1. Programming without BlueJ Week 12

2. CONCEPTS COVERED THIS WEEK
Programming without BlueJ
CONCEPTS COVERED THIS WEEK
Running Java without BlueJ
The main Method

3. The BlueJ directory structure
Programming with BlueJ
The BlueJ directory structure
project: clock-display
package.bluej
ClockDisplay.java
ClockDisplay.class
ClockDisplay.ctxt
NumberDisplay.java
NumberDisplay.class
NumberDisplay.ctxt
ClockDisplay
NumberDisplay
look at package in BlueJ
then look at directory
compare files in directory to classes in package

4. The BlueJ file structure
Programming with BlueJ
The BlueJ file structure
package.bluej Stores BlueJ project details
ClassName.ctxt BlueJ class context file
ClassName.java Java source code file
ClassName.class Java bytecode file
look at package in BlueJ
then look at directory
compare files in directory to classes in package

5. Programming without BlueJ
Standard Java Files
ClassName.java
Java source files contain the source code in readable form, as typed in by the programmer.
ClassName.class
Java class files contain bytecode (a machine readable version of the class). They are generated by the compiler from the source file.
look at package in BlueJ
then look at directory
compare files in directory to classes in package

6. Standard Java Edit-Compile-Run Cycle
Programming without BlueJ
Standard Java Edit-Compile-Run Cycle
011010
110101
1001 10
source file
class file
011010
110101
010001 1 1 1
0111
look at package in BlueJ
then look at directory
compare files in directory to classes in package
compiler
(javac)
editor
virtual machine (java)

7. Editing Java Source Files
Programming without BlueJ
Editing Java Source Files
A Java file can be edited using any text editor e.g. Notepad
Careful with using Word as, by default, Word does not save in text format
Be sure to save changes before compiling!
look at package in BlueJ
then look at directory
compare files in directory to classes in package

8. Command Line Invocation
Programming without BlueJ
Command Line Invocation
Compilation and execution of Java in JDK are done from a command line.
Windows: DOS shell, i.e. Command Prompt:
Ensure that the directory for the Java compiler (javac) and Java runtime (java) is set in the command path e.g.
set path=C:\Program Files (x86)\Java\jdk1.6.0_33\bin
look at package in BlueJ
then look at directory
compare files in directory to classes in package

9. Programming without BlueJ
Java Compilation
The name of the JDK compiler is javac
To compile a Java class definition: javac <source name>
javac compiles the source file as well as all other classes the source file depends upon.
Example usage of javac:
X:\>cd My Documents\Java\Picture
X:\My Documents\Java\Picture>javac Picture.java
look at package in BlueJ
then look at directory
compare files in directory to classes in package

10. Java Compilation (Error Messages)
Programming without BlueJ
Java Compilation (Error Messages)
Example usage of javac:
X:\>cd My Documents\Java\Picture
X:\My Documents\Java\Picture>javac Picture.java
Picture.java:14: ';' expected.
private square wall ^
1 error
X:\My Documents\Java\Picture>
look at package in BlueJ
then look at directory
compare files in directory to classes in package
The programmer has to open the Java file in the editor, find the line number, fix the error and recompile.

11. Java Program Execution
Programming without BlueJ
Java Program Execution
Example usage of java:
X:\My Documents\Java\Picture>java Picture
“java” starts the Java virtual machine (JVM).
The named class is loaded and execution is started.
Other classes are loaded as needed.
Only possible if class has been compiled.
look at package in BlueJ
then look at directory
compare files in directory to classes in package

12. Programming without BlueJ
Java Program Execution – Problem: Execute What?
Example usage of java:
X:\My Documents\Java\Picture>java Picture
Exception in thread "main" java.lang.NoSuchMethodError: main
How does the system know which of the methods of the class to execute?
look at package in BlueJ
then look at directory
compare files in directory to classes in package

13. Java Program Execution – The Main Method
Programming without BlueJ
Java Program Execution – The Main Method
Example usage of javac:
X:\My Documents\Java\Picture>java Picture
Exception in thread "main" java.lang.NoSuchMethodError: main
The solution: The Java system always executes a class method called main with a specific signature.
look at package in BlueJ
then look at directory
compare files in directory to classes in package

14. Java Program Execution – The Main Method
Programming without BlueJ
Java Program Execution – The Main Method
Fixed signature for the main method:
public static void main(String[] args) { ... }
For this to work, a main method must exist in one (and only one) of the classes being called!
It is possible to place all of an application’s code in a class with just a main method!
But this is absolutely NOT RECOMMENDED!
look at package in BlueJ
then look at directory
compare files in directory to classes in package

15. Java Program Execution – The Main Method
Programming without BlueJ
Java Program Execution – The Main Method
Example of a main method in a class called Game:
public static void main(String[] args) {
Game game = new Game();
game.play(); }
look at package in BlueJ
then look at directory
compare files in directory to classes in package
In general the main method should ONLY
create an object
call the first method

16. Creating a Java program
Writing the Java code
Any editor (such as Notepad) can be used to write a Java program. However, most professional programmers use a software application called an Integrated Development Environment or IDE
JCreator