Chapter 4
Grouping Objects

Modified:

Overview

Chapter 3 examined fundamental problem solving approaches of modularization and abstraction. Here we will examine some of the ways objects may be grouped together into collections.

4.1 Grouping objects in flexible-size collections

Many objects in life come in groups:

drawer of socks

list of people to invite to a wedding

days in a month

dozen eggs

Some groups are fixed in size while others are flexible size. We need programming features for modeling both cases.

4.2 A personal notebook

Stores notes
No limit on number of notes
Display individual notes
Display number of notes stored

Exercise 1 - NoteBook class
In BlueJ

Open project chapter04\notebook1.

Click View | Show Terminal

In Terminal window click Options | Record method calls

Create a NoteBook object.

Store two notes:

"Buy bread"

"Sell car"

Check the number of notes stored.

Show each note. The first note is numbered 0, the second 1, etc.

4.3 A first look at library classes

Collections - An object that can store a arbitrary number of other objects.

Examples:

drawer object can hold arbitrary number of other objects: socks, rocks, spoons, forks, photographs.

note book object can hold arbitrary number of other objects: shopping list, reminders, class notes, etc. Important that each page is arranged in numerical order, page 0, page 1, ...

ArrayList object can hold arbitrary number of other objects: Strings, ClockDisplay, etc.

Class libraries - A class that is already defined and can be used in your program. Similar classes are normally grouped together as a library.

Package - What Java calls a library of classes.

Example packages:

java.util - Utility classes.

java.net - Networking classes.

java.Math - Math classes.

4.3.1 An example using a library

import java.util.ArrayList;

public class Notebook
{

private ArrayList<String> notes;

   public NoteBook()
{
       notes = new ArrayList<String>();
}

   public void storeNote(String note)
{
       notes.add( note );
}

   public int numberOfNotes()
{
      return notes.size();
}

   public void showNote(int noteNumber)
{
      if(noteNumber < 0) {                                    // Not valid note number, so do nothing.
      }
      else if( noteNumber < numberOfNotes()) {      // Valid note number, so print it.

            System.out.println( notes.get( noteNumber ));
      }
      else {                                                        // Not valid note number, so do nothing.
      }

}

Using the ArrayList class:

import java.util.ArrayList; - Gain access to the library.

private ArrayList<String> notes; - Define field notes as type ArrayList class.

notes = new ArrayList<String>() - Create an instance of ArrayList.

ArrayList methods:

The ArrayList class supplies many useful methods, we will use four for now:

add - adds an object to the ArrayList. Example:

notes.add("Take vacation");

size - returns the number of objects in the ArrayList. Example (assuming 3 objects):

notes.size(); returns 3.

get - returns the ith object in ArrayList. Example:

notes.get( 2 ); returns the 2nd object from ArrayList notes.

remove - removes the ith object in the ArrayList. Example (assuming 3 objects):

notes.remove(1);

4.4 Object structures with collections

ArrayList object notes after the following statements are executed:

private ArrayList<String> notes;

notes = new ArrayList();

notes.add("Buy bread");

notes.add("Sell car");

Exercise 2 - NoteBook methods
In BlueJ

Inspect the NoteBook object.

Click on the notes field and Inspect button.

What is the value of field size?

What do you think is the value of accessor: notes.size()

Click on elementData and Inspect button.

What is the number of:

"Sell car"

"Buy bread"

Call method:

showNote to show note number 1.

numberOfNotes.

Store another note "11:30 mtg"

What do think is the number of this note?

What do you think is the value of: notes.size()?

In what order does ArrayList objects store objects as added?

Update the object diagram above by hand to include "11:30 mtg" note.

4.5 Generic classes

An ArrayList can have elements of any type of object. To define that the notes field must be <String> objects only.

ArrayList<String> notes;

To define a group of TicketMachines:

ArrayList<TicketMachine> tms;

Exercise 2.5
In your head

Declare a field named library of type ArrayList having elements of type Book.

