Compiling and the Java Virtual Machine JVM

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Compiling and the Java Virtual Machine (JVM)
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Working with a real language

• The syntax of Pseudocode is pretty loose
• visual validation encourages a permissive
  approach
• emphasized the importance of structure
• Compilers essentially require perfection.
• The procedure for writing your algorithms wont
  change (much!)
• Write a high level approach focusing on
  algorithm.
• Write the code concentrating on details.
• Check it by hand before compiling.
• Because we're introducing a real language at the
  same time as we're introducing the object
  oriented paradigm we will require you to just
  write some code.
• You may not understand it all immediately
• Eventually we'll explain!

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The First Part

• Initially well use Java to write procedural code
  just to let you get the feel of a real language.
• In fact, well start by programming some of the
  same types of modules that we used in the
  procedural pseudocode earlier in the semester.
• Then, as we introduce the Object Oriented
  paradigm well use Java as it was intended.

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About Java
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Introduction to Java

• What Java is
• A real professional programming language
• (which is good and bad...)
• Portable across any hardware platform that has a
  JVM interpreter (more later)
• An object-oriented language
• What Java is not
• The Ultimate Programming Language
• HTML or another web-content language
• Only useful for web applets
• Just Another Vacuous Acronym

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Real Languages

• Real languages require the programmer to write
  code using a very specific set of rules, keywords
  and syntax.
• Then this code is transformed into actual
  instructions that a specific machine can execute.
  Often a complex process
• A number of strategies have been invented to
  accomplish this process
• Assemblers
• Compilers
• Interpreters
• A traditional problem has been the necessity to
  have different versions of a program for
  different machines.

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A New Idea

• Java was specifically developed to be able to
  write code once and run it anywhere
• How is this magic accomplished?
• Using an intermediate language! Byte code.
• The Byte code is interpreted (executed) using a
  special piece of software (a program) called the
  Java Virtual Machine (JVM)

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Compilation
Source Code .java
Java compiler
Generic Byte Code .class
OS-specificJVM interpreter
OS-specific Object Code
Execute program
Need one of these for every different OS
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Structure of Java Programs

• Initially well write programs that fit in one
  file.
• Create the file with an editor (e.g. emacs)
• Compile with javac producing a .class file
• Execute by running java and sending it the .class
  file
• Our first (tiny) program will be roughly like a
  cross between an algorithm and a procedure.
• Lets take a look...

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Sample Application
We create a file (using an editor) called
HelloWorld.java

• public class HelloWorld
• public static void main(String argv)
• System.out.println(Hello World!)

We compile by typing (at the OS prompt) javac
HelloWorld.java Which produces HelloWorld.class Th
en we execute by typing java HelloWorld
Hello World!
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Demo

• gtjavac HelloWorld.java
• gtjava HelloWorld
• Hello World!
• gt

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Quick Trix

• The name of the file must match the name of the
  class EXACTLY!!!
• File Bubba.java
• Contains
• Everything must be EXACTLY correct!!!

Capitalization counts
class Bubba ...
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Eventually...

• Applications (normal computer programs)
• Each program consists of multiple files.
• Each file contains a class.
• Each class will contain modules which will
  resemble procedures and functions.
• THE MODULES WILL BE CALLED METHODS
• The .class file that you send to java must
  contain a method named main
• It will actually look like this
• public static void main(String argv ) ..
• the JVM will use the file naming convention to
  find the other classes required by this main
  program.

As opposed to Applets
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Some basic syntax issues

• Your TA is smarter than the java compiler
• Lines need to terminate with a
• Easier said than done
• Braces will be used to indicate "blocks" of code
• Which essentially act like a single line

public class HelloWorld public static void
main(String argv) System.out.println(He
llo World!)
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A look ahead...

• Pseudocode
• if some_boolean then
• a lt- a 1
• else
• a lt- a 2
• b lt- 7
• endif

• Java
• if(some_boolean)
• a a 1
• else
• a a 2
• b 7

Note placement
Note Indentation is used in both cases. Means
nothing to compiler. instead of lt- (more
later) Must have parens in Java
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