comp401sp14lec07arrays collections iterator

Arrays, List<E> and Iterator<E>

COMP 401, Spring 2014 Lecture 7 1/30/2014

Array Review •  Ordered sequence of elements –  Elements must be of the same type –  Fixed size once created –  Valid indices from 0 to length-‐1

•  Arrays are reference types – Have fields and methods like other object types –  In parUcular, size of array is available through length field

•  Declaring variables that can hold an array reference –  type[] variable_name

CreaUng Empty Arrays •  With new keyword as in:

–  int[] iarray = new int[10]; –  Point[] parray = new Point[7];

•  What do we mean by empty? –  For array of value of types

•  Elements get default value –  0 for numeric –  false for boolean

–  For array of reference types •  Elements set to null •  You need to set each element a`er you create the array to point to either

exisUng or new object. •  Can create and iniUalize array with literal syntax as in:

–  int[] iarray = new int[] {1, 2, 6, 8, 10}; –  Point[] parray = new Point[] {new Point(0,0), new Point(1,2), new

Point(3,4)};

Arrays as Reference Types •  Same reference, same array –  ImplicaUon for arrays passed to methods

•  When an array is passed to a method, any changes that the method makes to its elements is permanent.

–  PotenUal danger with object state •  If holding object state in an array, easy to accidentally break encapsulaUon and expose object state to alteraUon.

•  Array cloning –  Easy way to create a “shallow” copy of an array –  Just call clone() method

•  Result will be a new array of same size with same values or references

MulUdimensional Arrays •  MulUdimension array is simply an array of arrays

–  Fill out dimensions le` to right. int[][] marray = new int[5][];!for(int i=0; i<5; i++) {!

marray[i] = new int[10];!}!

•  Each Sub-‐dimension can have independent size. –  SomeUmes known as as a “ragged” or “uneven” array int[][] marray = new int[5][];!for (int i=0; i<5; i++) {!

marray[i] = new int[i+1];!}!

•  If each sub-‐dimension is same size, can create with a single new statement –  int[][] marray = new int[5][10];

Arrays uUlity class

•  Arrays is a library of useful funcUons for manipulaUng arrays – Note “s” in Arrays – Like Math class, all methods are staUc

•  binarySearch •  sort •  filling and copying subranges •  hnp://docs.oracle.com/javase/7/docs/api/java/uUl/Arrays.html

Java CollecUon Framework •  Arrays are not resizeable

–  O`en need a collecUon of items that acts like an array that can grow or shrink

•  Java CollecUon Framework –  In package java.uUl –  Defines a set of interfaces for resizeable collecUons

•  List –  Ordered by integer index

•  Set –  Unordered, no duplicate elements

•  Map –  Indexed by an arbitrary key –  SomeUmes known as a “dicUonary”

–  Provides a set of classes that implement these interfaces in specific ways •  Examples for List: ArrayList, LinkedList,

Java CollecUons as Generics

•  Java CollecUon Framework interfaces and classes are “generic” –  Interface/class name is modified to specify type in the collecUon. •  ArrayList<E> implements List<E> •  E is a placeholder •  Must be filled in by an actual type name when used.

– For now, just want to have working knowledge of List as an interface and ArrayList as a collecUon that implements the interface.

List<E> •  boolean add(E val)

–  Adds val to end of list and returns true •  void add(int index, E val)

–  Inserts val as posiUon index •  E get(int index)

–  Returns element at posiUon index •  E set(int index, E val)

–  Sets element at posiUon index, returns original value for element •  E remove(int index)

–  Removes element at posiUon index, list shrinks •  int size()

–  Returns current size of list •  boolean isEmpty()

–  Same as tesUng size() == 0 •  E[] toArray(E[] a)

–  Converts list to an array given current elements

ArrayList<E>

•  ArrayList<E> is an implementaUon of List<E> – Uses an array internally

•  lec07.ex1

Design SituaUon

•  Suppose we have an object that encapsulates some sort of collecUon. – SongLibrary

•  A collecUon of songs in an iTunes-‐like system

– PolygonModel •  A collecUon of polygons in a 3D modeling system

Design SituaUon

•  Now suppose we have code outside of this collecUon object that needs to examine each element of the underlying collecUon in turn. – SongFilter

•  A object that represents a search criteria to be applied to a collecUon of songs

– An intersecUon test in which each polygon of a PolygonModel needs to be evaluated

Strategy 1: Provide access to underlying collecUon as an array.

•  SongLibrary –  public Song[] getSongs()

•  PolygonModel –  public Polygon[] getPolygons()

•  Drawbacks? – May have to do a lot of work to create the array –  CollecUon may be result of generaUve process

•  There may be no “end” to the collecUon. •  Or the collecUon may be large so we don’t want to provide the whole thing at once.