4.6 Numbering within collections

index - Position of an object in a collection.

Examples:

Index 0 of notes ArrayList object is "Buy bread".

Index 1 of notes ArrayList object is "Sell car".

Exercise 3 - Indexes

For the object diagram:

What would be the index of the next note added?

What is:

notes.size();

notes.get(1);

notes.get(0);

notes.get(5);

System.out.println( notes.get( 1 ) );

4.7 Removing an item from a collection

remove - removes the ith object in the ArrayList. Example (assuming 3 objects):

Exercise 4 - remove

In your head

Using the object diagram, what is the index of "11:30 mtg" after:

notes.remove(1);

What is the value now of:

notes.size()

In BlueJ

Add the mutator method deleteNote that removes the ith index note from the notebook. Assume the ith note exists.

What is the method signature?

Does it need an information parameter?

Does it return anything?

Which of the signatures is most appropriate?

int deleteNote()

void deleteNote()

void deleteNote(int ith)

int deleteNote(int ith)

Create a NoteBook object with 3 notes.

Inspect the object.

Inspect notes.

Inspect elementData.

Use deleteNote to remove note at index 1.

Use deleteNote to remove note at index 4.

4.8 Processing a whole collection

listNotes - We need a NoteBook method that prints all notes along with their index numbers to the terminal window.

Exercise 5 - listAllNotes

Which of the signatures would be most appropriate for a method to print all notes:

public ArrayList<String> listAllNotes()

public void listAllNotes(int index)

public int listAllNotes()

public void listAllNotes()

In BlueJ

Add the listAllNotes method using the appropriate signature and the following as the method body. Index 0 holds the first note, index 1 the second, etc.

System.out.println( notes.get(0) );
System.out.println( notes.get(1) );
System.out.println( notes.get(2) );
:

Test the listAllNotes method.

What is wrong with the above as the body for listAllNotes method?

4.8.1 The for-each loop

for-each loop - Executes a block of statements repeatedly instead of programmer writing repeatedly. Loops scale very well.

Examples that produce the same result on each row:

Write repeatedly Execute repeatedly

int i = 0;
System.out.println( notes.get( i ) );
i = i + 1;
System.out.println( notes.get( i ) );
i = i + 1;
System.out.println( notes.get( i ) ); for ( String note : notes )
System.out.println( note );

Exercise 5.5
In your head

What is the result of the above for the object at right?

In BlueJ

Change listAllNotes to use a for-each loop.

4.8.2 The while loop

while loop - Executes a block of statements repeatedly instead of programmer writing repeatedly. Loops scale very well.

Examples that produce the same result on each row:

Write repeatedly Execute repeatedly

int i = 0;              // Add 1 to i 4 times
i = i + 1;
i = i + 1;
i = i + 1;
i = i + 1; int i = 0;               // Add 1 to i 4 times
while ( i < 4 ) {
     i = i + 1;
}

int i = 4;              // Subtract 1 from i 4 times
i = i - 1;
i = i - 1;
i = i - 1;
i = i - 1; int i = 4;               // Subtract 1 from i 4 times
while ( i >= 1 ) {
     i = i - 1;
}

int i = 0;
i = i + 1;
System.out.println( i );
i = i + 1;
System.out.println( i );
i = i + 1;
System.out.println( i );
i = i + 1;
System.out.println( i ); int i = 0;
while ( i < 4 ) {
     i = i + 1;
     System.out.println( i );
}

int i = 0;
System.out.println( i );
i = i + 1;
System.out.println( i );
i = i + 1;
System.out.println( i );
i = i + 1;
System.out.println( i );
i = i + 1; int i = 0;
while ( i < 4 ) {
     System.out.println( i );
     i = i + 1;
}

