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Using Java without BlueJ
5.0
BlueJ projects
• A BlueJ project is stored in a
directory on disk
• A BlueJ package is stored in several
different files
• Some files store the source code,
some store the compiled code,
some store additional information
• BlueJ uses standard Java format for
some files and adds some additional
files with extra information
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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The BlueJ directory structure
project: calculator
Calculator
UserInterface
CalcEngine
c:\bluej\calculator\
Calculator.java
Calculator.class
Calculator.ctxt
CalcEngine.java
CalcEngine.class
CalcEngine.ctxt
package.bluej
UserInterface.java
UserInterface.class
UserInterface.ctxt
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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The BlueJ file structure
• package.bluej – the package file.
Contains information about classes in the
package. One per package (project).
• *.java - standard Java source file (text).
One per class.
• *.class - standard Java code file. One
per class.
• *.ctxt - BlueJ context file. Contains
extra information for a class. One per class.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Standard Java files
• source files: *.java
Java source files contain the source code
in readable form, as typed in by the
programmer.
• class files: *.class
Java class files contain byte code (a
machine readable version of the class).
They are generated by the compiler from
the source file.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
5
The edit-compile-execute
Cycle
source file
class file
011010
110101
010001
011010
110101
1001
10
1
0111
0110110
editor
compiler
(javac)
virtual machine
(java)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Editing
• A file can be edited in any text editor
– Notepad, emacs, jEdit, PFE, vi, …
• Don't use Microsoft Word: by default,
Word does not save in text format
– Includes formatting (i.e. fonts, shapes, …)
• Make sure to save with a .java file
extension before compiling!
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Command line invocation
• Compilation and execution of Java in
JDK are done from a command line
– Windows: DOS shell
– Unix: Unix shell
• Must make sure that commands for
compiler (javac) and runtime (java)
are in the command path
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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• JDK compiler:Compiling
javac
• To invoke: javac ClassName.java
compiles source file and all classes that it depends on
• To find commands in path … either:
Change directory to location of commands
– cd C:\Program Files\Java\jdk1.7.0_65\bin
– javac ClassName.java
Ensure commands are in your command PATH
– Control Panel -> System -> Advanced ->
Environment Variables -> PATH
– Add “C:\Program Files\Java\jdk1.7.0_65\bin”
– javac ClassName.java
• Creates byte file: ClassName.class
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Compiler
error messages
C:\bluej\project> javac ClassName.java
ClassName.java:22: ';' expected.
private Parser parser
^
1 error
C:\bluej\project>
The programmer has to open the file in the editor,
find the line number, fix the error and recompile.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Execution
C:\bluej\project> java ClassName
• java starts the Java virtual machine
• The named class is loaded and
execution is started
• Other classes are loaded as needed
• Only possible if class has been compiled
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Problem
Execute what?
• If we try:
C:\bluej\project> java ClassName
• We get a compiler error:
Exception in thread "main"
java.lang.NoSuchMethodError: main
• The problem: there is NO instance object
How does the system know which method to
execute?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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The main method
• The java system will always execute a
method called main with a certain
signature:
public static void main(String[] args)
{
...
}
• If compiling and executing from the
command line, then the main method
must exist!
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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The main method
• main must exist
• main must be public
• main must be static (class
method)
• main must have a String[]
parameter
• Only main can be invoked
Example of a class method
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Main method
example
public static void main(String[] args)
{
Game game = new Game();
game.play();
}
• Consider placing in a separate class,
containing just this
• The main method should
– create an object
– call the first method
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Demo
TestingMain.java
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Command Line Execution
javac Driver.java
– compiles code in the Driver class
– produces Driver.class
java Driver
– executes the byte file Driver.class
– invokes main method (if it exists)
java Driver Hello world “that I live in”
– passes any String arguments into main
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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