Strategy 2: Provide index access to each underlying item in collecUon

•  SongLibrary –  public int getNumSongs(); –  public Song getSong(int song_idx);

•  PolygonModel –  public int getNumPolygons(); –  public Polygon getPolygon(int polygon_idx);

•  Drawbacks? – Doesn’t help with generaUve collecUons –  Imposes restricUons on how collecUon is represented and linearized

– Deteriorates encapsulaUon

Strategy 3: Internalize a “cursor”

•  SongLibrary – public void resetSongCursor(); – public Song getNextSong(); – public boolean isCursorAtEnd();

•  Drawbacks? – Can’t have two traversals going at the same Ume. – But, this does come close.

Iterator Design Panern

•  “Provide a way to access the elements of an aggregate object sequenUally without exposing its underlying representaUon” – Gang of Four, Design Pa8erns

•  Consider: for(int i=0; i<slist.size(); i++) {!

Song next_song = slist.get(i);!// Do something with next_song.!

}!

Iterator Design Panern •  Iterator object encapsulates details of item traversal. –  Understands details of the underlying collecUon. – Manages order of items

•  May want a traversal that is not just first to last. •  Underlying collecUon may not have a natural linear order.

– Manages state of traversal •  Allows traversal to be picked up again later.

•  AssumpUon: underlying collecUon is not changed or modified while the traversal is occurring. –  Iterator should be able to detect this and signal an error –  Variant of panern will have iterator provide methods that modify underlying collecUon safely

Components of Iterator Panern •  CollecUon object is “iterable” –  Provides a method that returns an object that acts as an iterator.

•  Iterator object provides access to the elements in turn. – At the very least:

•  A method to test whether more items exist. •  A method to retrieve the next item.

– Other possible features: •  Methods that remove an item safely. •  Method to “peek” at the next item. •  Method to reset the traversal.

Java Iterator Panern Interfaces

•  The Java CollecUons Framework defines two generic interfaces for supporUng the iterable design panern –  Implemented by the various collecUon types such as List<E>, Map<E>, Set<E>, etc.

•  Iterable<E> –  Interator<E> iterator()

•  Iterator<E>

Iterator<E>

•  boolean hasNext() – Are we at the end of the traversal?

•  E next() – Get the next item of the traversal. – Throws a runUme excepUon if no next item.

•  void remove() – Not supported by all implementaUons. – Safely removes last item retrieved by next() from the underlying collecUon.

Iterable examples

•  lec07.ex2 – Main1

•  Simple use

– Main2 •  Parallel iterators

– Main3 •  Simultaneous iterators

– Main4 •  for – each syntacUc sugar

Main1 Visualized (1) ArrayList<Song> slist

0 Words and Guitar

Dig Me Out

Jenny

Linle Babies

Buy Her Candy

1

2

3

4


0 Words and Guitar

Dig Me Out

Jenny

Linle Babies

Buy Her Candy

1

2

3

4

Iterator<Song> iter

0 next_idx

list


0 Words and Guitar

Dig Me Out

Jenny

Linle Babies

Buy Her Candy

1

2

3

4

Iterator<Song> iter

0 next_idx

list

public boolean hasNext() { if (next_idx < list.size()) { return true; } return false;

}

NOTE: This may or may not be how it is actually implemented, but it is effec:vely what is going on.


0 Words and Guitar

Dig Me Out

Jenny

Linle Babies

Buy Her Candy

1

2

3

4

Iterator<Song> iter

1 next_idx

list

public Song next() { Song s = list.get(next_idx); next_idx++; return s;

}

NOTE: Real implementaUon would first check to see if hasNext() is sUll true and throw an excepUon otherwise.

lec07.ex2.Main2

•  Parallel iteraUon – Processes two different lists

•  Iterator associated with each. •  Iterators advance unevenly

lec07.ex2.Main3

•  Simultaneous iteraUon – 2 Iterators, 1 List

•  Insert your own joke here.

for -‐ each •  Java provides “syntacUc sugar” for a parUcularly common use of iterators. –  for-‐each loop –  Supposing e_coll is Iterable<E>, then these are equivalent:

Iterator<E> iter = e_coll.iterator();!while (iter.hasNext()) {!

!E elem = iter.next();!!// Do something with element!

}!!

for (E elem : e_coll) {!!// Do something with elem!

}!

•  lec07.ex2.Main4

for-‐each with Array

•  The for-‐each construct also works with Arrays •  Useful if you need to process the elements of an array but do not need the index.

String[] names = new String[] {“Amy”, “Mike”,! “Cameron”, “Claire”};!for (String n : names) {!

System.out.println(n);!

}!

lec07.ex3

•  A more complicated iterator – Can build iterators that do things other than just go through every item. •  Prior examples made use of Iterator<E> built into List<E>, here we are going to implement our own specialized version of Iterator<E>

comp401sp14lec07arrays collections iterator

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