What s Wrong with This?

Transcription

1 More Sophisticated Behaviour 2 Using library classes to implement more advanced functionality. Also class (static( static) ) fields. What s Wrong with This? class Notebook private ArrayList<String> notes; public Notebook () ArrayList<String> notes = new ArrayList<String> (); public void addnote (String note) notes.add (note); What s Wrong with This? class Notebook private ArrayList<String> notes; Delete!!! public Notebook () ArrayList<String> notes = new ArrayList<String> (); public void addnote (String note) notes.add (note); 1

2 Keep going to the classes! Main Concepts to be Covered More library classes sets and maps (HashSet( and HashMap) Information hiding public versus private Class fields static and final Writing documentation Seen before Using lists import java.util.arraylist ArrayList; import java.util.iterator; ArrayList<String> mylist = new ArrayList<String> (); mylist.add ("one"); mylist.add ("two"); mylist.add ("one"); Iterator<String> it = mylist.iterator (); while (it.hasnext( ()) String s = it.next ();... do something with s 2

3 New Using sets import java.util.hashset HashSet; import java.util.iterator; HashSet<String> myset = new HashSet<String> (); myset.add ("one"); myset.add ("two"); myset.add ("one"); Iterator<String> it = myset.iterator (); while (it.hasnext( ()) String s = it.next ();... do something with s Using sets and lists import java.util.hashset HashSet; import java.util.iterator; All varieties of set and list are examples of collection classes. We can use for-each loops and Iterators on them. Of course, we can also use get (index) methods on them as well (or instead). One difference is that a set one holds one instance of objects that are equal i.e. if a and b have both been added to a set and a.equals(b),, then only one of them will be there. It is not guaranteed which one is kept. So, never search a set using ==. Using sets and lists import java.util.hashset HashSet; import java.util.iterator; All varieties of set and list are examples of collection classes. We can use for-each loops and Iterators on them. Of course, we can also use get (index) methods on them as well (or instead). A second difference is that the order in which elements are kept in a set is not defined. For example, the order of elements returned by a for-each loop or Iterator is not necessarily the same order in which they were added. 3

4 Using sets and lists import java.util.hashset HashSet; import java.util.iterator; Go look them up in the Java API documentation!!! Seen before Eliza Project SupportSystem InputReader See Chapter05, tech-support projects Seen before Eliza Main Loop boolean finished = false; while (!finished) ) This returns the line of text typed by the customer. String input = reader.getinput (); if (input.startswith( ("bye"))) finished = true; else String response = responder.generateresponse (); System.out.println (response); Chapter05, tech-support1, SupportSystem.start() 4

5 New Eliza Main Loop boolean finished = false; while (!finished) ) This returns the set of different words typed by the customer. HashSet<String> input = reader.getinput (); if (input.contains( ("bye"))) finished = true; Now, bye can be any else word not just the first. Go look this up String response = (java.util.hashset) responder.generateresponse (input); System.out.println (response); Chapter05, tech-support-complete, complete, SupportSystem.start() New Eliza Main Loop boolean finished = false; while (!finished) ) This is the first time the response is dependant upon the enquiry! HashSet<String> input = reader.getinput (); if (input.contains( ("bye"))) finished = true; else String response = responder.generateresponse (input); System.out.println (response); Chapter05, tech-support-complete, complete, SupportSystem.start() New Eliza InputReader public String getinput () System.out.print ("> "); String inputline = scanner.nextline(); return inputline; import java.util.scanner; private Scanner scanner; go look this up (java.util.scanner) This is a String Note: in the project code, scanner is (confusingly) called reader Chapter05, tech-support1, InputReader.getInput() 5

6 New Eliza InputReader public HashSet<String> getinput () private Scanner scanner; System.out.print ("> ") ; String inputline = scanner.nextline().trim().tolowercase(); ();... more stuff import java.util.scanner; Note: in the project code, scanner is (confusingly) called reader Chapter05, tech-support-complete, complete, InputReader.getInput() New Eliza InputReader public HashSet<String> getinput () private Scanner scanner; System.out.print ("> ") ; String inputline = scanner.nextline().trim().tolowercase(); ();... more stuff import java.util.scanner; This is a String Chapter05, tech-support-complete, complete, InputReader.getInput() New Eliza InputReader public HashSet<String> getinput () private Scanner scanner; System.out.print ("> ") ; String inputline = scanner.nextline().trim().tolowercase(); ();... more stuff import java.util.scanner; So is this Chapter05, tech-support-complete, complete, InputReader.getInput() 6

