Download Lecture 6: Java Software Solutions Foundations of Program Design

Lecture 6
Objects for Organizing Data
Instructors:
Fu-Chiung Cheng
(鄭福炯)
Associate Professor
Computer Science & Engineering
Tatung Institute of Technology
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Outline
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array declaration and use
arrays of objects
parameters and arrays
multidimensional arrays
the Vector class
• additional techniques for managing strings
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Arrays
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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
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scores
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2
3
4
5
6
7
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79 87 94 82 67 98 87 81 74 91
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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
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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
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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;
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Declaring Arrays
• Some examples of array declarations:
float[] prices = new float[500];
boolean[] flags;
flags = new boolean[20];
char[] codes = new char[1750];
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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
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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
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Array Declarations Revisited
• Array declarations: two ways
float[] prices; and
float prices[];
are essentially equivalent
• The first format is usually more readable
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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};
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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
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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
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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)
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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();
}
System.out.println();
roster.print();
} // method main
} // class RollCall
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;
}
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
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
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
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• See Array_Test.java
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
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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
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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
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
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?
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)
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");
System.out.println (band);
} // method main
} // class Beatles
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
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)
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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);
System.out.println (song);
authors.addElement ("George");
System.out.println (song);
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"));
System.out.println (song);
song.removeAllElements();
System.out.println (song);
} // method main
} // class ZZ_Top
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
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
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
("http://www.wpllabs.com/vision.html"); // main
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
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");
System.out.println (text1);
System.out.println (text2);
System.out.println (text3);
text3.reverse();
System.out.println (text3);
} // method main} // class Money
text2.insert (5, 'e');
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