Object Oriented Programming - Java

Object Oriented
Programming – Java
By Daniel :
- javac
-java
-javah
-javadoc
-appletviewer

javac
 javac stands for Java Compiler
 The javac tool reads class and interface definitions, written in the Java
programming language, and compiles them into bytecode class files.
 There are two ways to pass source code file names to javac:
o For a small number of source files, simply list the file names on the
command line.
o For a large number of source files, list the file names in a file,
separated by blanks or line breaks.

javac (Cont..)
 Source code file names must have .java suffixes, class file names must have .class
suffixes, and both source and class files must have root names that identify the class.
For example, a class called MyClass would be written in a source file called
MyClass.java and compiled into a bytecode class file called MyClass.class.
 Inner class definitions produce additional class files. These class files have names
combining the inner and outer class names, such as MyClass$MyInnerClass.class.
 You should arrange source files in a directory tree that reflects their package tree. For
example, if you keep all your source files in /workspace, the source code for
com.mysoft.mypack.MyClass should be in
/workspace/com/mysoft/mypack/MyClass.java.
 By default, the compiler puts each class file in the same directory as its source file. You
can specify a separate destination directory with -d option

After using the javac command
as follow:
$ Javac car.java
We get car.class file ready to
be used by the launcher
(Car.class)
The work of the javac

javac
Sets the destination directory for class files. The
destination directory must already exist; javac will not
create the destination directory.
If a class is part of a package, javac puts the class file in a
subdirectory reflecting the package name, creating
directories as needed.
For example, if you specify -d /home/myclasses and the
class is called com.mypackage.MyClass, then the class
file is called
/home/myclasses/com/mypackage/MyClass.class.

java
java - Java application launcher
 The java tool launches a Java application. It does this by starting a Java
runtime environment, loading a specified class, and invoking that class's main
method.
 The method must be declared public and static , it must not return any value, and
it must accept a String array as a parameter. The method declaration must look
like the following: public static void main(String dims[]) By default, the first
non-option argument is the name of the class to be invoked.
 A fully-qualified class name should be used. If the -jar option is specified, the
first non-option argument is the name of a JAR archive containing class and
resource files for the application, with the startup class indicated by the Main-
Class manifest header.
 The Java runtime searches for the startup class, and other classes used, in three
sets of locations: the bootstrap class path, the installed extensions, and the user
class path. Non-option arguments after the class name or JAR file name are passed
to the main function.

What does the java launcher do?
java is called as ‘java launcher because It does this by starting a Java
runtime environment, loading a specified class, and invoking that class's
main method. This method must have the following signature: public static
void main(String[]). It has to be static and public. This means that it’ll have
the job of loading the .class file and making it ready for execution by the
Java Virtual Machine (JVM).
$ java runCar --runCar if the class with the main method:

javah
javah - C header and stub file generator
 The javah command generates C header and source files that
are needed to implement native methods. The generated
header and source files are used by C programs to reference
instance variables of an object from native source code. The
.h file contains a structure definition whose layout parallels
that of the corresponding class. The fields in the structure
correspond to instance variables in the class.
 The name of the header file and the structure declared
within it are derived from the name of the class. If the class
passed to javah is inside a package, the package name is
prepended to both the header file name and the structure
name. Underscores ( _ ) are used as name delimiters.

Javah (cont..)
 By default, javah creates a header file for each class listed on
the command line and puts the files in the current directory.
Use the -stubs option to create source files. Use the -o
option to concatenate the results for all listed classes into a
single file.
 The new native method interface, Java Native Interface
(JNI), does not require header information or stub files. The
javah command can still be used to generate native method
function prototypes needed for JNI-style native methods.
javah produces JNI-style output by default, and places the
result in the .h file.
 The javah_g version is a non-optimized version of javah
suitable for use with debuggers like jdb.

$ Javah runCar
This will create an header file for the class runCar that
can help in implementing the java code in C language. And
this command must be done on the class with the main
method, for our case now it is runCar.
After $ javah runCar we get:

Now we’ll try viewing what contained in the runCar.h file
$ cat runCar.h
This is how the runCar.h file looks like. Interesting, isn’t it?

javadoc
javadoc - Java API documentation generator
 The Javadoc tool parses the declarations and documentation comments
in a set of Java source files and produces a corresponding set of HTML
pages describing (by default) the public and protected classes, nested
classes (but not anonymous inner classes), interfaces, constructors,
methods, and fields.
 You can run the Javadoc tool on entire packages, individual source files,
or both. In the first case, you pass in as an argument to javadoc a series
of package names. In the second case, you pass in a series of source
(.java) file names. See EXAMPLES at the end of this document.
 NOTE - When you pass in package names to the Javadoc tool, it
currently processes all .java classes in the specified package directories,
even if the
 java files are code examples or other classes that are not actually
members of the specified packages. It does not parse each .java file for
a package declaration; we may add this parsing in a future release.

javadoc (cont..)
During a run, the Javadoc tool automatically adds cross-
reference links to package, class and member names that are
being documented as part of that run. Links appear in several
places:
o Declarations (return types, argument types, field types)
o "See Also" sections generated from @see tags
o In-line text generated from {@link} tags
o Exception names generated from @throws tags
o Specified by links to members in interfaces and Overrides links
to members in classes
o Summary tables listing packages, classes and members
o Package and class inheritance trees

This is an example of a javadoc
class. Note that all the method
and classes are public in
nature. The comments starts as
/** and ends with */. To
generate documentation for
this code, do:
$ Javadoc MyClass.java
The documentation to
generated is per java standards
of documentation.
MyClass.java

After :
$ Javadoc MyClass.java
You should see:

Now our java documentation MyClass.html is ready to use:

appletviewer
appletviewer - Java applet viewer
The appletviewer command connects to the documents or
resources designated by urls and displays each applet referenced
by that document in its own window.
Note:
if the documents referred to by urls do not reference any applets
with the OBJECT, EMBED, or APPLET tag, appletviewer does
nothing. For details on the HTML tags that appletviewer
supports, see
http://java.sun.com/j2se/1.5.0/docs/tooldocs/appletviewertags
.html.
Command:
$ appletviewer MyClass.html

Thank you!
By Daniel Ilunga

Object Oriented Programming - Java

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