value for the data type.
• For example, for an integer array all the elements will be
initialized to 0 and for a String array all the elements will be
initialized to empty string.
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Programming with Java
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Using an Initializer to Create an
Array—Examples
int intDepartmentNumber[] = {423, 633, 672, 981};
String strDepartmentNames[] = {"Accounting", "Information Systems", "Marketing",
"Sales"};
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More on Subscripts
• A subscript must reference a valid element in the array.
• If an array has 10 elements in the list and you try and reference
the 15th element in the list, array will throw an exception.
• Java rounds up fractional subscripts such as 21/2.
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Iterating Through an Array
• You can use a for loop to iterate through all the elements in an
array.
• The following loop initializes a loop index to 0 to access the
first element in the fltSales array.
• As intIndex is incremented each element is added to the total.
for(int intIndex = 0; intIndex < 10; intIndex ++)
{
fltTotal += fltSales[intIndex];
}
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Using Array Elements for
Accumulators
• Array elements are regular variables and perform in the same
ways as all variables used so far.
• You may use subscripted variables for counters or
accumulators.
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Catching User Input Errors
• If the user enters an invalid number you can check he range
with an if statement or catch the exception.
• If you choose to catch an exception rather than test with an if
statement, check for an ArrayIndexOutOfBoundsException.
• Using the group number, as an index to the array is a technique
called direct reference.
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Shows the Applet and the Variables Used
intGroupNum
(txtGroup –1)
3
intTotal[] array
[0]
0
[1]
0
[2]
0
[3]
10
[4]
0
[5]
0
[6]
0
[7]
0
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txtGroup
txtSale
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Table Lookup
• Considering the last example of the troops, whose numbers were
from 1 to 8.
• It is not always that the groups have numbers that are sequentially
numbered.
• The best way is to declare two arrays. One array that holds the
group numbers and the other array holds the total sales.
• The user will type the group number and the sale.
• The array will look up the group number and the subscript of the
group number will be used in the array for the total sales.
• The technique used to find the subscript is called table lookup.
• The subscript will serve to keep track of the total for each group.
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Each Element in the intGroupNumArray Corresponds
to One Element of the intTotal Array
intGroupNumArray[ ]
McGraw-Hill/Irwin
intTotal[ ] array
[0]
101
[0]
101
[1]
103
[1]
103
[2]
110
[2]
110
[3]
115
[3]
115
[4]
121
[4]
121
[5]
123
[5]
123
[6]
130
[6]
130
[7]
145
[7]
145
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A Lookup Operation
intGroupSub
2
intGroupNumArray[ ]
intTotal[ ] array
intGroupNum
110
txtGroup
txtSale
[0]
101
[0]
0
[1]
103
[1]
0
[2]
110
[2]
10
[3]
115
[3]
0
[4]
121
[4]
0
[5]
123
[5]
0
[6]
130
[6]
0
[7]
145
[7]
0
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Flowchart of
the Logic of a
Table Lookup
Operation
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Using Lists with Arrays
• Instead of storing information in an array for the group
numbers, store them in a List or Choice component.
• This is would be a more friendly and efficient solution.
• The user can select the group number from the list and the
index can be used for the array.
• The getSelectedIndex method will determine the array
subscript.
• You can print the group numbers, using the getItem method.
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Allow the User to Select a Group
Using the Choice List
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Multidimensional Arrays
• You may need two or more subscripts to identify tabular data,
where data is arranged in rows and columns.
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The Declaration Statement for TwoDimensional Arrays—General Format
DataType ArrayName[NumberRows][NumberColumns];
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The Declaration Statement for TwoDimensional Arrays—Examples
String strName[][] = new String[3][4];
float fltRate[][] = {{1.0f, 1.5f, 1.65f, 1.85f},
{1.58f, 2.0f, 2.4f, 3.05f},
{1.71f, 2.52f, 3.10f, 4.0f},
{2.04f, 3.12f, 4.0f, 5.01f},
{2.52f, 3.75f, 5.10f, 7.25f}};
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Multidimensional Arrays
• The strName example has three rows and four columns.
