Transcript Objects First With Java - Chapter 4

Grouping objects
Collections and iterators
Main concepts to be covered
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Collections
Loops
Iterators
Arrays
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The requirement to group
objects
• Many applications involve collections of
objects:
• Personal organizers.
• Library catalogs.
• Student-record system.
• The number of items to be stored varies.
• Items added.
• Items deleted.
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A personal notebook
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Notes may be stored.
Individual notes can be viewed.
There is no limit to the number of notes.
It will tell you how many notes are stored.
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A personal notebook
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Class Libraries
• Learning to program is to learn how to
reuse code written by other. There is no
point in reinventing the wheel.
• Java comes with a library of useful classes,
which are referred to as the Java API
• They are organized in packages that follow
a directory structure
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Class Libraries
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Class libraries
• You can access the classes from your code,
using the import statement
• import java.util.Vector;
• import java.util.*;
• Grouping objects is a recurring requirement.
• The java.util package contains classes for
doing this.
• We will use ArrayList as a first example
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Class Libraries
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import java.util.ArrayList;
/**
* ...
*/
public class Notebook
{
// Storage for an arbitrary number of notes.
private ArrayList notes;
/**
* Perform any initialization required for the
* notebook.
*/
public Notebook()
{
notes = new ArrayList();
}
...
}
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Object structures with
collections
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Adding a third note
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Features of the collection
• It increases its capacity as necessary.
• It keeps a private count (size() accessor).
• It keeps the objects in order.
• Details of how all this is done are hidden.
• Does that matter? Does not knowing how
prevent us from using it?
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Using the collection
public class Notebook
{
private ArrayList notes;
...
public void storeNote(String note)
{
notes.add(note);
Adding a new note
}
public int numberOfNotes()
{
return notes.size();
}
Returning the number of notes
(delegation).
...
}
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Index numbering
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Retrieving an object
Index validity checks
public void showNote(int noteNumber)
{
if(noteNumber < 0) {
// This is not a valid note number.
}
else if(noteNumber < numberOfNotes()) {
System.out.println(notes.get(noteNumber));
}
else {
// This is not a valid note number.
}
}
Retrieve and print the note
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Removing an item
public void removeNote(int noteNumber)
{
if(noteNumber < 0) {
// This is not a valid note number, so do
nothing.
}
else if(noteNumber < numberOfNotes()) {
// This is a valid note number.
notes.remove(noteNumber);
}
else {
// This is not a valid note number, so do
nothing.
}
}
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Removal may affect
numbering
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Review
• Collections allow an arbitrary number of
objects to be stored.
• Class libraries usually contain tried-andtested collection classes.
• Java’s class libraries are called packages.
• We have used the ArrayList class from
the java.util package.
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Review
• Items may be added and removed.
• Each item has an index.
• Index values may change if items are
removed (or further items added).
• The main ArrayList methods are add,
get, remove and size.
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Iteration
• We often want to perform some actions an
arbitrary number of times.
• E.g., print all the notes in the notebook. How
many are there?
• Most programming languages include loop
statements to make this possible.
• Java has three sorts of loop statements.
• We will focus on its while loop to begin with.
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While loop pseudo code
General form of a while loop
while keyword
Boolean test
while(loop condition) {
loop body
}
Statements to be repeated
Pseudo-code example to print every note
while(there is at least one more note to be printed) {
show the next note
}
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A Java example
/**
* List all notes in the notebook.
*/
public void listNotes()
{
int index = 0;
while(index < notes.size()) {
System.out.println(notes.get(index));
index++;
}
}
Increment by one
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Increments
• i++ : the value of i is used first before it is
being incremented with one
• a = 5; b = a++;
• a == 6; b == 5;
• ++i: increments i with one, the new value of
i is then used.
• a = 5; b = ++a;
• a == 6; b == 6;
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Iterating over a collection
java.util.Iterator
Returns an Iterator
object
Iterator it = myCollection.iterator();
while(it.hasNext()) {
call it.next() to get the next object
do something with that object
}
public void listNotes()
{
Iterator it = notes.iterator();
while(it.hasNext()) {
System.out.println(it.next());
}
}
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Iterators
• An iterator is an object that provides functionality
to iterate over all elements of a collection or
container class
• java.util.Iterator
• two main methods: hasNext() to check if the
Iterator has more elements and next() to take the
next object from the Iterator.