System.out.println( notes.get(0) );
System.out.println( notes.get(1) );
System.out.println( notes.get(2) );
System.out.println( notes.get(3) );
int i = 0;
while ( i < 4 ) {
     System.out.println( notes.get( i ) );
     i = i + 1;
}

if (notes.size() >= 0)
    System.out.println( 0 + ":" + notes.get(0) );
if (notes.size() >= 1)
    System.out.println( 1 + ":" + notes.get(1) );
if (notes.size() >= 2)
    System.out.println( 2 + ":" + notes.get(2) );
if (notes.size() >= 3)
    System.out.println( 3 + ":" + notes.get(3) ); int i = 0;
while ( i < notes.size() ) {
    System.out.println( i + ":" + notes.get( i ) );
    i = i + 1;
}

Exercise 6 - listAllNotes

Which of the above would be most appropriate for the body of your listAllNotes method?

In BlueJ

Copy and paste the appropriate body for listAllNotes method into NoteBook class.

Create an instance with 2 notes and call the listAllNotes method.

Modify listAllNotes method to print the notes in reverse order.

Create an instance with 2 notes and call the listAllNotes method.

Modify deleteNote to print out an error message if the note number parameter is not valid.

Infinite loops

Looping is a powerful tool but has the danger of looping infinitely (i.e. the loop never stops). A simple example of an infinite loop that prints "Forever..." forever till stopped externally is:

while( true )
System.out.println( "Forever...");

Stopping - In BlueJ an infinite loop can often be stopped by:
- Click which runs the Debugger.
- Click Halt button.
Task Manager - When BlueJ cannot stop a run away program, Windows Task Manager can.
- Press Ctrl-Alt-Del keys simultaneously, to run the Task Manager.
- Select the Processes tab and locate the java.exe or javaw.exe process.
- Select the java.exe process and click End Process.
- This will end BlueJ.

Exercise 6.1 - Infinite loops
In BlueJ

Copy and paste the following method into the NoteBook class.
public void infinite()
{
     int i=0;
     while( true ) {
           System.out.println( i + " Forever...");
           i=i+1;
     }
}

Test by creating a NoteBook object and calling the method.

Stop by:

Pressing Ctrl-Shift-R keys simultaneously.

or

Right click . It may take a little time to respond.

Click Reset Virtual Machine.

or

Ctrl-Shift-Delete keys simultaneously and use the Task Manager.

End the java.exe process.

BlueJ can become unstable when reset. Suggest closing and restarting BlueJ.

4.8.3 Iterating over a collection

iterator - An object that provides the ability to iterate over every element in a collection.

Examples that all produce the same results (assuming notes.size() == 3):

System.out.println( notes.get(0) );
System.out.println( notes.get(1) );
System.out.println( notes.get(2) ); Iterator<String> i = notes.iterator();

System.out.println( i.next( ) );
System.out.println( i.next( ) );
System.out.println( i.next( ) );

int i = 0;

while ( i < notes.size() ) {
    System.out.println( notes.get( i ) );
    i = i + 1;
} Iterator<String> i = notes.iterator();

while ( i.hasNext() ) {
    System.out.println( i.next() );
}

Iterator<String> i - Defines i of type Iterator.
notes.iterator() - Returns an Iterator object for the notes ArrayList collections.
i.hasNext() - Returns true if there are more objects in the collection.
i.next() - Returns the current object in the collection and iterates to the next object.

Exercise 7 - Iterators

From the diagram at right, what is printed by:

Iterator<String> theIter = notes.iterator();

System.out.println( theIter.next( ) );

From the diagram at right, what is printed by:

Iterator<String> it = notes.iterator();

System.out.println( it.hasNext( ) );

What is printed by:

Iterator<String> it = notes.iterator();

System.out.println( it.next( ) );
System.out.println( it.next( ) );
System.out.println( it.next( ) );

System.out.println( it.hasNext( ) );

4.8.4 Index access versus iterators

index - notes.get( 3 ); The index allows any object in the collection to be accessed in any order.

iterator - notes.next( ); The iterator access is strictly sequential order.