7 New Eliza InputReader import java.util.scanner; public HashSet<String> getinput () go look these up private Scanner scanner; (java.lang.string) System.out.print ("> ") ; String inputline = scanner.nextline().trim().tolowercase(); ();... more stuff go look this up (java.util.scanner) So is this Chapter05, tech-support-complete, complete, InputReader.getInput() New Eliza InputReader public HashSet<String> getinput () go look this up (java.lang.string) System.out.print ("> "); String inputline = scanner.nextline().trim().tolowercase(); (); String[] wordarray = inputline.split (" "); HashSet<String> wordset = new HashSet<String> (); for (String word : wordarray) ) wordset.add (word); converts the which eliminates array to a set return wordset; repeated words Chapter05, tech-support-complete, complete, InputReader.getInput() Maps Maps are collections that contain pairs of values. A pair consists of a key and a value. Lookup works by supplying a key, and retrieving the corresponding value. An example: a telephone book. 7

8 Using Maps A map with Strings both as keys and values: phonebook:hashmap "Rick Blaine" "(531) " "Ilsa Lund" "(402) " "Victor Laszlo" "(998) " Using Maps HashMap<String, String> phonebook = new HashMap<String, String> (); key value ( "Rick Blaine", "(531) "); ( "Ilsa" Lund", "(402) "); ("Victor Laszlo", "(998) "); value key String phonenumber = phonebook.get ("Rick Blaine");... System.out.println (phonenumber); Using Maps HashMap<String, String> phonebook = new HashMap<String, String> (); key value ( "Rick Blaine", "(531) "); ( "Ilsa" Lund", "(402) "); ("Victor Laszlo", "(998) "); value key String phonenumber = phonebook.get ("Ricky Blaine");... Important: invoking get with a key that does not exist in the HashMap object returns null.. This can be useful! 8

9 Using Maps HashMap<String, String> phonebook = new HashMap<String, String> (); key value ( "Rick Blaine", "(531) "); ( "Ilsa" Lund", "(402) "); ("Victor Laszlo", "(998) "); value key String phonenumber = phonebook.get ("Ricky Blaine"); if (phonenumber( == null)... else... Important: invoking get with a key that does not exist in the HashMap object returns null.. This can be useful! Using Maps HashMap<String, String> phonebook = new HashMap<String, String> (); key value ( "Rick Blaine", "(531) "); ( "Ilsa" Lund", "(402) "); ("Victor Laszlo", "(998) "); value key String phonenumber = phonebook.get ("Ricky Blaine"); if (phonenumber( == null)... else... "Ricky Blaine" was not in the phonebook "Ricky Blaine" was in the phonebook Seen before Eliza Project SupportSystem InputReader See Chapter05, tech-support projects 9

10 Just seen Eliza InputReader public HashSet<String> getinput () go look this up (java.lang.string) System.out.print ("> "); String inputline = scanner.nextline().trim().tolowercase(); (); String[] wordarray = inputline.split (" "); HashSet<String> wordset = new HashSet<String> (); for (String word : wordarray) ) wordset.add (word); convert the which eliminates array to a set return wordset; repeated words Chapter05, tech-support-complete, complete, InputReader.getInput() Seen before Eliza Project SupportSystem InputReader See Chapter05, tech-support projects Just seen Eliza Main Loop boolean finished = false; while (!finished) ) This is the first time the response is dependant upon the enquiry! HashSet<String> input = reader.getinput (); if (input.contains( ("bye"))) finished = true; else String response = responder.generateresponse (input); System.out.println (response); Chapter05, tech-support-complete, complete, SupportSystem.start() 10

11 Seen before Eliza Project SupportSystem InputReader See Chapter05, tech-support projects Seen before Generating Random Responses public () Chapter05, tech-support2, randomgenerator = new Random (); responses = new ArrayList<String> (); fillresponses (); private void fillresponses () returns a random... number in the legal range for indexing responses public String generateresponse () int index = randomgenerator.nextint (responses.size ()); return responses.get (index); New Generating Better Responses Chapter05, tech-support-complete, complete, public () randomgenerator = new Random (); defaultresponses = new ArrayList<String> (); filldefaultresponses (); responsemap = new HashMap<String, String> (); fillresponsemap ();... private void filldefaultresponses ()... private void fillresponsemap () private String pickdefaultresponse () int index = randomgenerator.nextint (defaultresponses.size ()); return responses.get (index); Same as tech-support2 generateresponse 11

12 Generating Random Responses Chapter05, tech-support-complete, complete, private void filldefaultresponses () defaultresponses.add ("That sounds odd. Tell me more?"); defaultresponses.add ("No one else complained!"); defaultresponses.add ("Interesting. Tell me more?"); defaultresponses.add ("Do you have a dll conflict?"); defaultresponses.add ("Read the ******* manual!"); defaultresponses.add ("That's not a bug - it's a feature!");... etc. Same as tech-support2 fillresponses Just seen Generating Better Responses Chapter05, tech-support-complete, complete, public () randomgenerator = new Random (); defaultresponses = new ArrayList<String> (); filldefaultresponses (); responsemap = new HashMap<String, String> (); fillresponsemap ();... private void filldefaultresponses ()... private void fillresponsemap () private String pickdefaultresponse () int index = randomgenerator.nextint (defaultresponses.size ()); return responses.get (index); Same as tech-support2 generateresponse New Generating Relevant Responses Chapter05, tech-support-complete, complete, private void fillresponsemap () responsemap.put ("crash", "Well, it never crashes here!\n" + "Tell me about your configuration?"); responsemap.put ("slow", "Upgrade your processor then!\n" + "That should solve your problems."); responsemap.put ("windows", "This is a known bug in Windows!\n + "Please report it to Microsoft");... etc. 12

13 New Generating Better Responses Chapter05, tech-support-complete, complete, public String generateresponse (HashSet<String> words) Iterator<String> it = words.iterator (); while (it.hasnext( ()() String word = it.next (); String response = responsemap.get (word); if (response(!= null) return response; supply the key return pickdefaultresponse (); If the key is in the Map,, this returns the matching value otherwise returns null. Or Generating Better Responses Chapter05, tech-support-complete, complete, public String generateresponse (HashSet<String> words) for (String word : words) ) String response = responsemap.get (word); if (response(!= null) return response; supply the key return pickdefaultresponse (); If the key is in the Map,, this returns the matching value otherwise returns null. Revision Make sure you understand every statement on the next slide 13

14 Revision A class is a template from which objects can be constructed. A class defines the data fields held by its objects, together with logic for constructors of its objects and methods for accessing and operating on their data. Methods (and constructors) may declare and use their own local variables these exist only for the duration of their method (or constructor). On the other hand, data fields last for the duration of their object. Methods and constructors may be passed parameters these exist only for the duration of the method or constructor. Data fields, parameters and local variables may have class types (i.e. they are references to other objects) or primitive types (e.g. int, float, boolean, char, ). Data fields, constructors and methods may be declared public or private. Revision If you don t, you must find out e.g. read the book! Information hiding Public attributes (fields, constructors, methods) are accessible to other classes. Fields should not (usually) be public. Private attributes are accessible only within the same class. Only methods and constructors that are intended for use by other classes should be public. 14

15 Information hiding Data belonging to one object should be hidden from other objects. Know what an object can do, not how it does it. Information hiding increases the level of independence. Independence of modules ( low coupling ) is important for building and maintaining large systems. Chapter05, balls Class Fields New Class Fields A class defines the data fields held by its objects, together with logic for constructors of its objects and methods for accessing and operating on their data. A class may also define class fields,, which are shared by all its objects. The class itself holds a single instance of each of these fields its objects all work with the same instances. Compare this with data fields objects all work with their own (different) instances. Class fields are declared with the keyword: static. In the BouncingBall class, gravity is a class field whose common value is shared by all its instances the data fields xposition and yposition have values that are specific for each ball. 15

16 A Class Constant private static final int gravity = 3; private : specifies the visibilty of the declaration. static : specifies that all objects of this class share this field it belongs to the class. final : specifies that the value given cannot be changed it is a constant. Note: final can be applied to any field (class or data), method local variable or parameter. Writing class documentation Your own classes should be documented in the same way and to the same standard as the built- in library classes. Other people should be able to use your classes without reading their implementation. Make your class a library class Investigate Investigate Investigate!!!!!!!!! Java libraries are called packages Elements of documentation Documentation for a class should include: the class name a description of the overall purpose and characteristics of the class a version number author names documentation explaining each constructor and each method 16

17 Elements of documentation Documentation for each constructor and method should include: the name of the method the return type (if any) the parameter names and types a description of the purpose and function of each method a description of each parameter a description of the value returned (if any) javadoc - Class comments /** * The class represents a response * generator object. It is used to generate * an automatic response. * Michael Kölling and David J. Barnes 1.0 (30.Mar.2006) */ class... javadoc - Method comments /** * Change the colour of this shape. The new colour * becomes visible immediately. Legal colour strings * are "White", "Orange", "Blonde", "Pink", "Blue" * and "Brown". * colour The name of the new colour. true, if the colour was successfully changed; * false, if the colour name is invalid. */ public boolean changecolour (String colour)... 17

18 Review Java has an extensive class library (packages). A good programmer must know how to find things in the library and become familiar with those regularly used. The documentation tells us what we need to know to use a class (interface). The implementation is hidden (information hiding). We document our classes so that the interface can be read on its own (class comment, method comments). 18