1. 5 More sophisticated behavior
Objektorienterad programmering d2, förel. 5
5 More sophisticated behavior
Using library classes to implement some more advanced functionality
BK Chap. 6
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2. Main concepts to be covered
Objektorienterad programmering d2, förel. 5
Main concepts to be covered
Maps and sets
Random number generation
Equality vs identity
Class variables
Constants
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3. A Technical Support System
Objektorienterad programmering d2, förel. 5
A Technical Support System
A textual dialog system
Simulates on-line computer helpdesk
Idea based on ‘Eliza’ by Joseph Weizenbaum (MIT, 1960s)
Explore tech-support-complete …
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4. Objektorienterad programmering d2, förel. 5
Tech support
The system erased all my files
Does the query contain known
computer related keywords?
Yes No
Print a response containing
keyword(s) from the query
Print a random response
from a list of standard answers
Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1
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5. A simple ”knowledge base”
Objektorienterad programmering d2, förel. 5
A simple ”knowledge base”
Set of words
String
”W1”
”Wi”
”Wn”
”W1 ... Wn”
tokenize
For some Wi: does the table
contain Wi as a keyword?
Response table
key1
response1
...
keym
responsem
List of standard responses
response1
...
responsem
Yes, Wi = keyj
=> print responsej
No: print random
standard response
Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1
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6. Objektorienterad programmering d2, förel. 5
Data structures
String
HashSet<String>
Set of words
”W1 ... Wn”
”W1”
”Wi”
”Wn”
HashMap<String,String>
Response table
key1
response1
...
keym
responsem
ArrayList<String>
List of standard responses
response1
...
responsem
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7. Objektorienterad programmering d2, förel. 5
Main loop structure
boolean finished = false;
while(!finished) {
do something
if(exit condition)
finished = true;
else {
do something more }
A common iteration pattern.
Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1
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8. Alternative loop structure
Objektorienterad programmering d2, förel. 5
Alternative loop structure
While ( true ) {
do something
if(exit condition)
break;
else {
do something more }
A common iteration pattern.
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9. Objektorienterad programmering d2, förel. 5
Main loop body
String input = reader.getInput();
...
String response = responder.generateResponse();
System.out.println(response);
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10. Objektorienterad programmering d2, förel. 5
The exit condition
String input = reader.getInput();
if(input.startsWith("bye"))
break;
Look for ‘startsWith’ in java.lang.String
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11. Some methods for Strings
Objektorienterad programmering d2, förel. 5
Some methods for Strings
contains
startsWith
indexOf
substring
toUpperCase
trim
split
Beware: strings are immutable!
Kolla
StringBuffer och StringBuilder
Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1
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12. Random number generation
Objektorienterad programmering d2, förel. 5
Random number generation
The library class Random can be used to generate (pseudo) random numbers
import java.util.Random;
...
Random randomGenerator = new Random();
int index1 = randomGenerator.nextInt();
int index2 = randomGenerator.nextInt(100);
int dieValue = randomGenerator.nextInt(6)+1;
0 ≤ randomGenerator.nextInt(n) < n
Tärning: 35
Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1
DAT050, 16/17, lp 1

13. Generating random responses
Objektorienterad programmering d2, förel. 5
Generating random responses
public Responder() {
randomGenerator = new Random();
responses = new ArrayList<String>();
fillResponses(); }
public void fillResponses()
fill responses with a selection of response strings
public String generateResponse()
int index = randomGenerator.nextInt(responses.size());
return responses.get(index);
Konstruktorn anropar en metod som gör resten av jobbet
Borde denna vara privat?
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14. Objects First with Java
Sets and Maps
Further library classes
Set
Map
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15. Objektorienterad programmering d2, förel. 5
Using sets
import java.util.HashSet;
import java.util.Iterator;
...
HashSet<String> mySet = new HashSet<String>();
mySet.add("one");
mySet.add("two");
mySet.add("three");
Iterator<String> it = mySet.iterator();
while(it.hasNext()) {
call it.next() to get the next object
do something with that object }
Compare this to ArrayList code!
Hur många”Two” finns det i mängden?
I vilken ordning hamnar strängarna?
I den ordning de sattes in?
I bokstavsordning?
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16. Sets don’t store duplicates
Objektorienterad programmering d2, förel. 5
Sets don’t store duplicates
Example.
String[] words =
{"aaa","bb","aaa","ccc","d","aaa","bb","e","ccc"};
HashSet<String> set = new HashSet<String>();
for(String word : words)
set.add(word);
for(String s : set)
System.out.println(s);
program output bb
aaa
ccc e d
in arbitrary order (HashSet)
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17. Tokenizing strings (Splitting a string into words)
Objektorienterad programmering d2, förel. 5
Tokenizing strings (Splitting a string into words)
public HashSet<String> getInput() {
System.out.print("> ");
String inputLine =
reader.nextLine().trim().toLowerCase();
String[] wordArray = inputLine.split();
HashSet<String> words = new HashSet<String>();
for(String word : wordArray)
words.add(word);
return words; }
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18. Objektorienterad programmering d2, förel. 5
Maps
Maps are collections that contain pairs of values.