4.9 Grouping objects complete example - a party

The following classes define a Person with an age and name, and a Party class that groups Person objects. The class definitions follow:

public class Person
{
   private int age;
   private String name;

   public Person(int age, String newName) {
      this.age=age;
      name=newName;
   }

   public int getAge() {
      return age;
   }

   public void printPerson() {
      System.out.print("Name: " + name);
      Systen.out.println(" Age: " + age );
   }
} import java.util.ArrayList;
import java.util.Iterator;

public class Party
{
   private ArrayList <Person> guests;

   public Party() {
      guests = new ArrayList <Person> ();
   }

   public void arrive( Person partier ) {
      guests.add( partier );
   }

   public void indexPrintParty() {
     int i = 0;
     while (i < guests.size())
     {
       guests.get(i).printPerson();
       i = i + 1;
     }
   }

   public void iteratorPrintParty() {
      Iterator <Person> git = guests.iterator();
      while ( git.hasNext() )
         git.next().printPerson();
   }
}

Exercise 7.1 - Example
In your head

Give the object diagram for the following:

Person p0 = new Person(23, "George");
Person p1 = new Person(29, "Sally");
ArrayList <Person> guests = new ArrayList <Person> ();

guests.add( p0 );
guests.add( p1 );

In BlueJ

Create a new project named Exercise 7.1

Create a new class named Person

Copy and paste the Person class above.

Create a new class named Party

Copy and paste the Party class above.

Duplicate in BlueJ the results of the following:

Person p0 = new Person(23, "George");
Person p1 = new Person(29, "Sally");

Party wedding = new Party();
wedding.arrive( p0 );
wedding.arrive( p1 );

Give the object diagram for Question 4.

4.9.1 Anonymous objects

The following two examples produce identical results:

Person p0 = new Person(23, "George");
Person p1 = new Person(29, "Sally");
guests = new ArrayList <Person> ();

guests.add( p0 );
guests.add( p1 );
guests = new ArrayList <Person> ();

guests.add( new Person(23, "George") );
guests.add( new Person(29, "Sally") );

On the right, anonymous objects are created eliminating the need to define variables p0 and p1.

4.12 Fixed size collections

array - A collection that can store a fixed number of elements (objects or primitive-type). Uses a special syntax to access elements in the collection.
Operations - Only two operations are defined on fixed size array variables.
- Assignment - x[ index ] = y;
- Copy - y = x[ index ];

**Examples that produce the same result**
int x0; int x1; int x2; x0 = -3; x1 = -5; x2 = -4; int sum = x0 + x1 + x2;	int [ ] x = new int [ 3 ]; x[ 0 ] = -3; x[ 1 ] = -5; x[ 2 ] = -4; int sum = x[ 0 ] + x[ 1 ] + x[ 2 ];
String notes0 ; String notes1 ; String notes2 ; notes0 = "Buy bread"; notes1 = "Sell car"; notes2 = "11:30 mtg";	String notes[ ] = new String[3]; notes[ 0 ] = "Buy bread"; notes[ 1 ] = "Sell car"; notes[ 2 ] = "11:30 mtg";
ArrayList notes = new ArrayList( ); notes.add( "Buy bread" ); notes.add( "Sell car" ) ; notes.add( "11:30 mtg" ) ;	String notes[ ] = new String[3]; notes[ 0 ] = "Buy bread"; notes[ 1 ] = "Sell car"; notes[ 2 ] = "11:30 mtg";
System.out.println( notes.get(0) ); System.out.println( notes.get(1) ); System.out.println( notes.get(2) );	System.out.println( notes[ 0 ] ); System.out.println( notes[ 1 ] ); System.out.println( notes[ 2 ] );
int i = 0; while ( i < notes.size() ) { System.out.println( notes.get( i ) ); i = i + 1; }	int i = 0; while ( i < notes.length ) { System.out.println( notes[ i ] ); i = i + 1; }

Exercise 8 - Fixed size collections

String notes[ ] = new String[3];

notes[ 0 ] = "Buy bread";
notes[ 1 ] = "Sell car";
notes[ 2 ] = "11:30 mtg";

From the diagram at right, what is the result:

String s = notes[ 2 ];

notes[ 1 ] = notes[ 0 ];

notes[ 2 ] = "Take a break";

notes[ 3 ] = "Broke";

notes[ -1 ] = "Broke";

From the diagram at right, what is printed by:
int i = 2;

while ( i >= 0 ) {
System.out.println( i + notes[ i ] );
i = i - 1;
}

Define the object pages, an array 4 ints, see the figure at right.

