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Chapter 11 Arrays
Introduction
• Array: An ordered collection of values with two
distinguishing characters:
– Ordered and fixed length
– Homogeneous. Every value in the array must be of the
same type
• The individual values in an array are called elements.
• The number of elements is called the length of the
array
• Each element is identified by its position number in
the array, which is called index. In Java, the index
numbers begin with 0.
Array declaration
An array is characterized by
• Element type
• Length
type[ ] identifier = new type[length];
Default values in initialization
• numerics
0
• boolean
false
• objects
null
An array of objects
Elements of an array can be objects of any Java class.
Example: An array of 5 instances of the student class
Student[] topStudents = new Student[5];
Defining length
• Use named constant to declare the length of an array.
private static final in N_JUDGES = 5;
double[ ] scores = new double[N_JUDGES];
• Or read the length of an array from the user.
Selecting elements
Identifying an element
array[index]
• Index can be an expression
• Cycling through array elements
for (int i = 0; i < array.length; i++) {
operations involving the ith element
}
Human-readable index values
•
•
From time to time, the fact that Java starts index
numbering at 0 can be confusing. Sometimes, it
makes sense to let the user work with index
numbers that begin with 1.
Two standard ways:
1. Use Java’s index number internally and then add one
whenever those numbers are presented to the user.
2. Use index values beginning at 1 and ignore the first (0)
element in each array. This strategy requires allocating an
additional element for each array but has the advantage
that the internal and external index numbers correspond.
Internal representation of arrays
Student[] topStudents = new Student[2];
topStudents[0] = new Student(“Abcd”, 314159);
1000
length
2
1004
1008
topStudents[0]
null
100C
topStudents[1]
null
1010
heap
topStudents
1000
stack
FFB8
FFBC
FFC0
1000
1004
length
2
1008
topStudents[0]
1028
100C
topStudents[1]
null
1010
Student[] topStudents = new Student[2];
topStudents[0] = new Student(“Abcd”, 314159);
1014
1018
length
4
101C
A
b
1020
c
d
1024
1028
102C
studentName
studentID
creditsEarned
paidUp
1014
1030
314159
1034
0.0
false
1038
topStudents
1000
FFB8
103C
FFBC
1040
FFC0
Passing arrays as parameters
• Recall: Passing objects (references) versus primitive
type (values) as parameters.
• Java defines all arrays as objects, implying that the
elements of an array are shared between the callee
and the caller.
swapElements(array[i], array[n – i – 1]) (wrong)
swapElements(array, i, n – i – 1)
private void swapElements(int[] array, int p1, int p2) {
int tmp = array[p1];
array[p1] = array[p2];
array[p2] = tmp;
}
• Every array in Java has a length field.
private void reverseArray(int[] array) {
for (int i = 0; i < array.length / 2; i++) {
swapElements(array, i, array.length – i – 1);
}
}
Using arrays
Example: Letter frequency table
• Design a data structure for the problem
Array: letterCounts[ ]
index: distance from ‘A’
index = Character.toUpperCase(ch) – ‘A’
letterCounts[0] is the count for ‘A’ or ‘a’
A convenient way of initializing an array:
int[ ] digits = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
private static final String[ ] US_CITIES_OVER_ONE_MILLION = {
“New York”,
“Los Angeles”,
“Chicago”,
“Huston”,
“Philadelphia”,
“Phoenix”,
“San Diego”,
“San Antonio”,
“Dallas”,
}
Arrays and graphics
• Arrays turn up frequently in graphical programming. Any time
that you have repeated collections of similar objects, an array
provides a convenient structure for storing them.
• As an aesthetically pleasing illustration of both the use of
arrays and the possibility of creating dynamic pictures using
nothing but straight lines the text presents YarnPattern
program, which simulates the following process:
–
–
–
–
Place a set of pegs at regular intervals around a rectangular border
Tie a piece of colored yarn around the peg in the upper left corner
Loop that yarn around that peg a certain distance DELTA ahead
Continuing moving forward DELTA pegs until you close the loop
Two-dimensional arrays
Each element of an array is an array (of the same
dimension)
int[][] A = new int[3][2]
An array of three arrays of dimension two
A[0][0] A[0][1]
A[1][0] A[1][1]
A[2][0] A[2][0]
3-by-2 matrix
Memory allocation (row orientation)
A[0][0]
A[0][1]
A[1][0]
A[1][1]
A[2][0]
A[2][1]
Initializing a two-dimensional array
Static int A[3][2] = {
{1, 4},
{2, 5},
{3, 6}
};
A 3-by-2 matrix
The ArrayList Class
• Although arrays are conceptually important as a data structure,
they are not used as much in Java as they are in most other
languages. The reason is that the java.util package
includes a class called ArrayList that provides the standard
array behavior along with other useful operations.
• ArrayList is a Java class rather than a special form in the
language. As a result, all operations on ArrayLists are
indicated using method calls. For example,
– You create a new ArrayList by calling the ArrayList constructor.
– You get the number of elements by calling the size method rather than
by selecting a length field.
– You use the get and set methods to select individual elements.
Methods in the ArrayList class
Figure 11-12, p. 443, where <T> is the base type.
boolean add(<T> element)
<T> remove(int index)
int indexOf(<T> value)
An ArrayList allows you to add new elements to the end of a list.
By contrast, you can’t change the size of an existing array
without allocating a new array and then copying all the
elements from the old array into the new one.
Linking objects
• Objects in Java can contain references to other objects. Such
objects are said to be linked. Linked structures are used quite
often in programming.
• An integer list:
public class IntegerList {
public IntegerList(int n, IntegerList link) {
value = n;
next = link;
}
/* Private instance variables */
private int value;
private IntegerList next;
}
Linked structures
• Java defines a special value called null to represent a
reference to a nonexistent value and can be assigned to any
variable that holds an object reference. Thus you can assign
null to the next field of the last element to signify the end
of the list.
• You can insert or remove an element from a list. The size of a
linked structure can change. Also, elements of a linked
structure can be objects.
• A simple example: SignalTower class, Figure 7-3, p. 242.
Arrays vs. linked lists
• The two attributes that define a data type are: domain and a set
of operations.
• An array is a collection of items of the same type. It is efficient
to select an element. The addresses of array[i] is the
address of array + sizeof(overhead) +
i*sizeof(type). For example, if the type is int, then
sizeof(int) is 4. Since the array size is fixed, it is hard to
insert or delete an element.
• The items on a list can have different types. Linked lists can
represent general structures such as tree. Items can be inserted
to or removed from a list. However, to select an element, you
have to follow the links starting from the first item on the list
(sequential access).