1. 4 Grouping objects (cont.)
Objektorienterad programmering d2
4 Grouping objects (cont.)
Iteration Arrays
BK chap ,
DAT050, 18/19, lp 1

2. Main concepts to be covered
Objektorienterad programmering d2
Main concepts to be covered
Iteration with Loops
Collection traversal with Iterators
Fixed-size collections - Arrays
Rep. F3 bild 30: ArrayList, MusicOrganizer
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

3. Objektorienterad programmering d2
Iteration
We often want to perform some actions an arbitrary number of times.
E.g., print all the elements in a collection.
Most programming languages include loop statements to make this possible.
Java has several sorts of loop statement.
We will start with its for-each loop.
With collections, we often want to repeat things once for every object in a particular collection.
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DAT050, 18/19, lp 1

4. For-each loop pseudo code
Objektorienterad programmering d2
For-each loop pseudo code
General form of the for-each loop
for keyword
loop header
for(ElementType element : collection) {
loop body }
Statement(s) to be repeated
Pseudo-code expression of the actions of a for-each loop
For each element in collection, do the things in the loop body.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

5. Objektorienterad programmering d2
A Java example
/**
* List all file names in the organizer. */
public void listAllFiles() {
for(String filename : files) {
System.out.println(filename); }
for each filename in files, print out filename
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

6. Objektorienterad programmering d2
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 can use a boolean condition to decide whether or not to keep going.
A while loop provides this control.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

7. Objektorienterad programmering d2
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
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

8. Objektorienterad programmering d2
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++; }
Increment index by 1
while the value of index is less than the size of the collection, print the next file name, and then increment index
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

9. Using a non-generic list
Objektorienterad programmering d2
Using a non-generic list
private ArrayList files;
// ”Old” Java style // the list elements have type Object
/**
* List all file names in the organizer. */
public void listAllFiles() {
int index = 0;
while(index < files.size()) {
String filename = (String)files.get(index); System.out.println(filename);
index++; }
type cast Object to String
Elementen i en lista har typen Object (ung. void* i C)
Det man stoppar in behåller sin egen typ
N.B. This style is obsolete
Don’t use it!
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

10. Objektorienterad programmering d2
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.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

11. Searching a collection
Objektorienterad programmering d2
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.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

12. Objektorienterad programmering d2
Finishing a search
How do we finish a search?
Either there are no more items to check (failure): index >= files.size()
Or the item has been found (success): found == true
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

13. Loop condition for searching
Objektorienterad programmering d2
Loop condition for searching
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() && !found
while ( cond ) { statements; } // post condition: !cond
Detta gäller inte om vi avbryter loopen
på något annat sätt, t.ex. med break
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

14. Searching a collection using a boolean
Objektorienterad programmering d2
Searching a collection using a boolean
int index = 0;
boolean found = false;
while(index < files.size() && !found) {
String filename = files.get(index);
if(filename .contains(searchString)) {
// We don't need to keep looking.
found = true; }
else {
index++;
// Either we found it (found is true), // or we searched the whole collection // (found is false).
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

15. Searching a collection using break
Objektorienterad programmering d2
Searching a collection using break
int index = 0;
while(index < files.size()) {
String filename = files.get(index);
if(filename.contains(searchString)) {
// We don't need to keep looking.
break; }
else {
index++;
// Either we found it: index < files.size()
// or we searched the whole collection:
// index == files.size();
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

16. Searching a collection using a smarter condition
Objektorienterad programmering d2
Searching a collection using a smarter condition
int index = 0;
while( index < files.size() && ! files.get(index).contains(searchString) )
index++;
// Either we found it: index < files.size()
// or we searched the whole collection:
// index == files.size();
Denna variant är att föredra
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

17. Don’t forget the simple cases!
Objektorienterad programmering d2
Don’t forget the simple cases!
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.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

18. Objektorienterad programmering d2 Iterators
Grouping objects
Iterators
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19. Iterator and iterator()
Objektorienterad programmering d2
Iterator and iterator()
Collections have an iterator() method.
iterator() returns an Iterator object.
Iterator<E> has three methods:
boolean hasNext()
E next()
void remove()
DAT050, 18/19, lp 1

20. Moving away from String
Objektorienterad programmering d2
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
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

21. Using an Iterator object
Objektorienterad programmering d2
Using an Iterator object
Iterator<ElementType> it = myCollection.iterator();
while(it.hasNext()) {
call it.next() to get the next object
do something with that object }
java.util.Iterator
returns an Iterator object
public void listAllFiles() {
Iterator<Track> it = files.iterator();
while(it.hasNext()) { Track tk = it.next();
System.out.println(tk.getDetails()); }
- Vad är vitsen med detta?
- Iteratorer används för många
olika datastrukturer
- Med samma syntax!
OBS. Glöm inte elementtypen!
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

22. Objektorienterad programmering d2
Iterator mechanics
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DAT050, 18/19, lp 1

23. Objektorienterad programmering d2
myList:List
myList.iterator()
:Element
:Element
:Element
:Element
:Iterator
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

24. Objektorienterad programmering d2
myList:List
:Element
:Element
:Element
:Element
= iterator.next();
:Iterator
:Iterator ✔
hasNext()?
Element e
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

25. Objektorienterad programmering d2
myList:List
:Element
:Element
:Element
:Element
:Iterator
:Iterator ✔
hasNext()?
next()
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

26. Objektorienterad programmering d2
myList:List
:Element
:Element
:Element
:Element
:Iterator
:Iterator ✔
hasNext()?
next()
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

27. Objektorienterad programmering d2
myList:List
:Element
:Element
:Element
:Element
:Iterator
:Iterator
hasNext()? ✔
next()
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