Initialize index 2 of pages to 15.

Give the statements to print the figure at right.

4.12.a Declaring array variables

Examples:

Declare an array variable hours that can refer to an array of ints:

int[ ] hours;

Declare an array variable months that can refer to an array of Strings:

String[ ] months;

4.12.b Creating array objects

Examples:

Create an array of 24 ints and assign to hours:
hours = new int[24];

Create an array of 12 Strings and assign to months:
months = new String[12];

Exercise 9 - Arrays

Write a declaration for an array variable people that can refer to an array of Person objects.

Assign variable people an array of 30 Person objects.

Write a declaration for an array variable vacant that can refer to an array of booleans.

Assign variable vacant an array of 13 booleans.

Write a declaration for an array variable dim that can refer to an array of ints.

Assign dim an array of 12 ints.

4.12.c Using array objects

Indexing - Accessing individual variables of an array using the [ ].

Examples:

Assign hours index 20 value 54:
hours[20] = 54;

Assign months index 2 value "March":
months[2] = "March";

Add hours index 23 value to variable count:
count = count + hours[23];

Print months index 2 value:
System.out.println(months[2]);

Exercise 10 - Using Arrays

Initialize each variable (i.e. index) of dim with the number of days in January, February, etc. for this year.

Sum index 5, 6 and 7 of dim to variable summer.

Print index 5, 6 and 7 of dim.

4.12.d The for-loop

for-loop - Iterates from a starting value to an ending value in designated steps.

Examples that produce the same result:

ArrayList <String> notes = new ArrayList <String> (); notes.add( "Buy bread" ); notes.add( "Sell car" ) ; notes.add( "11:30 mtg" ) ;	String notes[ ] = new String[3]; notes[ 0 ] = "Buy bread"; notes[ 1 ] = "Sell car"; notes[ 2 ] = "11:30 mtg";
int i = 0; while ( i < notes.size() ) { System.out.println( notes.get( i ) ); i = i + 1; }	int i = 0; while ( i < notes.length ) { System.out.println( notes[ i ] ); i = i + 1; }
int i; for ( i=0; i < 3 ; i++) { System.out.println( notes.get( i ) ); }	int i; for ( i=0; i < 3; i++ ) { System.out.println( notes[ i ] ); }
int i; for ( i=0; i < notes.size() ; i++) { System.out.println( notes.get( i ) ); }	int i; for ( i=0; i < notes.length; i++ ) { System.out.println( notes[ i ] ); }

Examples:

Print all the integers from 3 to 12

int i;

for ( i = 3; i <=12; i++)

    System.out.println( i );

Total all the hourCounts variables to sumHours:

int sumHours = 0;

for ( int i = 0; i < 24; i++)

    sumHours = sumHours + hourCounts[ i ];

Print all variables in array hourCounts:

for ( int i = 0; i < 24; i++)

     System.out.println( hourCounts[ i ]);



Find largest value in array dim:

int largest = dim[0];

for ( int i = 0; i < 12; i++)

     if ( dim[ i ] > largest )

           largest = dim[ i ];

Find smallest value in array dim:

int smallest = dim[0];

for ( int i = 0; i < 12; i++)

     if ( dim[ i ] < smallest )

           smallest = dim[ i ];

Exercise 11 - for loop with Arrays

Print all the integers from 12 to 3.

Print all variables in array hourCounts in reverse order.

Print all the variables in array dim.

Print all the variables in array dim in reverse order.

Total all the variables in array dim.

Find smallest value in array hourcounts.

