1. COS 260 DAY 8
Tony Gauvin

2. Agenda Questions? 3rd Mini quiz Assignment 2 posted
No password required
Assignment 2 posted
Due Oct 5 prior to class
Continue to Discuss Grouping objects

3. Assignment 2
Problem 1 (24 points) Complete exercises 3.31 and 3.32 on page 81 of the textbook. Zip the two resulting projects into two separate zip files named Problem1a and problem1b Problem 2 (36 points) Complete exercises 4.40, 4.41 and 4.42 on page 123. Save the results of these three exercise to a new project called club_ver1, Zip the project directory, name the zip file "problem2" and upload. Problem 3 (40 points) Complete exercises 4.54 and 4.55 on page 137, begin with the project file created in Problem 2. Save the results of these two exercises to a new project called club_ver2, Zip the project directory, name the zip file "problem3" and upload.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

4. Objects First with Java Introduction to collections
Grouping objects
Introduction to collections
5.0
© David J. Barnes and Michael Kölling

5. Objects First with Java
Review
Collections allow an arbitrary number of objects to be stored.
Class libraries usually contain tried-and-tested collection classes.
Java’s class libraries are called packages.
We have used the ArrayList class from the java.util package.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

6. Objects First with Java
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.
ArrayList is a parameterized or generic type.
Can store any primitive or object type
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

7. Objects First with Java
Review
Loop statements allow a block of statements to be repeated.
The for-each loop allows iteration over a whole collection.
Music-organizer-v3
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

8. Objects First with Java
Selective processing
Statements can be nested, giving greater selectivity:
public void findFiles(String searchString) {
for(String filename : files) {
if(filename.contains(searchString)) { System.out.println(filename); } }
© David J. Barnes and Michael Kölling

9. Critique of for-each Easy to write. Termination happens naturally.
The collection cannot be changed.
There is no index provided.
Not all collections are index-based.
We can’t stop part way through;
e.g. find-the-first-that-matches.
It provides ‘definite iteration’ – aka ‘bounded iteration’.

10. Objects First with Java Indefinite iteration - the while loop
Grouping objects
Indefinite iteration - the while loop
© David J. Barnes and Michael Kölling

11. Main concepts to be covered
Objects First with Java
Main concepts to be covered
The difference between definite and indefinite (unbounded) iteration.
The while loop
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

12. Search tasks are indefinite
We cannot predict, in advance, how many places we will have to look.
Although, there may well be an absolute limit – i.e., checking every possible location.
‘Infinite loops’ are also possible.
Through error or the nature of the task.

13. Objects First with Java
The while loop
A for-each loop repeats the loop body for each object in a collection.
Sometimes we require more variation than this.
We use a boolean condition to decide whether or not to keep going.
A while loop provides this control.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

14. Objects First with Java
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 expression of the actions of a while loop
while we wish to continue,
do the things in the loop body
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

15. Looking for your keys Or: while(the keys are missing) {
look in the next place; }
Or:
while(not (the keys have been found)) {

16. Suppose we don’t find them?
Looking for your keys
Sentential value
boolean searching = true;
while(searching) {
if(they are in the next place) {
searching = false; }
Suppose we don’t find them?

17. Objects First with Java
A Java example
/**
* List all file names in the organizer. */
public void listAllFiles() {
int index = 0;
while(index < files.size()) { String filename = files.get(index);
System.out.println(filename);
index++; }
while the value of index is less than the size of the collection, get and print the next file name, and then increment index
Increment index by 1
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

18. Increment/decrement Operators
++ a; prefix
a++; postfix
--b;
b--;
a = 2;
c = ++a + a++ + a;
C =?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

19. Elements of the loop We have declared an index variable.
The condition must be expressed correctly.
We have to fetch each element.
The index variable must be incremented explicitly.

20. Objects First with Java
for-each versus while
for-each:
easier to write.
safer: it is guaranteed to stop.
while:
we don’t have to process the whole collection.
doesn’t even have to be used with a collection.
take care: could be an infinite loop.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

21. Searching a collection
A fundamental activity.
Applicable beyond collections.
Necessarily indefinite.
We must code for both success and failure – exhausted search.
Both must make the loop’s condition false.
The collection might be empty.

22. Finishing a search How do we finish a search?
Either there are no more items to check: index >= files.size()
Or the item has been found: found == true found == ! searching

23. Continuing a search
With a while loop we need to state the condition for continuing:
So the loop’s condition will be the opposite of that for finishing: index < files.size() && ! found index < files.size() && searching
NB: ‘or’ becomes ‘and’ when inverting everything.

