MSc IT

1
MSc IT Computing Object Oriented Programming

• Java
• Lecture 2

2
Topics Covered

• Packages
• Javadoc
• Scope
• Review
• Phase test 1 - Formative

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Packages
4
What is a Package?

• Java allows you to group classes together in a
  collection called a package
• Packages are used to
• organise work
• separate your work from libraries provided by
  others
• most importantly to avoid naming clashes

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Using Packages

• We have used packages throughout our programming
• importing classes from java.awt, java.util, etc
• We have also used java.lang, but without an
  import statement
• that is assumed
• An import statement merely allows us to use a
  class name without its package name
• e.g. use Hashtable rather than java.util.hashtable

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Importing a Package

• We may import an entire package makes all the
  classes defined there available
• import java.util.
• Or separate classes
• import java.util.Hashtable
• There is no overhead at compile or run time in
  using .
• the only possible problem would be when two
  classes from different packages have the same
  name
• then only the classes required are imported
• if more than one with the same name are actually
  needed, the full names are used

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Naming Packages

• All our own work so far has gone into the default
  package
• Packages are named hierarchically
• all java packages start with java.
• except swing, which retains the javax.
  (extension) package name
• from when it was an extension
• Sun recommends that using your companys domain
  name, reversed, as a package prefix
• e.g. uk.ac.dmu.mypackage

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• The first line of a file specifies the package to
  which its classes belong
• Eg the first line of the Class file for
  GregorianCalender.java is
• package java.util
• Therefore the first line of a file creating
  mypackage would be
• package uk.ac.dmu.mypackage

9
Locating Packages

• The default package has no package path
• if the current directory (.) is part of the class
  path
• for practical use when developing programs, it
  should be
• then the classes without a package declaration in
  the current directory automatically become part
  of the default package
• JDK stores packages in subdirectories or ZIP or
  JAR files
• e.g all files belonging to java.util are in a
  subdirectory java\util (or java/util on UNIX)
• Subdirectories need not branch off the root, they
  can branch off at any point in the class path
• e.g if the java packages are in K\JDK.1.2.1\BIN
  then java.util will be found in
  K\JDK1.2.1\BIN\JAVA\UTIL

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Compiling

• When the java compiler searches for a class it
  firstly examines all files in the default
  package, those are in all directories directly on
  the class path
• It then examines the imported packages, by
  concatenating the class file name and package
  names with all the directories on the class path
• e.g. suppose a class path of k\jdk1.2.1\bin\
• there are imports of java.util and java.awt and
  the compiler is looking for a class called
  Example.class

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• It searches all files in the bin directory, then
  the current directory (.) for Example.class
• Then it looks for
• k\jdk1.2.1\bin\java\util\Example.class
• k\jdk1.2.1\bin\java\awt\Example.class
• .\java\util\Example.class
• .\java\awt\Example.class
• The java.lang package is always searched.
• You must be sure that you place the class file of
  any package you write in a suitable subdirectory
  from the class path
• e.g. \uk\ac\dmu\mypackage
• In practise you will work in the default package
  until your code assumes its final form
• If you distribute your application, dont forget
  to install the accompanying package

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JavaDoc
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What is Javadoc

• Accurate documentation of code is essential
• The javadoc utility produces documentation in the
  same form as the API documentation
• examines the source code
• for classes, methods and / ./
• the API documentation is produced by javadoc
• Output is produced as HTML files
• Code should always be self documenting
• separate documentation will tend to be forgotten
  as code is updated
• use of javadoc ensures that the documentation can
  be read separately

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Using Javadoc

• Comments are placed within / /, and placed
  before the feature described
• Comment the following features
• Package
• Public class
• Public interface
• Public and protected methods
• Public and protected variables and constants
• Comments consist of free form text starting with
  a summary statement
• Some HTML tags may be included, such as ltBgt lt\Bgt
  for bold
• dont use headings

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• You may then place tags such as
• _at_see packagenamemethodname(parameterlist)
• note separates the class name from the method
  name
•  
• /
• A class providing keyboard reader facilities
• _at_version 1
• _at_author Susan
• /

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• Method comments include tags
• _at_param
• _at_return
• _at_throws
•  
• /
• method reads from keyboard and returns a String
• _at_return String
• /

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• To run javadoc you may work in the directory
  containing the code and specify the directory for
  the results
• javadoc d docDirectory nameOfPackage
• To document a single class in the current
  directory
• javadoc KeyboardInput.java
• See online documentation for more details
• See KeyboardInput.java example

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Scope
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Scope

• Identifiers are only valid within the block of
  code in which they are created
• Class variables are global
• ie valid in all methods within the class
• Method variables are valid only within the method
• Loop Control Variables (eg for() loops) are only
  valid within the loop
• Correct code indentation assists in understanding
  scope

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Eg KeyboardInput.java

• private BufferedReader in new BufferedReader
  (new InputStreamReader(System.in))
• in is a global identifier
• public synchronized boolean readBoolean()
• String input ""
• try
• input in.readLine()
• catch (IOException e) //nothing done with
  error!
• boolean val
• val (Boolean.valueOf(input)).booleanValue()
• return val
• input val are local to the readBoolean() method
• Reused within the other methods

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Review
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We have covered

• Statements
• Classes
• Objects Instantiation
• Loops
• for(), while(), do while()
• Arrays
• 1- and 2- dimensional
• Using UML
• Constructors Overloading

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References Exercises

• References
• http//java.sun.com/j2se/javadoc/writingdoccomment
  s/index.html
• http//tinf2.vub.ac.be/dvermeir/java/other_doc/Ja
  vaPackages.html
• Exercises
• Read through Chapter 3 of Java Gently (much of
  this will be revision, but there is a useful
  section on boolean expressions)
• Do problem 3.3 on page 109, and problem 3.18 on
  page 112
• If you complete all of this, try any (or all) of
  the remaining problems at the end of Chapter 3

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Summary

• Packages
• organise work
• separate your work from libraries provided by
  others
• most importantly to avoid naming clashes
• Javadoc
• produces documentation in HTML format
• easy to update/maintain
• Scope
• validity of identifiers