4.12.1 A log-file analyzer

Web-servers log client visits to web pages.

Analyzing the log file can determine:

popular pages

broken links (missing pages)

server usage

busy periods

We have a web log file in the format of:

year     month day   hour minute

2009         5     01     00     19
2009         5     01     01     27
2009         5     01     02     17

public class LogAnalyzer
{

private int[ ] hourCounts;
private LogfileReader reader;

   public LogAnalyzer()
{
       hourCounts = new int[24];
       reader = new LogfileReader();
   }

   public void analyzeHourlyData()
{
       while(reader.hasMoreEntries() ) {
              LogEntry entry = reader.nextEntry();
              int hour = entry.getHour();
              hourCounts[hour]= hourCounts[hour] + 1;
       }
}

public void printHourlyCounts()
{
       System.out.println("Hr: Count");
       for(int hour = 0; hour < hourCounts.length; hour=hour+1)
       {
              System.out.println(hour + ": " + hourCounts[hour]);
       }
}

   public void printData()
{
       reader.printData();
}

}

Web log file format:
year     month day   hour minute

2009         3     01     00     19
2009         3     01     01     17
2009         3     01     01     22
2009         3     01     01     25
2009         3     01     01     42
2009         3     01     01     57
2009         3     01     02     17

LogAnalyzer - Constructor

hourCounts - Assigned object of fixed array of 24 ints.

reader - Assigned LogfileReader object.

analyzeHourlyData - Fills hoursCount array with total of the number of visits
within an hour period.

For example, adds 1 to array index 5 of hourCounts for a log entry
at 5 o'clock. Consider when hour = 5:

     int hour = entry.getHour();
     hourCounts[hour]=hourCounts[hour]+1;

printHourlyCounts - Prints each of the 24 variables in the hourCounts array.

The hoursCount array prints as:

     0:0
     1:5
     2:0
     3:0
     4:2
     :
     :

4.12.2 Declaring array variables

4.12.3 Creating array objects

The following creates the array at right:

private int[ ] hourCounts;

   public LogAnalyzer()
{
       hourCounts = new int[24];
    }

4.12.4 Using array objects

4.12.5 Analyzing the log file

printHourlyCounts - Prints each of the 24 variables in the hourCounts array.

The hoursCount array at right would print:

     0:0
     1:5
     2:0
     3:0
     4:2
       :

public void printHourlyCounts()
{
       System.out.println("Hour: Count");

       for( int hour = 0; hour < hourCounts.length; hour++)
       {
              System.out.println(hour + ": " + hourCounts[ hour ]);
       }
}

4.12.5.1 Arrays of objects

Fixed size arrays can hold:

primitive types (int, double, boolean) as seen above

references to objects as seen below

Array define - The following defines tms as a TicketMachine array object at right, assuming the TicketMachine definition of Chapter 2):

TicketMachine [] tms;

Array create and assign - The following creates a fixed size array of 3 TicketMachines and assigns the object reference to tms:

tms = new TicketMachine [ 3 ];

Object create and assign - The following creates 3 TicketMachines and assigns each to one of the indexed variables of the array:

tms [ 0 ] = new TicketMachine ( 40 );
tms [ 1 ] = new TicketMachine ( 50 );
tms [ 2 ] = new TicketMachine ( 60 );

Array use - The following calls the insertMoney(), emptyMachine() and printTicket() methods using TicketMachine numbered 2.

tms [ 2 ].insertMoney ( 70 );
tms [ 2 ].printTicket ( );
System.out.println( tms [ 2 ].emptyMachine( ) );

Exercise 12 - Array of objects
In your head

What ticket is printed in the previous example?

What is printed by:

System.out.println( tms [ 2 ].emptyMachine( ) );

4.12.6 The for loop

Exercise 13 - for loop
In your head

Complete the numberOfAccesses method below to count the total number of accesses recorded in the log file. Complete using a for loop iterating over the hourCounts array.

public int numberOfAccesses()
{
    int total = 0;

// total each variable of hourCounts

    return total;
}

In BlueJ

Add numberOfAccesses method to LogAnalyzer class and test.