• The second example uses an initializer to set values, one row at a
time.
• You must always use two subscripts when referring to individual
elements of the table.
• Specify the row with first subscript and the column with second
subscript.
• The elements of the array may be used in the same ways as any
other variable – in accumulators, counts, reference field,
displaying text and conditions.
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Summing a Two-Dimensional Table
• You can find the sum of a table in various ways.
• You may sum either the columns or the rows of the table; or as
in a cross-foot, or you may sum the figures in both directions
and double-check the totals.
• To sum the array in both directions each column needs one total
field, and each row needs one total field.
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Two One-Dimensional Arrays Hold
Totals for the Two-Dimensional Array
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Lookup Operation for TwoDimensional Tables
• When you look up items in a two-dimensional table, you can
use the same techniques discussed with single dimensional
arrays: direct reference and table lookup. The limitations are
the same.
• Direct reference – row and column subscripts must be readily
available.
• A table lookup – most common technique; require additional
one-dimensional arrays or list to aid in the lookup process.
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This Shipping Rate Table in a Two-Dimensional Array
Can Be Used to Look Up the Correct Shipping Charges
Shipping Rate Table
fltRate array
intWeightSub
1
Weight (not
to Exceed)
intWeightSub
uses the index
of 1stWeight
intZoneSub
3
Zone
A
Zone
B
Zone
C
Zone
D
1 lb.
1.00
1.50
1.65
1.85
3 lb.
1.58
2.00
2.40
3.05
5 lb.
1.71
2.52
3.10
4.00
10 lb.
2.04
3.12
4.00
5.01
>10 lb.
2.52
3.75
5.10
7.25
intZoneSub
uses the index
of 1stZone
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Using TextFields
• If you are using text fields for data entry rather than lists, the
input requires more validation.
• You must validate both the entries before you can find the
correct value.
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Creating an Array of Objects
• You can create arrays of classes or create arrays of components.
• Using a class requires the keyword new to create an array of
objects.
For example: Course Courses[] = new Course[3];
Label lblGroupNumber[] = new Label[8];
• The objects in the array are not automatically instantiated.
• Use a loop to instantiate each object, once again using the keyword
new.
• Usually you will place the following code in the init.
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FindCourseApplet Looks Up the Course Number
in the Courses Array and Fills in the Room and
Unit Information
Courses array
strDesc
Courses[0].strRoom
strRoom
fltUnits
Courses[0] CIS 10A
26D-308
3.5
Courses[1] COMP 16
17-11A
3.5
Courses[2] CIS 28
17-13
4.0
Courses[0].fltUnits
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An Array of Course Objects
• Instead, you can refer to objectName-dot-variableName
(Courses[0].strRoom).
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An Array of Components
• You can also create arrays of components.
• This technique is handy when you need several components of
the same type, especially if you need to reference the
components with an index.
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Use Two Arrays of Labels to
Display the Group Numbers
and the Total for Each Group
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Java Arrays for C++ and Visual
Basic Programmers
• Arrays in Java must have defined limits differing from those in
C++.
• Java does not allow pointer arithmetic to access array elements.
• Both features enhance security in Java.
• In Visual Basic, you can declare array with subscripts
beginning with any number, such as (1 to 10) or (-10 to 10),
which Java does not allow.
• Visual Basic also has dynamic arrays, which can be resized
during program execution.
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Vectors
• In reality, Java does have resizable or dynamic arrays – they
are called vectors.
• A Vector is a class that is similar to an array.
• It holds multiple values and is referenced by an index.
• In addition, a vector automatically grows when more
elements are needed.
• The Vector class has many methods to aid creating the
elements, adding and removing elements, and searching for
particular elements.
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