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

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

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

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

The edit-compile-execute Cycle 011010 110101 1001 10 source file class file 011010 110101 010001 1 1 1 0111 0110110 compiler (javac) editor virtual machine (java) Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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

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

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

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

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

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

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

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

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

Demo TestingMain.java