Pairs consist of a key and a value.
Lookup works by supplying a key, and retrieving a value.
An example: a telephone book.
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19. Objektorienterad programmering d2, förel. 5
Using maps
A map with Strings as keys and values
:HashMap
“Uno Holmer"
“+46(0) "
“Peter Lundin"
"+46(0) "
“Javaakuten"
“999999"
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20. Objektorienterad programmering d2, förel. 5
Using maps
HashMap <String, String> phoneBook = new HashMap<String,String>();
phoneBook.put("Uno Holmer", "+46(0) ");
phoneBook.put("Arne Linde", "+46(0) ");
phoneBook.put("Javaakuten", "999999");
String phoneNumber = phoneBook.get("Javaakuten");
System.out.println(phoneNumber);
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21. Generating “smart” responses
Objektorienterad programmering d2, förel. 5
Generating “smart” responses
private HashMap<String,String> responseMap; ... public String generateResponse(HashSet<String> words)
{ for(String word : words) {
String response = responseMap.get(word);
if(response != null)
return response; } return pickDefaultResponse(); }
Words är de ord användaren skrev in i sin fråga
Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1
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22. Objektorienterad programmering d2, förel. 5
Using containsKey
private HashMap<String,String> responseMap; ... public String generateResponse(HashSet<String> words)
{ for(String word : words) {
if ( responseMap.containsKey(word) )
return responseMap.get(word); } return pickDefaultResponse(); }
Words är de ord användaren skrev in i sin fråga
Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1
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23. Objektorienterad programmering d2, förel. 5
List, Map and Set
Alternative ways to group objects.
Varying implementations available:
ArrayList, LinkedList
HashSet, TreeSet
HashMap, TreeMap
The first word reveals the underlying data representation (data structure).
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24. Objektorienterad programmering d2, förel. 5
Code completion
The BlueJ editor supports lookup of methods.
Use Ctrl-space after a method-call dot to bring up a list of available methods.
Use Return to select a highlighted method.
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25. Code completion in BlueJ
Objektorienterad programmering d2, förel. 5
Code completion in BlueJ
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26. Objektorienterad programmering d2, förel. 5
Some further topics
Equality vs identity
Class variables
Constants
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27. Side note: Equality vs identity
Objektorienterad programmering d2, förel. 5
Side note: Equality vs identity
A == B is true iff A and B refer to the same object
A.equals(B) is true iff A and B refer to objects with equal content
A == B implies A.equals(B)
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.
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28. Objektorienterad programmering d2, förel. 5
Equality vs identity 1
:Person
:Person 20 30
person1
person2
== is not true here (of course)
person1 == person2 ?
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29. Objektorienterad programmering d2, förel. 5
Equality vs identity 2
:Person
:Person 20 20
person1
person2
== is still not true here (different objects, == tests identity)
person1 == person2 ?
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30. Objektorienterad programmering d2, förel. 5
Identity vs equality 3
:Person
:Person 20 20
person1
person2
== is true now (same object)
person1 == person2 ?
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31. Objektorienterad programmering d2, förel. 5
String equality
tests identity
if(input == "bye") {
... }
if(input.equals("bye")) {
Always use equals for testing text equality.
tests equality
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.
Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1
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32. Objektorienterad programmering d2, förel. 5
String equality 1
String input = reader.getInput();
if ( input == "bye" ) {
... }
== tests identity
:String
:String == ?
"bye"
"bye"
== is still not true here (different objects, == tests identity)
input
 (may be) false!
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33. Objektorienterad programmering d2, förel. 5
String equality 2
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
 true!
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34. The problem with Strings
Objektorienterad programmering d2, förel. 5
The problem with Strings
The compiler stores identical String literals in shared memory:
The result is reference equality for apparently distinct String objects. Eg. ”Hello!” == ”Hello!” is true
But this cannot be done for identical strings that arise outside the program’s code
e.g., from user input.
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35. Class variables and constants
Objektorienterad programmering d2, förel. 5
Class variables and constants
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36. Objektorienterad programmering d2, förel. 5
Class variables
A class variable is shared between all instances of the class.
In fact, it belongs to the class and exists independent of any instances.
Designated by the static keyword.
Public static variables are accessed via the class name; e.g.
Thermometer.boilingPoint
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37. Objektorienterad programmering d2, förel. 5
Class variables
Tärning: 12
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38. Objektorienterad programmering d2, förel. 5
Constants
A variable, once set, can have its value fixed.
Designated by the final keyword.
final int max = list.size();
Final fields must be set in their declaration or the constructor.
Combing static and final is common.
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39. Objects First with Java
Class constants
static: class variable
final: constant
private static final int gravity = 3;
Public visibility is less of an issue with final fields.
Upper-case names often used for class constants:
public static final int BOILING_POINT = 100;
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