24. And  !Or! a && b is the same as !(!a || !b) a b a&&b !a||!b !(!a||!b)
true
false
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

25. Searching a collection
Objects First with Java
Searching a collection
int index = 0;
boolean found = false;
while(index < files.size() && !found) {
String file = files.get(index);
if(file.contains(searchString)) {
// We don't need to keep looking.
found = true; }
else {
index++;
// Either we found it at index,
// or we searched the whole collection.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

26. Indefinite iteration
Does the search still work if the collection is empty?
Yes! The loop’s body won’t be entered in that case.
Important feature of while:
The body will be executed zero or more times.

27. public int findFirst(String searchString)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

28. While without a collection
Objects First with Java
While without a collection
// Print all even numbers from 2 to 30.
int index = 2;
while(index <= 30) {
System.out.println(index);
index = index + 2; }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

29. The String class The String class is defined in the java.lang package.
It has some special features that need a little care.
In particular, comparison of String objects can be tricky.

30. Side note: String equality
Objects First with Java
Side note: String equality
tests identity
(are they the same string object ?)
if(input == "bye") {
... }
if(input.equals("bye")) {
Always use .equals for text equality.
tests equality
(are the strings lexicologically equal?)
if desired, include more detailed discussion of identity vs equality here.
(That's the next 5 slides, up to "Identity vs equality (Strings)". Skip these if this
is not needed now.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

31. Objects First with Java
Identity vs equality 1
Other (non-String) objects:
:Person
:Person
“Fred”
“Jill”
== is not true here (of course)
person1
person2
person1 == person2 ?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

32. Objects First with Java
Identity vs equality 2
Other (non-String) objects:
:Person
:Person
“Fred”
“Fred”
== is still not true here (different objects, == tests identity)
person1
person2
person1 == person2 ?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

33. Objects First with Java
Identity vs equality 3
Other (non-String) objects:
:Person
:Person
“Fred”
“Fred”
== is true now (same object)
person1
person2
person1 == person2 ?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

34. Identity vs equality (Strings)
Objects First with Java
Identity vs equality (Strings)
String input = reader.getInput();
if(input == "bye") {
... }
== tests identity
:String
:String == ?
"bye"
"bye"
== is still not true here (different objects, == tests identity)
input
à (may be) false!
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

35. Identity vs equality (Strings)
Objects First with Java
Identity vs equality (Strings)
String input = reader.getInput();
if(input.equals("bye")) {
... }
equals tests equality
:String
:String ?
equals
"bye"
"bye"
== is still not true here (different objects, == tests identity)
input
à true!
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

36. The problem with Strings
The compiler merges identical String literals in the program code.
The result is reference equality for apparently distinct String objects.
Done to save memory
But this cannot be done for identical strings that arise outside the program’s code;
e.g., from user input.

37. Moving away from String
Our collection of String objects for music tracks is limited.
No separate identification of artist, title, etc.
A Track class with separate fields:
artist
title
Filename
Music-organizer-v5

38. Objects First with Java Iterators (for processing collections)
Grouping objects
Iterators
(for processing collections)
also explain: how to pass an ArrayList for testing
© David J. Barnes and Michael Kölling

39. Iterator and iterator()
Collections have an iterator() method.
This returns an Iterator object.
Iterator<E> has three methods:
boolean hasNext()
E next()
void remove()

40. Using an Iterator object
Objects First with Java
Using an Iterator object
returns an Iterator object
java.util.Iterator
Iterator<ElementType> it = myCollection.iterator();
while(it.hasNext()) {
call it.next() to get the next object
do something with that object }
public void listAllFiles() {
Iterator<Track> it = files.iterator();
while(it.hasNext()) { Track tk = it.next();
System.out.println(tk.getDetails()); }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

41. Objects First with Java
Iterator mechanics
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

42. Objects First with Java
myList:List
myList.iterator()
:Element
:Element
:Element
:Element
:Iterator
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

43. Objects First with Java
myList:List
:Element
:Element
:Element
:Element
:Iterator
:Iterator ✔
hasNext()?
next()
Element e = iterator.next();
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

44. Objects First with Java
myList:List
:Element
:Element
:Element
:Element
:Iterator
:Iterator ✔
hasNext()?
next()
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

45. Objects First with Java
myList:List
:Element
:Element
:Element
:Element
:Iterator
:Iterator ✔
hasNext()?
next()
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

46. Objects First with Java
myList:List
:Element
:Element
:Element
:Element
:Iterator
:Iterator
hasNext()? ✔
next()
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

47. Objects First with Java
myList:List
:Element
:Element
:Element
:Element
:Iterator ✗
hasNext()?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

48. Objects First with Java
Index versus Iterator
Ways to iterate over a collection:
for-each loop.
Use if we want to process every element.
while loop.
Use if we might want to stop part way through.
Use for repetition that doesn't involve a collection.
Iterator object.
Often used with collections where indexed access is not very efficient, or impossible.
Use to remove from a collection.
Iteration is an important programming pattern.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

49. Removing from a collection
Iterator<Track> it = tracks.iterator();
while(it.hasNext()) {
Track t = it.next();
String artist = t.getArtist();
if(artist.equals(artistToRemove)) {
it.remove(); }
Use the Iterator’s remove method.