• You can access an iterator for all java collections.
• Not all collections can be accessed using indexes.
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The auction project
• The auction project provides further
illustration of collections and iteration.
• Two further points to follow up:
• The null value.
• Casting. Used to store the result of get into a
variable:
• String message = (String) notes.get(0);
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The auction project
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The Lot Class
public void bidFor(Person bidder, long value)
{
if((highestBid == null) ||
(highestBid.getValue() < value)) {
// This bid is the best so far.
setHighestBid(new Bid(bidder, value));
}
else {
System.out.println("Lot number: " + getNumber() +
" (" + getDescription() + ")" +
" already has a bid of: " +
highestBid.getValue());
}
}
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The null Keyword
• highestBid == null;
• The Java keyword null is used to mean ‘no
object’ when a variable is not currently holding
a reference to a particular object.
• setHighestBid(new Bid(bidder, value));
• two things are done here:
• we create a new object Bid
• we pass this new object immediately to the method
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The Auction Class
public void showLots()
{
Iterator it = lots.iterator();
while(it.hasNext()) {
Lot lot = (Lot) it.next();
System.out.println(lot.getNumber() + ": " +
lot.getDescription());
// Include any details of a highest bid.
Bid highestBid = lot.getHighestBid();
if(highestBid != null) {
System.out.println("
Bid: " +
highestBid.getValue());
}
else {
System.out.println("
(No bid)");
}
}
}
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Type Casting
• Lot lot = (Lot) it.next();
• the return value of the Iterator method next() is
an object of type Object. To store this in an
object of type Lot we need to explicitly convert
it that type. This is called Casting
• This can only been done if the objects we have
added to the container were originally of type
Lot.
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Type Casting
• Collections in Java allow for the storage of
any type of objects. In order to do so it
transforms everything you add into an
object of type Object.
• When retrieving objects from a collection
we normally cast them back into their
original types.
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Wrapper Classes
• Container classes in Java can only contain
objects.
• Primitive types can therefore not be added.
• To solve this, Java provides wrapper
classes: Integer, Float, Double
• Integer a = new Integer(10);
• int b = a.intValue();
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Fixed-size collections
• Sometimes the maximum collection size
can be pre-determined.
• Programming languages usually offer a
special fixed-size collection type: an array.
• Java arrays can store objects or primitivetype values.
• Arrays use a special syntax.
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The weblog-analyzer project
• Web server records details of each access.
• Supports webmaster’s tasks.
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Most popular pages.
Busiest periods.
How much data is being delivered.
Broken references.
• Analyze accesses by hour.
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The weblog-analyzer project
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Creating an array object
public class LogAnalyzer
{
private int[] hourCounts;
private LogfileReader reader;
Array variable declaration
public LogAnalyzer()
{
hourCounts = new int[24];
reader = new LogfileReader();
}
...
Array object creation
}
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The hourCounts array
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Using an array
• Square-bracket notation is used to access an array
element: hourCounts[...]
• The number of elements of an array can be
obtained via: hourCounts.length
• Elements are used like ordinary variables.
• On the left of an assignment:
• hourCounts[hour] = ...;
• In an expression:
• adjusted = hourCounts[hour] – 3;
• hourCounts[hour]++;
• Arrays are passed by reference while their
elements are passed by value.
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The for loop
• Similar to a while loop.
• Often used to iterate a fixed number of
times.
• Often used to iterate over an array.
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For loop pseudo-code
General form of a for loop
for(initialization; condition; post-body action) {
statements to be repeated
}
Equivalent in while-loop form
initialization;
while(condition) {
statements to be repeated
post-body action
}
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A Java example
for loop version
for(int hour = 0; hour < hourCounts.length; hour++) {
System.out.println(hour + ": " + hourCounts[hour]);
}
while loop version
int hour = 0;
while(hour < hourCounts.length) {
System.out.println(hour + ": " + hourCounts[hour]);
hour++;
}
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Review
• Arrays are appropriate where a fixed-size
collection is required.
• Arrays use special syntax.
• For loops offer an alternative to while loops
when the number of repetitions is known.
• For loops are often used to iterate over
arrays.
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Concepts
• Java API
• packages
• for
• while
• import
• array
• ArrayList
• Iterator
• a++
• ++a
• type cast
• null
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