Slide 1Array BasicsCreating Empty ArraysProcessing ArraysArrays as Reference TypesMultidimensional ArraysArrays utility classAssignment 2Java Collection FrameworkList<E>ArrayList<E>Iterator Design PatternIterator Design PatternIterator<E>Iterable exampleArrays, List<E> and Iterator<E>COMP 401, Spring 2013Lecture 61/29/2013Array Basics•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 particular, size of array is available through length field•Declaring variables that can hold an array reference–type[] variable_nameCreating 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 after you create the array to point to either existing or new object.•Can create and initialize 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)};•lec6.v1Processing Arrays•Often need to loop over all values of an array–Common pattern for doing this:for (int i=0; i<a.length; i++) {// Deal with a[i]}•for – each construct can be used if index variable is not actually needed for anything other than retrieving the elementfor (Point p : parray) {// Loop variable p set to each element// of parray in turn.}•lec6.v2Arrays as Reference Types•Same reference, same array–Saw this a little bit in lec6.v1–Implication for arrays passed to methods•When an array is passed to a method, any changes that the method makes to its elements is permanent.–Potential danger with object state•If holding object state in an array, easy to accidentally break encapsulation and expose object state to alteration.•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•Equivalent to array copy code example in lec6.v2•lec6.v3Multidimensional Arrays•Multidimension array is simply an array of arrays–Fill out dimensions left 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.–Sometimes known as as a “ragged” or “uneven” arrayint[][] 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];•lec6.v4Arrays utility class•Arrays is a library of useful functions for manipulating arrays–Note “s” in Arrays–Like Math class, all methods are static•binarySearch•sort•filling and copying subrangesAssignment 2•Be sure to clone array of Point objects passed to PolygonImpl constructor.•Similarly, be sure to clone array to return as a result of getPoints(). Don’t refer back to original internally held array!!•Notes about area and centroid formulas•Tester will be released tomorrow.Java Collection Framework•Arrays are not resizeable–Often need a collection of items that acts like an array than can grow or shrink•Java Collection Framework–In package java.util–Defines a set of interfaces for resizeable collections•List–Ordered by integer index•Set–Unordered, no duplicate elements•Map–Indexed by an arbitrary key–Sometimes known as a “dictionary” –Provides a set of classes that implement these interfaces in specific ways•Examples for List: ArrayList, LinkedList,•Java Collection Framework interfaces and classes are “generic” –Interface/class is modified to specify type of object in collection–For now, just want to have working knowledge of List and ArrayListList<E>•boolean add(E val)–Adds val to end of list and returns true•void add(int index, E val)–Inserts val as position index•E get(int index)–Returns element at position index•E set(int index, E val)–Sets element at position index, returns original value for element•E remove(int index)–Removes element at position index, list shrinks•int size()–Returns current size of list•boolean isEmpty()–Same as testing size() == 0•E[] toArray(E[] a)–Converts list to an array given current elementsArrayList<E>•ArrayList<E> is an implementation of List<E>–Uses an array internally•lec6.v5Iterator Design Pattern•“Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation”–Gang of Four, Design Patterns•Consider:for(int i=0; i<slist.size(); i++) {Song next_song = slist.get(i);// Do something with next_song.}COMP 401 :: Spring 2012 12Iterator Design Pattern•Iterator object encapsulates details of item traversal.–Understands details of the underlying collection.–Manages order of items•May want a traversal that is not just first to last.•Underlying collection may not be linear.–Manages state of traversal•Allows traversal to be picked up again later.•Assumption: underlying collection is not changed or modified while the traversal is occurring.COMP 401 :: Spring 2012 13Iterator<E>•boolean hasNext()–Are we at the end of the traversal?•E next()–Get the next item of the traversal.–Throws a runtime exception if no next item.•void remove()–Not supported by all implementations.–Safely removes last item retrieved by next from the underlying collection.•Iterable<E>–Has an iterator() method that provides an Iterator<E> COMP 401 :: Spring 2012 14Iterable example•lec6.v6–Main1•Simple use–Main2•Parallel iteratorsCOMP 401 :: Spring 2012
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