1. Using Java without BlueJ
5.0

2. 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

3. The BlueJ directory structure
c:\bluej\calculator\
project: calculator
Calculator.java
Calculator.class
Calculator.ctxt
CalcEngine.java
CalcEngine.class
CalcEngine.ctxt
package.bluej
UserInterface.java
UserInterface.class
UserInterface.ctxt
Calculator
UserInterface
CalcEngine
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

4. 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

5. 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

6. The edit-compile-execute Cycle
011010
110101
1001 10
source file
class file
011010
110101
010001 1 1 1
0111
compiler
(javac)
editor
virtual machine (java)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

7. 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

8. 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

9. Compiling JDK compiler: 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”
Creates byte file: ClassName.class

10. 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

11. C:\bluej\project> java ClassName
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

12. 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

13. 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

14. 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
(may be invoked without an class instance)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

15. 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

16. Demo
TestingMain.java