Java Background v1.1

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Java Backgroundv1.1
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topics

• simplest Java program
• Java platform overview
• getting the software you need
• why did Sun create Java?
• Files (.java, .class, .jar)

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1 the simplest Java program
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simplest program in C and Java
include int main()
printf(Hello in C\n") return 0

• public class Hello
• public static void main(String args)
• System.out.println("Hello in Java")

Like C, Java is case-sensitive, and statements
end with You need a console to see the
results. Java file name must be Hello.java.
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simplest program in Java using Window from
library

• import java.awt.
• import javax.swing.
• public class Hello
• public static void main(String args)
• JFrame myWindow new JFrame()
• myWindow.setSize(new Dimension(400,275))
• myWindow.setTitle("Hello in Java")
• myWindow.setVisible(true)

Uses the JFrame class in the library to create a
Window. Java file name must be Hello.java.
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simplest program in Java extending Window from
library

• import java.awt.
• import javax.swing.
• public class Hello extends JFrame
• public static void main(String args)
• new Hello().run()
• private void run()
• this.setSize(new Dimension(400,275))
• this.setTitle("Hello in Java")
• this.setVisible(true)

Extends the JFrame class in the library to create
a Window.
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2 Java platform overview
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Java is a platform

• Java programs require a Java Virtual Machine
  (JVM) to run
• the JVM is available for Linux, Windows, Mac and
  Solaris
• commonly also called the Java Runtime Environment
  (JRE)
• free download from Sun
• the JRE is pre-installed in Mac OS X
• Windows users have to download and install it
  themselves
• NOTE the JRE does not include the Java compiler
• developers need the JDK instead (or in addition
  to)
• the Java compiler is in the Java Development Kit
  (JDK)
• installing the JDK gives you the compiler and the
  JRE
• the JDK does not include an integrated
  development environment (IDE)
• you can one of several free IDEs for Java
• Eclipse (free from IBM)
• Netbeans (free from Sun)
• BlueJ (free from Monash University)

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Just in Time (JIT) compiler in Java runtime (JVM)
compilation
before installation, or the first time each
method is called
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the JDK includes lots of libraries

• Java programs can call a huge body of pre-written
  code
• these reusable components are called the Class
  Libraries
• sometimes they are also called packages
• they are also called the Java APIs
• the libraries are designed to be used identically
  regardless of OS

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who implements JVMs?

• Sun Microsystems
• Java HotSpot Virtual Machine for Windows, Linux
  and Solaris
• Hewlett-Packard
• Java runtime for HP-UX, OpenVMS, Tru64,
  Reliant(Tandem) UNIX)
• IBM
• Java runtime for MVS, AIX, OS/400, z/OS
• Apple Computer
• MacOS Runtime for Java (MRJ)
• the full version of J2SE, version 1.4.2, is part
  of Mac OS X
• includes the JDK (compilers, etc.) and the
  HotSpot Virtual Machine
• BEA Systems
• JRockit (for their web server)

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3 getting the software you need
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versions of Java

• Java 1 - 1996
• Java 1.0 original, not a very good version
• Java 1.1 greatly revised and improved (1997)
• Java 2 - 1998
• Java 1.2 includes Swing (for graphical user
  interfaces)
• Java 1.3 more new packages, no new syntax (2000)
• Java 1.4 introduced the assert statement (2002)
• Java 5 - 2004
• Java 1.5 quite a bit of new syntax, including
  generics
• I am using JDK v5.0 Update 9 of the J2SE (or SE)
• Update 9 is the current sub-version as of
  October 2006

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editions

• Java SE (Standard Edition) is for desktop
  applications (a.k.a. J2SE)
• Java EE (Enterprise Edition) is for web servers
  (a.k.a. J2EE)
• Java ME (Micro Edition) is for devices such as
  pagers and mobile phones (a.k.a. J2ME).

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what languages require a JVM?

• Java used to be pretty much the only language
• there are now efforts by other languages to use
  JVM
• Jython (formerly known as JPython)
• except for some standard modules, Jython programs
  use Java classes instead of Python modules
• Jython was fairly complete
• its development stopped in 2005 when Microsoft
  hired its creator to work on IronPython ( which
  targets .NET instead)
• JRuby
• tightly integrates Ruby with Java, with full
  two-way access between the java and the Ruby
  code.
• this year (Sept.), Sun Microsystems hired JRuby's
  two creators to work on JRuby full time
• Groovy is a new dynamic language targeted at
  the JVM

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the three programs you need to write Java

• The Java Runtime Environment (JRE)
• this is the virtual machine that executes Java
  programs
• the Java compiler and associated utilites
• a way to edit source code
• youll want an integrated development environment
  (IDE) with
• an editor for source code
• a debugger, to help you find your mistakes
• a viewer to see the parts of your program
• an easy way to compile and run programs
• an easy way to view documentation
• some good IDEs for Java
• BlueJ (free) ? I used this
  one first
• Eclipse (free from IBM) ? I use this one now
• Netbeans (free from Sun) ? well regarded I
  havent used it

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what I am using

• Java Development Kit (JDK) 5.0 of the J2SE
• from http//java.sun.com/javase/downloads/
• get the JDK, not the JRE
• get the Standard Edition (not the Enterprise
  Edition)
• the JDK gives you both the runtime and the
  compiler
• its OK if your machine already has a JRE
  installed
• if your computer cant run Java 5, get Java
  version 1.4.2
• Eclipse IDE
• from http//eclipse.org/ (current version is
  3.2.1)
• unzip into a folder
• run eclipse.exe
• more details at http//harbormist.com/AcmJava
• NOTE
• Java SE (Standard Edition) is for desktop
  applications (a.k.a. J2SE)
• Java EE (Enterprise Edition) is for web servers
  (a.k.a. J2EE)
• Java ME (Micro Edition) is for devices such as
  pagers and mobile phones.

