Download 09.0.1_Java01_Compiling

Compiling and the Java
Virtual Machine (JVM)
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 won’t
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!
The First Part
• Initially we’ll use Java to write
procedural code just to let you get the
feel of a real language.
• In fact, we’ll 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 we’ll use Java as it was intended.
About Java
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
HTML
Only
Just
Ultimate Programming Language”
or another web-content language
useful for web applets
Another Vacuous Acronym
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.
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)
Compilation
“Source
Code” [.java]
Execute
program
Java
compiler
OS-specific
“Object
Code”
Generic
“Byte Code” [.class]
OS-specific
JVM
interpreter
Need one of these
for every different
OS
Structure of Java Programs
• Initially we’ll 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.
• Let’s take a look...
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
Then we execute by typing:
java HelloWorld
Hello World!
Demo
>javac HelloWorld.java
>java HelloWorld
Hello World!
>
Quick Trix
• The name of the file must match the name of
the class EXACTLY!!!
• File: Bubba.java
• Contains:
Capitalization counts
class Bubba
{...
• Everything must be EXACTLY correct!!!
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
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(“Hello World!”);
}
}
A look ahead...
• Pseudocode
• Java
if some_boolean then
a <- a + 1
else
a <- a + 2
b <- 7
endif
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 <- (more later)
Must have parens in Java