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Lecture 6Objects for Organizing Data

Instructors: Fu-Chiung Cheng

(鄭福炯 )Associate Professor

Computer Science & EngineeringTatung Institute of Technology

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Outline

• 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

• 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 90 1 2 3 4 5 6 7 8 9

scores 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, no

t 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 w

ith arrays and objects• See RollCall.java (next 4 pages)

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

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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

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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

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

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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)

import java.util.Vector; // page 825class 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"); text2.insert (5, 'e');

System.out.println (text1); System.out.println (text2); System.out.println (text3); text3.reverse(); System.out.println (text3); } // method main} // class Money

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