Exercise 14 - for loop
In your head

Three TicketMachines below are:

defined

created

money inserted

emptied and totaled

TicketMachine t0, t1, t2;

t0 = new TicketMachine( 40 );
t1 = new TicketMachine( 50 );
t2 = new TicketMachine( 60 );

t0.insertMoney( 100 );
t1.insertMoney( 75 );
t2.insertMoney( 85 );

int sum = 0;

sum = sum + t0.emptyMachine();
sum = sum + t1.emptyMachine();
sum = sum + t2.emptyMachine();

Complete the following to obtain exactly the same results as above but using a fixed size array and for-loop.

TicketMachine [ ] tms;
tms = new TicketMachine[3];

tms [ 0 ] = new TicketMachine( 40 );
tms [ 1 ] = new TicketMachine( 50 );
tms [ 2 ] = new TicketMachine( 60 );

tms [ 0 ].insertMoney( 100 );
tms [ 1 ].insertMoney( 75 );
tms [ 2 ].insertMoney( 85 );

int sum = 0;
        :
        :

Exercise 15
In BlueJ

Open project chapter02/better-ticket-machine.

Create the new class Subway.

Copy the following for Subway class.

public class Subway
{
     private TicketMachine [ ] tms;

     public Subway()
    {

    }

    public int totalPrice()
   {

   }
}

In the Subway constructor, assign tms an array of 100 TicketMachine's.

In the Subway constructor, use a for-loop to assign each variable of tms to a TicketMachine with price 50 cents:

tms[ i ] = new TicketMachine(50)

Complete totalPrice() method to return the total price of all 100 machines.

Test the totalPrice() method.

Add a method averagePrice() to compute the average price of a ticket.

4.13 Summary

Exercise 16
In your head

Differences between ArrayList and fixed size arrays of Strings

Define:

An ArrayList variable of type String named: people.

A fixed size array variable of type String named: dow.

Create (construct):

people an ArrayList object of type String.

dow a String array of 7 variables.

Assign:

people the String objects "Tom", "Dick" and "Harry".

dow the String objects "Sunday", "Monday", etc.

Access:

Print all objects of people.

Print all objects of dow.

Access:

Change object at index 1 of people to be "Sally".
      set(int index, Object element)
          Replaces the element at the specified position in this list with the specified element.

Change object at index 1 of dow to be "Friday".

Exercise 17
In your head

Differences between ArrayList and fixed size arrays of TicketMachines

Define:

An ArrayList variable of type TicketMachine named: bus.

A fixed size array variable of type TicketMachine named: plane.

Create (construct):

bus an ArrayList of type TicketMachine object.

plane a TicketMachine array of 3 variables.

Assign:

bus the TicketMachine objects with price 50, 150, 200.

plane the TicketMachine objects with price 50, 150, 200.

Access. Use void insertMoney(int amount):

Insert 25 cents into all objects of bus.

Insert 25 cents into all objects of plane.

Access. Use int getPrice():

Find the highest price of all objects of bus.

Find the highest price of all objects of plane.

Access. Use int emptyMachine():

Empty and total all objects of bus.

Empty and total all all objects of plane.

Exercise 18
In your head

Differences between ArrayList and fixed size arrays of Sound

Define:

An ArrayList variable of Songs named: songs.

A fixed size array variable of type Sound named: days.

Create (construct):

songs an ArrayList of Songs object.

days a Sound array of 3 variables.

Assign:

songs the Sound objects:
new Sound("2U", "ICanSing.wav", 3);
new Sound("Elvis", "ICanMakeUSing.wav", 4);
new Sound("Rain", "Sleep.wav", 2);
.

days the Sound objects:
new Sound("Sunday", "Sunday.wav", 3);
new Sound("Monday", "Monday.wav", 4);
new Sound("Tuesday", "Tuesday.wav", 2);

Access, using the Sound void playSound() method:

Play all objects of songs.

