Introduction to Java 2 Programming

1
Introduction to Java 2 Programming

• Lecture 4
• Writing Java Applications, Java Development Tools

2
Overview

• Java Programming Tools
• Command-Line Applications
• Practical Exercises

3
Java Programming Tools

• Javac the Java Compiler
• Java the Java Executable (virtual machine)
• Javadoc Java Documentation generator
• Jar Java ARchive creator
• All found in JAVA_HOME\bin

4
Javac the Java Compiler

• Generates bytecode (.class files) from source
  code (.java files)
• One class file per source file
• Usage example
• javac MyApplication.java
• javac com\ldodds\intro2java\Robot.java
• Javac expects source code to be organised into
  directories according to package structure
• One directory per dotted portion of package

5
Sample Directory Structure

• E.g
• package com.ldodds.intro2java

6
Java the Virtual Machine

• Executes the virtual machine and loads one or
  more classes into it
• Specify the class name (including packages) as a
  parameter
• Dont indicate the class file, but the class name
• Examples
• java MyApplication
• java com.ldodds.intro2java.Robot

7
Javadoc the Documentation Generator

• Automatically generates HTML documentation from
  Java source code
• Very efficient way to produce development
  documentation for your application.
• E.g
• javadoc com\ldodds\intro2java\Robot.java
• Output can be customised in a number of different
  ways,
• usually by adding special tags to the source
  code
• List of useful tags in the Tools Reference
  handout

8
Javadoc Example

• /
• A ltigtsimplelt/igt Calculator
• _at_author Leigh Dodds
• /
• public class Calculator
• /
• Adds two numbers together and returns the
  result
• /
• public int plus(int x, int y)
• return x y

9
Jar Tool

• Java ARchive
• Basically a zip file, used to package java
  classes
• E.g. for delivery as an applet or application
• Usually contain a Manifest
• An index of the contents of a jar file
• Major benefit is indicating which class holds the
  main method
• Allows an application to be launched
  automatically from a jar file
• E.g. by double-clicking the archive

10
The CLASSPATH

• The CLASSPATH is
• How Java finds compiled classes
• A system property
• A list of directories and/or jar files (similar
  to PATH)
• A common source of frustration!
• E.g
• CLASSPATHc\classesc\applications\app.jar
• Classify Java tools into three groups
• File based
• CLASSPATH based
• And mixed mode (I.e. use both)

11
File based tools

• Javadoc, jar
• Accept command-line parameters referring to
  files.
• Read the directory structure to find related
  files
• javadoc c\intro2java\src\intro2java\.java
• Result in file-based errors

12
CLASSPATH based tools

• java, javap
• Refer to classes and not files
• Ignore the file-system, except for those
  directories mentioned in the CLASSPATH
• Starts at those directories and walks down to
  find the correct .class files
• Can look inside JAR files to do the same
• Result in exceptions or errors, e.g.
  ClassNotFoundException NoClassDefFoundErrors

13
CLASSPATH based tools

• Example
• E.g.
• CLASSPATHc\src
• java com.ldodds.intro2java.Robot

14
Mixed mode tools

• javac
• Accept parameters referring to files
• BUT, Read the CLASSPATH to find related classes
• javac c\intro2java\src\intro2java\.java
• Results in file errors (relating to parameters),
  and cannot resolve symbol errors (relating to
  missing classes)

15
CLASSPATH Tips

• Always add the current working directory to the
  CLASSPATH
• SET CLASSPATHCLASSPATH.
• Good idea keep classes and source separate (e.g.
  bin and src directories)
• But need to be careful when compiling
• Use a global classes directory, e.g. c\classes
• Add this to the CLASSPATH
• Always compile into that directory
• SET CLASSPATHCLASSPATHc\classes
• javac d c\classes .java

16
Overview

• Java Programming Tools
• Command-line Applications
• Practical Exercises

17
Command Line Applications

• Java has no notion of executable
• Just loads classes into the virtual machine
• Starting a Java application involves calling a
  method
• Which may then create objects, call other
  methods, etc, etc
• There needs to be a starting point to trigger the
  application to run

18
The main method

• To turn a class into an application, give it a
  main method
• public static void main(String args)
• Must be of this format (otherwise Java cant find
  it)
• Can then be invoked from the command-line
• Do all the work to initialise the app (create
  objects, etc) in this method
• Minimise the amount of code in there just create
  an object or two, and call their methods.

19
Command Line Parameters

• Parameters are passed in as an array
• E.g.
• java MyApplication param1 param2 param3
• Can also use System Properties
• Parameters global to the whole virtual machine
• Get them with the System.getProperty() method
• //the command line
• java DpropertyNamepropertyValue MyApplication
• //In the code
• String value System.getProperty(propertyValue)
  

20
Overview

• More Syntax
• Constants, Strings, Arrays
• The Object Lifecycle
• Java Programming Tools
• Practical Exercises

21
The Calculator