Grouping objects

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Grouping objects

• Arrays, Collections and Iterators

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Main concepts to be covered

• Arrays
• Collections
• Iterators

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Requirement to group objects

• Many applications for collections of objects
• Personal organizers
• Library catalogs
• Student-record system
• The number of items to be stored varies
• Items added
• Items deleted

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Fixed-size collections

• Programming languages usually offer a special
  fixed-size collection type an array
• Arrays are built-in, use syntax
• Java arrays can store objects or primitive-type
  values
• Maximum collection size must be fixed at Array
  creation time
• How is Array creation time more dynamic than in
  other programming languages, such as C?

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Creating an array object
public class LogAnalyzer private int
hourCounts private LogfileReader reader  
public LogAnalyzer() hourCounts
new int24 reader new
LogfileReader() ...
Array variable declaration
Array object creation
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The hourCounts array
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Using an array

• Square-bracket notation is used to access an
  array element hourCountshour
• Elements are used like ordinary variables
• In an expression
• adjusted hourCountshour 3
• hourCountshour

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Class libraries

• Collections of useful classes
• Encourages reuse of design and code
• Java organizes its libraries in packages
• The java.util package includes classes for
  grouping objects in collections

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A personal notebook

• Notes may be stored
• No limit to the number of notes
• It tells how many notes are stored

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import java.util.ArrayList / ...
/ public class Notebook // Storage for an
arbitrary number of notes. private ArrayList
notes   / Perform any
initialization required for the notebook.
/ public Notebook() notes
new ArrayList() ...
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Object structures with collections
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Features of ArrayList

• It keeps the objects in order
• It increases its capacity as necessary
• It keeps a private count size() accessor
  retrieves it

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Using ArrayList collection
public class Notebook private ArrayList
notes ...   public void storeNote(String
note) notes.add(note)  
public int numberOfNotes() return
notes.size() ...
Adding a new note
Returning the number of notes (delegation).
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Adding a third note
myBook.add(1130 meet John)
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Retrieving an object
public void showNote(int noteNumber)
if(noteNumberlt0 noteNumber gt numberOfNotes())
// This is not a valid note number.
else System.out.println(notes.get(noteNumber))

Retrieve and print the note
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Removal may affect numbering
myBook.remove(1)
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Review ArrayList

• Items may be added and removed
• Each item has an index
• Index values may change if items are removed or
  further items added
• The main ArrayList methods are add, get, remove
  and size
• For more methods, see API document

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Collections framework

• Goals comparable to Cs STL
• Data structures
• Vector, LinkedList, HashSet, etc.
• Algorithms
• Sorting, searching, shuffling, etc.

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Loops and bugs

• Why are loops often a cause of bugs?
• What kind of bugs?
• Wouldnt be nice to avoid these bugs?

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Iterating over a collection
Returns an Iterator object
java.util.Iterator
Iterator it myCollection.iterator() while(it.ha
sNext()) call it.next() to get the next
object do something with that object
public void listNotes() Iterator it
notes.iterator() while(it.hasNext())
System.out.println(it.next())
How does Iterator help avoid bugs?
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Review loops and Iterator

• Java while and for loops are similar to C
• Similar bugs, too!
• Collection classes have special Iterator objects
  that simplify iteration over the whole collection
• Bounds-checking methods help avoid bugs