Play all objects of days.

Access, using the Sound int getDuration() accessor method:

Compute the total duration of songs.

Compute the total duration of days.

Access, using the Sound int getDuration() accessor method:

Find the longest duration of all objects of songs.

Find the longest duration of all objects of days.

Access, using the Sound int getDuration() accessor method:

Find the index of the longest duration of all objects of songs.

Find the index of the longest duration of all objects of days.

Write repeatedly	Execute repeatedly
int i = 0; // Add 1 to i 4 times i = i + 1; i = i + 1; i = i + 1; i = i + 1;	int i = 0; // Add 1 to i 4 times while ( i < 4 ) { i = i + 1; }
int i = 4; // Subtract 1 from i 4 times i = i - 1; i = i - 1; i = i - 1; i = i - 1;	int i = 4; // Subtract 1 from i 4 times while ( i >= 1 ) { i = i - 1; }
int i = 0; i = i + 1; System.out.println( i ); i = i + 1; System.out.println( i ); i = i + 1; System.out.println( i ); i = i + 1; System.out.println( i );	int i = 0; while ( i < 4 ) { i = i + 1; System.out.println( i ); }
int i = 0; System.out.println( i ); i = i + 1; System.out.println( i ); i = i + 1; System.out.println( i ); i = i + 1; System.out.println( i ); i = i + 1;	int i = 0; while ( i < 4 ) { System.out.println( i ); i = i + 1; }
System.out.println( notes.get(0) ); System.out.println( notes.get(1) ); System.out.println( notes.get(2) ); System.out.println( notes.get(3) );	int i = 0; while ( i < 4 ) { System.out.println( notes.get( i ) ); i = i + 1; }
if (notes.size() >= 0) System.out.println( 0 + ":" + notes.get(0) ); if (notes.size() >= 1) System.out.println( 1 + ":" + notes.get(1) ); if (notes.size() >= 2) System.out.println( 2 + ":" + notes.get(2) ); if (notes.size() >= 3) System.out.println( 3 + ":" + notes.get(3) );	int i = 0; while ( i < notes.size() ) { System.out.println( i + ":" + notes.get( i ) ); i = i + 1; }

Write repeatedly	Execute repeatedly
int i = 0; System.out.println( notes.get( i ) ); i = i + 1; System.out.println( notes.get( i ) ); i = i + 1; System.out.println( notes.get( i ) );	for ( String note : notes ) System.out.println( note );

System.out.println( notes.get(0) ); System.out.println( notes.get(1) ); System.out.println( notes.get(2) );	Iterator<String> i = notes.iterator(); System.out.println( i.next( ) ); System.out.println( i.next( ) ); System.out.println( i.next( ) );
int i = 0; while ( i < notes.size() ) { System.out.println( notes.get( i ) ); i = i + 1; }	Iterator<String> i = notes.iterator(); while ( i.hasNext() ) { System.out.println( i.next() ); }

Chapter 4 Grouping Objects

Overview

4.1 Grouping objects in flexible-size collections

4.2 A personal notebook

4.3 A first look at library classes

4.3.1 An example using a library

4.4 Object structures with collections

4.5 Generic classes

4.6 Numbering within collections

4.7 Removing an item from a collection

4.8 Processing a whole collection

4.8.1 The for-each loop

4.8.2 The while loop

Infinite loops

4.8.3 Iterating over a collection

4.8.4 Index access versus iterators

4.9 Grouping objects complete example - a party

4.9.1 Anonymous objects

4.12 Fixed size collections

4.12.a Declaring array variables

4.12.b Creating array objects

4.12.c Using array objects

4.12.d The for-loop

4.12.1 A log-file analyzer

4.12.2 Declaring array variables

4.12.3 Creating array objects

4.12.4 Using array objects

4.12.5 Analyzing the log file

4.12.5.1 Arrays of objects

4.12.6 The for loop

4.13 Summary

Chapter 4
Grouping Objects