Introduction to Java 2 Programming

Introduction to Java 2 Programming Lecture 4 Writing Java Applications, Java Development Tools

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

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

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

Sample Directory Structure E.g: package com.ldodds.intro2java;

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 • Don’t indicate the class file, but the class name Examples: java MyApplication java com.ldodds.intro2java.Robot

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

Javadoc Example /** * A <i>simple</i> Calculator * * @author Leigh Dodds */ public class Calculator { /** * Adds two numbers together and returns the result */ public int plus(int x, int y) { return x + y; } }

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

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: CLASSPATH=c:\classes;c:\applications\app.jar • Classify Java tools into three groups: • File based • CLASSPATH based • And “mixed mode” (I.e. use both)

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

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

CLASSPATH based tools • Example: • E.g. CLASSPATH=c:\src java com.ldodds.intro2java.Robot

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)

CLASSPATH Tips • Always add “the current working directory” to the CLASSPATH SET CLASSPATH=%CLASSPATH%;. • 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 CLASSPATH=%CLASSPATH%;c:\classes javac –d c:\classes *.java

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

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

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 can’t 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.

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 –DpropertyName=propertyValue MyApplication //In the code String value = System.getProperty(“propertyValue”);

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

The Calculator

Introduction to Java 2 Programming