5 More sophisticated behavior

Transcription

1 Main concepts to be covered 5 More sophisticated behavior Using library classes to implement some more advanced functionality BK Chap. 6 Maps and sets Random number generation Equality vs identity Class variables Constants Förel. 5 2 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 Tech support The system erased all my files Does the query contain known computer related keywords? Print a response containing keyword(s) from the query Print a random response from a list of standard answers Förel. 5 3 Förel. 5 4 A simple knowledge base String W 1 W n Response table key 1 response 1 key m response tokenize W 1 W i W n For some W i : does the table contain W i as a keyword? List of standard responses Set of words m response 1 Yes, W i = key j No: print random => print response j standard response response m String W 1 W n Response table key 1 response 1 key m response m Data structures Set of words W 1 W i W n HashMap<String,String> List of standard responses response 1 response m HashSet<String> ArrayList<String> Förel. 5 5 Förel. 5 6 DAT050, 16/17, lp 1 1

2 Main loop structure Alternative loop structure boolean finished = false; while(!finished) do something if(exit condition) finished = true; else do something more A common iteration pattern. While ( true ) do something if(exit condition) break; else do something more A common iteration pattern. Förel. 5 7 Förel. 5 8 Main loop body The exit condition String response = responder.generateresponse(); System.out.println(response); if(input.startswith()) break; Look for startswith in java.lang.string Förel. 5 9 Förel Some methods for Strings contains startswith indexof substring touppercase trim split Beware: strings are immutable! 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 Förel Förel DAT050, 16/17, lp 1 2

3 Generating random responses Sets and Maps public Responder() randomgenerator = new Random(); responses = new ArrayList<String>(); fillresponses(); public void fillresponses() fill responses with a selection of response strings Further library classes Set Map public String generateresponse() int index = randomgenerator.nextint(responses.size()); return responses.get(index); Förel Förel 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! 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 in arbitrary order (HashSet) e d Förel Förel 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); 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. return words; Förel Förel DAT050, 16/17, lp 1 3

4 Using maps A map with Strings as keys and values :HashMap Uno Holmer" +46(0) " Peter Lundin" "+46(0) " Javaakuten" " 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); Förel Förel. 5 Generating smart responses Using containskey 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(); 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(); Förel Förel 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). 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. Förel Förel DAT050, 16/17, lp 1 4

5 Code completion in BlueJ Some further topics Equality vs identity Class variables Constants Förel Förel Side note: Equality vs identity Equality vs identity 1 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 30 A == B implies A.equals(B) person1 person2 person1 == person2? Förel Förel Equality vs identity 2 Identity vs equality 3 person1 person2 person1 person2 person1 == person2? person1 == person2? Förel Förel DAT050, 16/17, lp 1 5

6 String equality String equality 1 if(input == ) tests identity if ( input == ) == tests identity if(input.equals()) tests equality Always use equals for testing text equality. ==? input (may be) false! Förel Förel input String equality 2 if ( input.equals() ) equals tests equality equals? true! 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. Förel Förel Class variables Class variables and constants 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 Förel DAT050, 16/17, lp 1 6

7 Class variables 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. Förel Förel 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; Förel DAT050, 16/17, lp 1 7