Lecture 12 1

CS110 Lecture 12Tuesday, March 9, 2004

• Announcements– hw5 due Thursday– Spring break next week

• Agenda– questions– ArrayList– TreeMap

Lecture 12 2

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 12 3

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

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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 12 5

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:

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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!

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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: 617exchange: 287

PhoneNumber

number: 5000

Lecture 12 8

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)

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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

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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

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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 12 12

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

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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 12 14

TreeMap boxes and arrows

entries:

"shape"TreeMap

Dictionary

TreeMap

"quadrilateral"

"a shape with four sides"

entries:

"a geometric object in a plane"

•••

entries:

Definition

Stringdefinition:

entries:

Definition

Stringdefinition:

Object Object

Object Object

Lecture 12 15

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 <word> <word> … all

• Lookup.java sends toString messages to a Definition object (line 53) and to a Dictionary object (line 103)

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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()

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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

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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

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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 12 20

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( )

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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 12 22

Figure 4.5Object structure of a 2x3 Screen

0:

char[][]

1:

char[]

‘ ‘

5

1:

2:

0:

‘ ‘

‘ ‘ intwidth:

intheight:

char[][]pixels:

2

3

char[]

‘ ‘

5

1:

2:

0:

‘ ‘

‘ ‘

Screen

Lecture 12 23

hw6

• Due Thursday after Spring break

• TreeMap practice– class Directory– little Bank, using a TreeMap