Transcript Arrays of Objects

Chapter 9 (part of)
Single-dimensional Arrays
Knowledge Goals
• Understand the difference between atomic
and composite data types
• Understand the difference between
unstructured and structured composite data
types
• Know how Java implements arrays
• Know how an array is passed as an
argument
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Skill Goals
• Declare and instantiate a one-dimensional
array
• Access and manipulate the individual
components in a one-dimensional array
where the elements are
• Atomic types
• Composite types
• Use an initializer list to instantiate a onedimensional array
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Java Data Types
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Java Data Types
Try expressing these definitions in words
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Java Data Types
Composite data type
A data type that allows a collection of values to be
associated with an identifier of that type
Unstructured data type
A collection of components that are not organized with
respect to one another
Structured data type
An organized collection of components; the organization
determines the means used to access individual
components
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Is a class structured?
Java Data Types
class Example
{
int field1;
int field2;
double field3;
}
class Example
{
double field3;
int field2;
int field1;
}
Changing the order does not change the access
Is a class structured?
Did you change your answer?
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One-Dimensional Arrays
Data structure
The implementation of a composite data type
Note the difference between a data structure
(implementation of any composite type) and a
structured data type (a composite type that is
structured)
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One-Dimensional Arrays
One-dimensional array
A structured collection of components, all of the same
type, that is given a single name; each component is
accessed by an index that indicates the component's
position within the collection
Class
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Array
 composite, unstructured
 composite, structured
 heterogeneous
 homogeneous
 access by field name
 access by position
One-Dimensional Arrays
Declare
Instantiate
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One-Dimensional Arrays
int[] numbers = new int[4];
What
type of
values
can be
stored in
each cell
?
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One-Dimensional Arrays
float realNumbers[] = new float[10];
How
do you
get
values
into the
cells
?
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One-Dimensional Arrays
Array Initializers
int numbers[] = {4.93, -15.2, 0.5, 1.67};
Initializers
do the
instantiation
and
storing in
with the
declaration
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One-Dimensional Arrays
Accessing Individual Components
Indexing expression
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One-Dimensional Arrays
Indexing
Expression
Place into which
a value is stored;
value is changed
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Indexing
Expression
Place from which
a value is extracted;
value is not changed
One-Dimensional Arrays
What
happens
if you
try to
access
value[1000]
?
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One-Dimensional Arrays
Out-of-bounds array index
An index that is either less than 0 or greater
than the array size minus 1, causing an
ArrayIndexoutOfBoundsException to be thrown
Length
A public instance variable associated with each
instantiated array, accessed by array name
.length
Use length to avoid out-of-bounds indexes
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One-Dimensional Arrays
Aggregate Array Operations
What does the following expression return?
numbers == values
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One-Dimensional Arrays
Now, what does the following expression return?
numbers == values
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One-Dimensional Arrays
System provides two useful array methods
first = second.clone();
// duplicates second
import java.util.Arrays;
Arrays.equals(first, second); // item-by-item check
System.out.println(first == second);
System.out.println(Arrays.equals(first, second);
What is printed?
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More Examples
What does this
code segment do?
totalOccupants = 0;
for (int aptNo = 0; aptNo < occupants.length; aptNo++)
totalOccupants = totalOccupants +occupants[aptNo];
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More Examples
What does
this code
fragment
do
?
if ((letter >= 'A' && letter <= 'Z' ||
letter >= 'a' && letter <= 'z'))
{
index = (int)Character.toUpperCase(letter) - (int)'A';
lettrCount[index] = letterCount[index] + 1;
}
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Arrays of Objects
String groceryItems[] = new String[10];
for (index = 0; index < grocerItems.length;
{
groceryItems[index] = inFile.nextLine();
}
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index++)
Arrays of Objects
Expression
groceryItems
array
groceryItems[0]
groceryItems[0].charAt()
groceryItems[10]
Class/Type
Reference to an
Reference to a string
A character
Error
Base address
The memory address of the first element of the array
"Reference to" is the base address
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Arrays of Objects
How do Array.equals and clone operate
with arrays of objects?
Array.equals uses the == operator to
compare arrays, so the addresses are
compared
clone uses the = operator, so the addresses
are copied
Array.equals(numbers, numbers.clone()) ?
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Arrays of Objects
Date bigEvents[] = new Date[10];
bigEvents
Expression
Class/Type
bigEvents
Array
bigEvents[0]
Date
bigEvents[0].month
String
bigEvents[0].day
int
bigEvents[0].year
int
bigEvents[0].month.charAt(0)
char
Date
[0]
[1]
.
.
.
.
Date
.
.
.
.
.
Date
[9]
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.
.
.
Arrays of Objects
length is the number of slots assigned to the
array
What if the array doesn’t have valid data in each
of these slots?
Keep a counter of how many slots have valid
data and use this counter when processing the
array
More about this type of
processing in Chapter 11
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Arrays of Objects
public static double average(int grades[])
// Calculates and returns the average grade in an
// array of grades.
// Assumption: All array slots have valid data.
{
int total = 0;
for (int i = 0; i < grades.length; i++)
total = total + grades[i];
return (double) total / (double) grades.length;
}
What is passed as an argument?
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