1. Lecture 6 Objects for Organizing Data
Instructors:
Fu-Chiung Cheng
(鄭福炯)
Associate Professor
Computer Science & Engineering
Tatung Institute of Technology 1 1

2. Outline array declaration and use arrays of objects
parameters and arrays
multidimensional arrays
the Vector class
additional techniques for managing strings 1 1

3. Arrays An array is an ordered list of values
Each value has a numeric index
An array of size N is indexed from zero to N-1
The following array of integers has a size of 10 and is indexed from 0 to 9
scores 2

4. Arrays Array reference: scores[4] the 5th value in the array
refers to the value 67
It can be assigned a value, printed, used in a calculation 3

5. Arrays An array stores multiple values of the same type
That type can be primitive types or objects
We can create an array of integers, or an array of characters, or an array of String objects, etc.
Array is an object
The name of the array is a object reference variable, and the array itself is instantiated separately 4

6. Declaring Arrays The scores array could be declared as follows:
int[] scores = new int[10];
Type: int[] (an array of integers)
Size: 10 (array of 10 integers)
Unknown size: a handle to an array object
int[] scores; 5

7. Declaring Arrays Some examples of array declarations:
float[] prices = new float[500];
boolean[] flags;
flags = new boolean[20];
char[] codes = new char[1750]; 6

8. Bounds Checking Once an array is created, it has a fixed size
An index used in an array reference must specify a valid element
That is, they must be in bounds (0 to N-1)
The Java interpreter will throw an exception if an array index is out of bounds
This is called automatic bounds checking
Its common to inadvertently introduce off-by-one errors when using arrays 7

9. Bounds Checking
Each array object has a public constant called length that stores the size of the array
It is referenced through the array name (just like any other object):
scores.length
Note that length holds the number of elements, not the largest index
See Reverse_Numbers.java and Adjust_Test_Scores.java 8

10. Array Declarations Revisited
Array declarations: two ways
float[] prices; and
float prices[];
are essentially equivalent
The first format is usually more readable 9

11. Initializer Lists
An initializer list can be used to instantiate and initialize an array in one step
The values are delimited by braces and separated by commas
Examples:
char[] letterGrades = {'A', 'B', 'C', 'D', 'F'};
int[] primes = {2, 3, 5, 7, 11, 13, 17, 19}; 10

12. Initializer Lists Note that when an initializer list is used:
the new operator is not used
no size value is specified
The size of the array is determined by the number of items in the initializer list
An initializer list can only be used in the declaration of an array 11

13. Arrays of Objects The elements of an array can be object references
The declaration
String[] words = new String[25];
reserves space to store 25 references to String objects
It does NOT create the String objects themselves
Each object stored in an array must be instantiated separately 12

14. Arrays of Objects Objects can have arrays as instance variables
Fairly complex structures can be created simply with arrays and objects
See RollCall.java (next 4 pages) 13

15. class RollCall {
public static void main (String[] args) {
Membership roster = new Membership();
Member person;
System.out.println();
person = roster.findMember(26911);
if (person == null)
System.out.println ("No match found");
else {
System.out.println (”Found the following member:");
person.print(); }
roster.print();
} // method main
} // class RollCall

16. class Membership {
private Member[] memberList;
public Membership() {
memberList = new Member[4];
memberList[0] = new Member ("Johnny Storm", 70469);
memberList[1] = new Member ("Sue Richards", 69048);
memberList[2] = new Member ("Reed Richards", 26911);
memberList[3] = new Member ("Ben Grimm", 89696);
} // constructor Membership
public Member findMember (int target) {
int index = 0;
while (index < memberList.length) {
if (memberList[index].getMembershipNumber() == target)
return memberList[index];
index++; }
return null;

17. public Member findMember (int target) {
int index = 0;
while (index < memberList.length) {
if (memberList[index].getMembershipNumber() == target)
return memberList[index];
index++; }
return null;
} // method findMember
public void print() {
System.out.println ("Member\t\tId #");
for (int person=0; person < memberList.length; person++)
memberList[person].print();
} // method print
} // class Membership

18. class Member {
private String name;
private int membershipNumber;
public Member (String memberName, int idNumber) {
name = memberName;
membershipNumber = idNumber;
} // constructor Member
public int getMembershipNumber() {
return membershipNumber;
} // method getMembershipNumber
public void print() {
System.out.println (name + "\t" + membershipNumber);
} // method print
} // class Member

19. Arrays as Parameters
An entire array can be passed to a method as a parameter
The reference to the array is passed, making the formal and actual parameters aliases of each other
Changing an array element in the method changes the original
An array element can be passed to a method as well, and follow the parameter passing rules of that element's type
See Array_Test.java 14

20. Multidimensional Arrays
A one-dimensional array stores a simple list of values
A two-dimensional array can be thought of as a table of values, with rows and columns
A two-dimensional array element is referenced using two index values
A two-dimensional array in Java is an array of arrays, therefore each row can have a different length 15

21. Multidimensional Arrays
An initializer list can be used to create and set up a multidimensional array
Note that each array dimension has its own length constant
See MultiArrayTest.java (Next page)
See Class MultiArray 16

22. class MultiArrayTest {
public static void main (String[] args) {
MultiArray chart = new MultiArray();
chart.print();
System.out.println();
for (int column=0; column < 4; column++)
System.out.println ("Sum of column "
+ column + ": "
+ chart.sumColumn (column));
} // method main
} // class MultiArrayTest

