2. 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.
3. 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
4. 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];
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.
6. 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
}
7. 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.
9. Student[] topStudents = new Student[2];
topStudents[0] = new Student(“Abcd”, 314159);
1000
1004
1008
100C
1010
1014
1018
101C
1020
1024
1028
102C
1030
1034
1038
103C
1040
4
A b
c d
1014
314159
0.0
false
2
1028
null
length
topStudents[0]
topStudents[1]
length
studentName
studentID
creditsEarned
paidUp
1000
topStudents FFB8
FFBC
FFC0
10. 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)
11. 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);
}
}
12. 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’
13. 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”,
}
14. 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
15. 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
18. 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.
19. 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.
20. 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;
}
21. 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.
22. 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).