50. Objects First with Java
Review
Loop statements allow a block of statements to be repeated.
The for-each loop allows iteration over a whole collection.
The while loop allows the repetition to be controlled by a boolean expression.
All collection classes provide special Iterator objects that provide sequential access to a whole collection.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

51. Objects First with Java
The auction project
The auction project provides further illustration of collections and iteration.
Examples of using null.
Anonymous objects.
Chaining method calls.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

52. The auction project

53. null Used with object types. Used to indicate, 'no object'.
We can test if an object variable holds the null value: if(highestBid == null) …
Used to indicate ‘no bid yet’.

54. Anonymous objects
Objects are often created and handed on elsewhere immediately: Lot furtherLot = new Lot(…); lots.add(furtherLot);
We don’t really need furtherLot: lots.add(new Lot(…));

55. Chaining method calls Methods often return objects.
We often immediately call a method on the returned object. Bid bid = lot.getHighestBid(); Person bidder = bid.getBidder();
We can use the anonymous object concept and chain method calls: lot.getHighestBid().getBidder()

56. Chaining method calls
Each method in the chain is called on the object returned from the previous method call in the chain.
String name = lot.getHighestBid().getBidder().getName();
Returns a Bid object from the Lot
Returns a Person object from the Bid
Returns a String object from the Person

57. Objects First with Java Arrays
Grouping objects
Arrays
© David J. Barnes and Michael Kölling

58. Fixed-size collections
Objects First with Java
Fixed-size collections
Sometimes the maximum collection size can be pre-determined.
A special fixed-size collection type is available: an array.
Unlike the flexible List collections, arrays can store object references or primitive-type values.
Arrays use a special syntax.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

59. The weblog-analyzer project
Objects First with Java
The weblog-analyzer project
Web server records details of each access.
Supports analysis tasks:
Most popular pages.
Busiest periods.
How much data is being delivered.
Broken references.
Analyze accesses by hour.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

60. Creating an array object
Objects First with Java
Creating an array object
Array variable declaration — does not contain size
public class LogAnalyzer {
private int[] hourCounts;
private LogfileReader reader;
public LogAnalyzer()
hourCounts = new int[24];
reader = new LogfileReader(); }
...
Array object creation — specifies size
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

61. Objects First with Java
The hourCounts array
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

62. Objects First with Java
Using an array
Square-bracket notation is used to access an array element: hourCounts[...]
Elements are used like ordinary variables.
The target of an assignment:
hourCounts[hour] = ...;
In an expression:
hourCounts[hour]++;
adjusted = hourCounts[hour] – 3;
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

63. Objects First with Java
Standard array use
private int[] hourCounts;
private String[] names;
...
hourCounts = new int[24];
hourcounts[i] = 0;
hourcounts[i]++;
System.out.println(hourcounts[i]);
declaration
creation
use
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

64. Objects First with Java
Array literals
The size is inferred from the data.
private int[] numbers = { 3, 15, 4, 5 };
declaration, creation and initialization
Array literals in this form can only be used in declarations.
Related uses require new:
numbers = new int[] { , 15, 4, 5 };
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

65. Objects First with Java
Array length
private int[] numbers = { 3, 15, 4, 5 };
int n = numbers.length;
no brackets!
NB: length is a field rather than a method!
It cannot be changed – ‘fixed size’.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

66. Objects First with Java
The for loop
There are two variations of the for loop, for-each and for.
The for loop is often used to iterate a fixed number of times.
Often used with a variable that changes a fixed amount on each iteration.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

67. Objects First with Java
For loop pseudo-code
General form of the 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 }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

68. Objects First with Java
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++; }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

69. Objects First with Java
Practice
Given an array of numbers, print out all the numbers in the array, using a for loop.
int[] numbers = { 4, 1, 22, 9, 14, 3, 9};
for ...
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

70. Objects First with Java
Practice
Fill an array with the Fibonacci sequence.
int[] fib = new int[100];
fib[0] = 0;
fib[1] = 1;
for ...
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

71. for loop with bigger step
Objects First with Java
for loop with bigger step
// Print multiples of 3 that are below 40.
for(int num = 3; num < 40; num = num + 3) {
System.out.println(num); }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling

72. Objects First with Java
Review
Arrays are appropriate where a fixed-size collection is required.
Arrays use a special syntax.
For loops are used when an index variable is required.
For loops offer an alternative to while loops when the number of repetitions is known.
Used with a regular step size.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling