Lecture 121 CS110 Lecture 12 Tuesday, March 9, 2004 Announcements –hw5 due Thursday –Spring break next week Agenda –questions –ArrayList –TreeMap

Lecture 122 ArrayList A Collection class in the Java API Like an array: stores elements at positions 0,1,… Can grow as needed! Unlike an array in other ways too –stores only objects (not primitive types) –can store different kinds of objects simultaneously –you must cast when retrieving –send explicit messages instead of using [ ] for access Read ArrayListDemo.java

Lecture 123 Casting Syntax: (Bar)foo tells the compiler to treat object foo as something of type Bar ArrayList get returns an anonymous Object for (int i = 0; i<myList.size(); i++ ){ SimpleObject foo = (SimpleObject)myList.get(i); System.out.println (i + "\t" + foo.name); } Perhaps the compiler ought to know, but it doesn’t

Lecture 124 ArrayList API can store: any Object, not primitive types declaration: ArrayList myList creation: myList = new ArrayList( ) put at end: myList.add( obj ) put at index: myList.add( index, obj ) –moves later entries down the list get: (Type)myList.get( index ) –cast to proper Type after get replace: myList.set( index, obj ) remove: myList.remove( index ) size: myList.size( ) looping: for( ; ; )

Lecture 125 Boxes and arrows for ArrayList ArrayList Object 0: String SimpleObject name: String SimpleObject name: String SimpleObject name: Object 1: Object 2: "zero " " one " " two " ArrayList myList:

Lecture 126 hw5 array practice (sorting) ArrayList practice: modify Bank.java so that it uses an ArrayList of BankAccounts instead of an array –BankAccounts have numbers 0, 1, 2, … –Banker can open as many accounts as she likes!

Lecture 127 Maps arrays and ArrayLists locate entries by index –index is an integer position, starting at 0 A Map locates entries by key –key is often a String (think dictionary, phone book) A map stores key-value pairs –key: “Java” value: “a modern OO language” –key: “UMass” value: areaCode: 617 exchange: 287 PhoneNumber number: 5000

Lecture 128 Duplicates? The same value may appear more than once in a collection (array, ArrayList or Map) –my wife and I have the same phone number –“field” and “instance variable” have the same definition –in an array, foo[3] may == foo[7] In a Map, keys are unique If you want to arrange for one person to have more than one phone number or one word to have more than one definition you need to work harder (see last slides, JOI Chapter4)

Lecture 129 TreeMap Java API provides class TreeMap Don’t ask why it’s called that Key can be any Object (but our keys will always be String objects) TreeMaps, like ArrayLists –can grow –store Objects (references, not primitive types) –are heterogeneous can hold objects of different types need to cast what you get from a TreeMap

Lecture 1210 Directory Model windows folder Contains TextFiles (not other Folders - wait for Chapter 5) API: create, add file to, get file from, get size, get owner, get create/mod date Design: Directory object has a TreeMap field storing TextFile objects keyed by String filename You write this for homework

Lecture 1211 Dictionary Model a real dictionary A collection of definitions of words –A word is a String –A definition will be an instance of class Definition Design: Dictionary object has a TreeMap field storing Definition objects keyed by String words API: create, add entry, look up entry, get size, arrange for printing the whole Dictionary

Lecture 1212 Dictionary.java Architecture: a Dictionary object is a thin wrapper around a TreeMap Prepare to use classes in java.util package (line 6) private TreeMap instance variable –declared on line 18 –created in constructor (line 26) Put a Definition in this Dictionary: addEntry method (36) –parameters: String word, reference to Definition instance –delegate to TreeMap put method

Lecture 1213 Dictionary (continued) Look up a Definition: getEntry (line 48) –delegate to TreeMap get method, passing String word as the argument (line 50) –cast retrieved anonymous object to Definition –return the Definition ( null if not found) How large is this Dictionary? getSize (line 61) –delegate to TreeMap size method What’s a Definition? –just one String field, set in constructor, access by public getter (toString) –would be richer in a real dictionary (pronunciation, etymology, …)

Lecture 1214 TreeMap boxes and arrows entries: "shape" TreeMap Dictionary TreeMap "quadrilateral" "a shape with four sides" entries: "a geometric object in a plane" entries: Definition String definition: entries: Definition String definition: Object

Lecture 1215 Class Lookup Dictionary has no unit test Class Lookup is a client for Dictionary (and tests it thoroughly) All of Lookup is static > java Lookup … all Lookup.java sends toString messages to a Definition object (line 53) and to a Dictionary object (line 103)

Lecture 1216 Looping on a TreeMap To print the whole Dictionary, Lookup sends a toString message, invoking Dictionary toString method (line 70) Subtle, since there’s no index to loop with Uses an Iterator object - Java tool custom designed for looping Iterator API has just two methods: –boolean hasNext() –Object next()

Lecture 1217 Set allWords = entries.keySet(); Iterator wordIterator = allWords.iterator(); ask the entries field (a TreeMap) for its keySet ask the keySet to give you an Iterator wI is like a list of the keys in the entries Map You can infer from this code that –Set and Iterator are classes in the Java API –keySet is a method in class TreeMap; it returns a Set –iterator is a method in class Set; it returns an Iterator Getting an Iterator

Lecture 1218 while ( wordIterator.hasNext() ) { word = (String)wordIterator.next(); definition = this.getEntry( word ); str += word + ":\n" + definition.toString() + "\n"; } hasNext() returns false when at end of list next() returns a reference to the next Object in the list cast that to a String since that’s what the key is Using an Iterator

Lecture 1219 while ( wordIterator.hasNext() ) { word = (String)wordIterator.next(); definition = this.getEntry( word ); str += word + ":\n" + definition.toString() + "\n"; } use the key to look up a Definition send the Definition a toString message add two lines to the String str we are building to represent the whole Dictionary Building a multiline String

Lecture 1220 TreeMap summary declaration: TreeMap mapName; creation: new TreeMap( ); put: mapName.put(Object key, Object obj) get: (Type)mapName.get(Object key) cast to proper Type length: mapName.size( ) looping: use Iterator mapName.keySet( ).iterator( )

Lecture 1221 Collections of collections Dictionary might map a word to an ArrayList of definitions Screen maintains a private field that’s an array of arrays of char: private char[][] pixels;

Lecture 1222 Figure 4.5 Object structure of a 2x3 Screen 0: char[][] 1: char[] ‘ 5 1: 2: 0: ‘ int width: int height: char[][] pixels: 2 3 char[] ‘ 5 1: 2: 0: ‘ Screen

Lecture 1223 hw6 Due Thursday after Spring break TreeMap practice –class Directory –little Bank, using a TreeMap