23. class MultiArray {
int[][] table = { {28, 84, 47, 72}, {69, 26}, {91, 40, 28},
{42, 34, 37}, {13, 26, 57, 35} };
public void print() {
for (int row=0; row < table.length; row++) {
for (int column=0; column < table[row].length; column++)
System.out.print (table[row][column] + " ");
System.out.println(); }
} // method print
public int sumColumn (int column) {
int sum = 0;
for (int row=0; row < table.length; row++)
if (column < table[row].length)
sum += table[row][column];
return sum;
} // method sumColumn
} // class MultiArray

24. What is wrong with this class?
class Soda_Scores {
private final int RESPONDENTS = 10;
private final int SODAS = 4;
private int[][] results = { {3, 4, 5, 2, 1, 4, 3, 2, 4, 4},
{2, 4, 3, 4, 3, 3, 2, 1, 2, 2},
{3, 5, 4, 5, 5, 3, 2, 5, 5, 5},
{1, 1, 1, 3, 1, 2, 1, 3, 2, 4} };
public int num_sodas() { return SODAS; }
public boolean worthy (int soda, int level) {
int count = 0;
for (int person=0; person < results[soda].length; person++)
if (results[soda][person] >= level)
count++;
return (count > RESPONDENTS/2);
} // method worthy
} // class Soda_Scores
What is wrong with this class?

25. The Vector Class
An object of class Vector is similar to an array in that it stores multiple values
However, a vector
only stores objects
does not have the indexing syntax that arrays have
Service methods are used to interact with a vector
The Vector class is part of the java.util package
See Beatles.java (next page)

26. import java.util.Vector;
class Beatles {
public static void main (String[] args) {
Vector band = new Vector();
band.addElement ("Paul");
band.addElement ("Pete");
band.addElement ("John");
band.addElement ("George");
System.out.println (band);
band.removeElement ("Pete");
band.addElement ("Ringo");
} // method main
} // class Beatles

27. The Vector Class
An important difference between an array and a vector is that a vector can change its size as needed.
Each vector initially has a certain amount of memory space reserved for storing elements
If an element is added that doesn't fit in the existing space, more room is automatically acquired

28. The Vector Class A vector is implemented using an array
Whenever new space is required, a new, larger array is created, and the values are copied from the original to the new array
To insert an element, existing elements are first copied, one by one, to another position in the array
Therefore, the implementation of Vector in the API is not very efficient
See ZZ_Top.java (Next) 19

29. import java.util.Vector; // page 825
class ZZ_Top {
public static void main (String[] args) {
Vector song = new Vector();
String name = new String ("ZZ Top's Greatest Hits");
Integer track = new Integer (6);
String title = "Cheap Sunglasses";
Double price = new Double (15.95);
Vector authors = new Vector (2);
authors.addElement ("Gibbons");
authors.addElement ("Hill");
song.addElement (track);
song.addElement (title);
song.addElement (authors);
song.addElement (price);
song.insertElementAt (name, 0);

30. System.out.println (song);
authors.addElement ("George");
System.out.println ("song size: " + song.size());
System.out.println ("authors size: " + authors.size());
System.out.println ("song begins: " + song.firstElement());
System.out.println ("authors begins: "
+ authors.firstElement());
System.out.println ("song ends: " + song.lastElement());
System.out.println ("authors ends: "
+ authors.lastElement());
authors.setElementAt ("Frank",
authors.indexOf ("George"));
song.removeAllElements();
} // method main
} // class ZZ_Top

31. The StringTokenizer Class Revisited
We've seen a StringTokenizer object separate a string into separate tokens
By default, those tokens are delimited by white space
By using other StringTokenizer constructors, we can define the delimiters used to define a token
We can also set whether we want the delimiters themselves returned as tokens
See Voltaire.java and URL_Tokens.java

32. import java.util.StringTokenizer; // page 805
public class Voltaire {
public static void main (String[] args) {
String quote = "Use, do not abuse; neither abstinence " +
"nor excess renders a man happy.";
StringTokenizer words = new StringTokenizer (quote);
System.out.println ("Characters: " + quote.length());
System.out.println ("Tokens: " + words.countTokens());
while (words.hasMoreTokens()) {
System.out.println (words.nextToken()); }
} // method main
} // class Voltaire

33. class URL_Tokenizer {
private String protocol;
private String address;
private String resource;
public URL_Tokenizer (String URL_Text) {
StringTokenizer URL = new StringTokenizer (URL_Text, ":");
protocol = URL.nextToken();
address = URL.nextToken (":/");
resource = URL.nextToken ("");
} // constructor URL_Tokenizer
public String get_protocol() { return protocol; }
public String get_address() { return address; }
public String get_resource() { return resource; }
} // class URL_Tokenizer …
URL_Tokenizer url = new URL_Tokenizer
(" // main

34. The StringBuffer Class
Recall that the value of a String object is immutable; once set it cannot be changed
The StringBuffer class can be used to define a character string whose value can change
It's service methods include the ability to append and insert characters
See Money.java
However, most functionality defined by the StringBuffer class can be accomplished with String objects and string concatenation

35. public class Money {
public static void main (String[] args) {
StringBuffer text1 = new StringBuffer(); // page 803
StringBuffer text2 = new StringBuffer(" m");
StringBuffer text3 = new StringBuffer ("1 dollar");
text1.append (1); text1.append (" p");
text1.append ('e'); text1.append ('n');
text1.append ("ny");
text2.insert (0, 1); text2.insert (2, "di");
text2.insert (5, 'e');
System.out.println (text1);
System.out.println (text2);
System.out.println (text3);
text3.reverse();
} // method main} // class Money

36. Conclusion Java Array is an object. Array size of N: index 0 to N-1
Bounds checking
Mutlidimensional array ==> array of arrays
each element could have a different length.
Vector (similar to Array) can dynamically change size
StringTokenizer class (good for compiler and other app.
StringBuffer class