28. Objektorienterad programmering d2
myList:List
:Element
:Element
:Element
:Element
:Iterator ✗
hasNext()?
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

29. Wrong use of Iterator object
Objektorienterad programmering d2
Wrong use of Iterator object
Iterator<Integer> it = myCollection.iterator();
while(it.hasNext()) {
if( it.next() > 0 ) {
System.out.println(it.next()); }
What’s wrong here?
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

30. Objektorienterad programmering d2
Correct use of Iterator object
Iterator<Integer> it = myCollection.iterator();
while(it.hasNext()) {
int element = it.next();
if( element > 0 ) {
System.out.println(element); }
Normally next() is called once in each iteration.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

31. Objektorienterad programmering d2
side note
a for each loop ...
for (ElementType x : myCollection) {
...
do something with x }
... is in principle automatically translated
into an iterator loop by the compiler
Iterator<ElementType> it = myCollection.iterator();
while(it.hasNext()) {
ElementType x = it.next();
...
do something with x }
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

32. Removing from a collection
Objektorienterad programmering d2
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.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

33. Objektorienterad programmering d2
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.
Iteratorer är mer generella
– man kan byta datastruktur
och ändå behålla koden intakt.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

34. Collections and primitive types
Objektorienterad programmering d2
Collections and primitive types
Java collections such as ArrayList can only store object types – not primitive types.
Each primitive type has a corresponding wrapper class that stores a primitive value as an object (Boxing).
ArrayList<Integer> al = new ArrayList<Integer>();
int i = 123;
al.add(new Integer(i));
i = (al.get(0)).intValue();
box an int
Type Wrapper
int Integer
long Long
short Short
float Float
double Double
char Character
boolean Boolean
...
TAVLAN
Rita en figur som visar mappningen
mellan en int och en Integer
unbox the Integer
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

35. Collections and primitive types
Objektorienterad programmering d2
Collections and primitive types
Java 5 (and later versions) performs boxing and unboxing automatically!
ArrayList<Integer> al = new ArrayList<Integer>();
int i = 123;
al.add(i); // auto boxing
i = files.get(0); // auto unboxing
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

36. Fixed-size collections
Objektorienterad programmering d2
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.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

37. The weblog-analyzer project
Objektorienterad programmering d2
The weblog-analyzer project
Web server records details of each access.
Supports webmaster’s tasks.
Most popular pages.
Busiest periods.
How much data is being delivered.
Broken references.
Analyze accesses by hour.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

38. Creating an array object
Objektorienterad programmering d2
Creating an array object
public class LogAnalyzer {
private int[] hourCounts;
private LogfileReader reader;
public LogAnalyzer()
hourCounts = new int[24];
reader = new LogfileReader(); }
...
Array variable declaration
Array object creation
Man kan skriva int hourCounts[] också (som i C).
Storleken kan beräknas dynamiskt:
- fältstorleken är inte låst vid compile-time som i C.
Ett fält är ett objekt!
Div. standardfunktioner i java.util.Arrays
(TAVLAN)
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DAT050, 18/19, lp 1

39. Objektorienterad programmering d2
The hourCounts array
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DAT050, 18/19, lp 1

40. Objektorienterad programmering d2
Using an array
Square-bracket notation is used to access an array element: hourCounts[...]
Elements are used like ordinary variables.
As target of an assignment:
hourCounts[hour] = ...;
In an expression:
adjusted = hourCounts[hour] – 3;
hourCounts[hour]++;
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

41. Objektorienterad programmering d2
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
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

42. Objektorienterad programmering d2
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[] { 3, 15, 4, 5 };
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

43. Objektorienterad programmering d2
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’.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

44. Objektorienterad programmering d2
Array utilities
Initialization ElemType[] arr = {value1,value2,...} arr = new ElemType[]{value1,value2,...}
Number of elements
arr.length
Asignment and copy
Shallow copy: arr2 = arr1 // reference copy
Deep copy (=cloning):
arr2 = (ElemType[])arr1.clone()
System.arraycopy(arr1,pos1,arr2,pos2,n)
Comparison
arr1 == arr // identity
Arrays.equals(arr1,arr2) // equality
See java.util.Arrays for more array utilities
Arrays.equals(a1,a2) om
a1 och a2 har samma typ
och samma storlek (kapacitet)
och elementen är parvis lika
Arrays.equals(null,null) -> true
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

45. Array traversal with for-loops
Objektorienterad programmering d2
Array traversal with for-loops
for loop version
for(int hour = 0; hour < hourCounts.length; hour++) {
System.out.println(hour + ": " + hourCounts[hour]); }
for-each loop version
int hour = 0;
for(int count : hourCounts) {
System.out.println(hour++ + ": " + count); }
Med for-each kan man bara läsa av
fält som innehåller enkla värden.
styrvariabeln blir en kopia av
fältelementen
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

46. Objektorienterad programmering d2
The auction project
The auction project provides further illustration of collections and iteration.
Examples of using null.
Anonymous objects.
Chaining method calls.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

47. Objektorienterad programmering d2
The auction project
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

48. Objektorienterad programmering d2
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’.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

49. Objektorienterad programmering d2
Anonymous objects
Objects are often created and handed on elsewhere immediately: Lot oneMoreLot = new Lot(…); lots.add(oneMoreLot);
We don’t really need oneMoreLot: lots.add(new Lot(…));
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

50. Objektorienterad programmering d2
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()
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

51. Objektorienterad programmering d2
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
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

52. Objektorienterad programmering d2
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.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1

53. Objektorienterad programmering d2
Review
Arrays are appropriate where a fixed-size collection is required.
Arrays use 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.
Objektorienterad programmering, DAT050, DAI2, 18/19, lp 1
DAT050, 18/19, lp 1