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the Eclipse welcome screen
go to the workbench
overview
tutorials
samples
whats new
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the Java perspective
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projects in the workspace

• In Eclipse, classes are organized into projects
  (complete programs)
• each project is kept in a folder
• Eclipse projects are kept in a workspace, which
  is just a folder containing project folders
• when you first start Eclipse, you will be asked
  to choose a workspace (folder)
• you can set this as default, so you use the same
  workspace every time
• you can tell Eclipse to ask you for the workspace
  each time it starts up

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starting a new project

• choose File ? New ? Project... ? Java Project and
  give your new project a name
• this will also be the name of the folder that is
  created in the workspace
• choose File ? New ? Class and give your new class
  a name
• you can also check the box that asks Eclipse to
  generate a main method for you
• start editing!

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compiling and running

• you dont compile your program Eclipse does that
  automatically as you type
• it also points out errors as you go along
• the first time you run your program, choose Run ?
  Run As... ? Java Application
• select the default package first, then
  right-click
• after that, you can just click the green circle
  with a triangle in it
• or, you can use the drop-down list to the right
  of the green circle to choose a particular
  program to run

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importing code into Eclipse

• if you have a program that is outside Eclipse
• on the file system, create a folder for the
  project in your Eclipse workspace folder
• move all the .java source files to that folder
• choose File ? New ? Project... ? Java Project and
  type in the name of your new folder
• Eclipse will recognize it and ask if you want to
  make the folder into a new project just click
  Finish
• alternative method
• In Eclipse, create an empty project with the
  desired name
• Choose File ? Import and follow instructions to
  import .java files from the file system into
  Eclipse

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4 why did Sun create Java?
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C and C perceived common problems

• pointers are dangerous
• memory leaks (failing to free memory correctly)
• function pointers (jumping to the wrong place)
• data pointers (pointing to the wrong place)
• manual garbage collection is a lot of work
• multiple inheritance (C) can get very
  complicated
• ambiguities like the diamond problem (a.k.a.
  diamond of death)
• not easily portable across platforms, even with
  re-compile and discipline

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a few ways Java improved on C

• instead of pointers, Java has references
• references are similar to pointers, but with
  protections (cannot jump into illegal parts of
  memory)avoids segmentation fault problems
• automatic garbage collection
• memory is reclaimed from the heap
  automaticallyavoids memory leaks
• single inheritance
• ambiguity and complexity of multiple inheritance
  avoided
• encapsulation
• all code must be in a classintended to encourage
  information hiding
• array bounds checking
• libraries
• many common tasks already coded and available for
  reuse by means of inheritance
• many interfaces (behaviors) already coded

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5 Files (.java, .class, .jar)
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What is a .class file?

• javac takes a text file as input, and creates a
  binary file as output
• .class files contain two main things
• bytecode
• instructions for the Java Virtual Machine
• at runtime, bytecode gets JIT-compiled into
  native code on the fly
• metadata
• a catalog of resources
• a description of the class contents that will be
  available at runtime by means of reflection

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what is a .jar file?

• Java Archive files (.jar extension) contain
• one or more .class files
• resources
• strings
• images
• Sounds
• .jar files may also contain .java source files
  (optional)
• if a JRE is present on a system, .jar files
  should be registered to run on the JRE
• double-clicking the .jar should run the JRE with
  the .jar file as input
• the JVM must be in the search path
• the /lib and /bin folders for the JRE must be in
  the CLASSPATH
• a .jar may reference other .jars if they can be
  found

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the single-file application

• monolithic application
• all source code compiled into one .JAR file

APP.jar
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component-based application

• component-based application
• .JAR file one or more other .JAR files

compute.jar
APP.jar
data.jar
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where do Java library .JAR files reside on the
file system?

• CLASSPATH environment variable
• user-level, not system-level, if on Windows
• programmers must tell the compiler which .JAR
  files a program will need
• in Eclipse, you do this by opening the project
  Properties
• add one or more External .JAR files to the Java
  Build Path
• (demo)

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jargon checklist

• what is a .jar file? .java? .class?
• what is metadata?
• What is bytecode?
• What is the JVM?
• what is the JRE?
• what is JDK?
• What are J2SE and J2EE?
• What is an IDE?
• What is a GUI?

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what we just covered

• simplest Java program
• Java platform overview
• getting the software you need
• why did Sun create Java?
• Files (.java, .class, .jar)

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