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 DAT050, 16/17, lp 1

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 Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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 … Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Objektorienterad programmering d2, förel. 5 Tech support The stupid @#*! 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 DAT050, 16/17, lp 1

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 DAT050, 16/17, lp 1

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 Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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 DAT050, 16/17, lp 1

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. Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Objektorienterad programmering d2, förel. 5 Main loop body String input = reader.getInput(); ... String response = responder.generateResponse(); System.out.println(response); Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Objektorienterad programmering d2, förel. 5 The exit condition String input = reader.getInput(); if(input.startsWith("bye")) break; Look for ‘startsWith’ in java.lang.String Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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 DAT050, 16/17, lp 1

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

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? Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Objects First with Java Sets and Maps Further library classes Set Map Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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? Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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) Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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; } Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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. Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Objektorienterad programmering d2, förel. 5 Using maps A map with Strings as keys and values :HashMap “Uno Holmer" “+46(0)317725730" “Peter Lundin" "+46(0)317722416" “Javaakuten" “999999" Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Objektorienterad programmering d2, förel. 5 Using maps HashMap <String, String> phoneBook = new HashMap<String,String>(); phoneBook.put("Uno Holmer", "+46(0)317725730"); phoneBook.put("Arne Linde", "+46(0)317721683"); phoneBook.put("Javaakuten", "999999"); String phoneNumber = phoneBook.get("Javaakuten"); System.out.println(phoneNumber); Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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 DAT050, 16/17, lp 1

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 DAT050, 16/17, lp 1

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). Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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. Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Code completion in BlueJ Objektorienterad programmering d2, förel. 5 Code completion in BlueJ Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Objektorienterad programmering d2, förel. 5 Some further topics Equality vs identity Class variables Constants Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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. Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Objektorienterad programmering d2, förel. 5 Equality vs identity 1 :Person :Person 20 30 person1 person2 == is not true here (of course) person1 == person2 ? Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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 ? Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Objektorienterad programmering d2, förel. 5 Identity vs equality 3 :Person :Person 20 20 person1 person2 == is true now (same object) person1 == person2 ? Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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 DAT050, 16/17, lp 1

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! Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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! Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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. Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Class variables and constants Objektorienterad programmering d2, förel. 5 Class variables and constants DAT050, 16/17, lp 1

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 Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

Objektorienterad programmering d2, förel. 5 Class variables Tärning: 12 Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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. Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1

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; Objektorienterad programmering, DAT050, DAI2, 16/17, lp 1 DAT050, 16/17, lp 1