# Difference between revisions of "Java package"

Octave is an easy to use but powerful environment for mathematical calculations, which can easily be extended by packages. Its features are close to the commercial tool Matlab so that it can often be used as a replacement. Java on the other hand offers a rich, object oriented and platform independent environment for many applications. The core Java classes can be easily extended by many freely available libraries. This document refers to the package java, which is part of the GNU Octave project. This package allows you to access Java classes from inside Octave. Thus it is possible to use existing class files or complete Java libraries directly from Octave.

This description is based on the Octave package java-1.2.8. The java package usually installs its script files (.m) in the directory .../share/Octave/packages/java-1.2.8 and its binary (.oct) files in .../libexec/Octave/packages/java-1.2.8. You can get help on specific functions in Octave by executing the help command with the name of a function from this package:

octave> help javaObject


You can view the whole doc file in Octave by executing the info command with just the word java:

octave> doc java


Note on calling Octave from Java: the java package is designed for calling Java from Octave. If you want to call Octave from Java, you might want to use a library like javaOctave [1] or joPas [2].

# FAQ

## How to distinguish between Octave and Matlab?

Octave and Matlab are very similar, but handle Java slightly different. Therefore it may be necessary to detect the environment and use the appropriate functions.

## How to make Java classes available to Octave?

Java finds classes by searching a classpath. This is a list of Java archive files and/or directories containing class files. In Octave and Matlab the classpath is composed of two parts:

• the static classpath is initialized once at startup of the JVM, and;
• the dynamic classpath which can be modified at runtime.

Octave searches the static classpath first, then the dynamic classpath. Classes appearing in the static as well as in the dynamic classpath will therefore be found in the static classpath and loaded from this location.

Classes which shall be used regularly or must be available to all users should be added to the static classpath. The static classpath is populated once from the contents of a plain text file named classpath.txt when the Java Virtual Machine starts. This file contains one line for each individual classpath to be added to the static classpath. These lines can identify single class files, directories containing class files or Java archives with complete class file hierarchies. Comment lines starting with a # or a % character are ignored.

The search rules for the file classpath.txt are:

• First, Octave searches for the file classpath.txt in your home directory, If such a file is found, it is read and defines the initial static classpath. Thus it is possible to build an initial static classpath on a "per user" basis.
• Next, Octave looks for another file classpath.txt in the package installation directory. This is where javaclasspath.m resides, usually something like
...\share\Octave\packages\java-1.2.8.
you can find this directory by executing the command pkg list. If this file exists, its contents is also appended to the static classpath. Note that the archives and class directories defined in this file will affect all users.

Classes which are used only by a specific script should be placed in the dynamic classpath. This portion of the classpath can be modified at runtime using the javaaddpath and javarmpath functions. Example:

octave> base_path = "C:/Octave/java_files";
octave> % add two JARchives to the dynamic classpath
octave> javaaddpath ([base_path, "/someclasses.jar"]);
octave> javaaddpath ([base_path, "/moreclasses.jar"]);
octave> % check the dynamic classpath
octave> p = javaclasspath;
octave> disp (p{1});
C:/Octave/java_files/someclasses.jar
octave> disp (p{2});
C:/Octave/java_files/moreclasses.jar
octave> % remove the first element from the classpath
octave> javarmpath ([base_path, "/someclasses.jar"]);
octave> p = javaclasspath;
octave> disp (p{1});
C:/Octave/java_files/moreclasses.jar
octave> % provoke an error
octave> disp (p{2});
error: A(I): Index exceeds matrix dimension.


Another way to add files to the dynamic classpath exclusively for your user account is to use the file .octaverc which is stored in your home directory. All Octave commands in this file are executed each time you start a new instance of Octave. The following example adds the directory octave to Octave’s search path and the archive myclasses.jar in this directory to the Java search path.

% content of .octaverc:


## How to create an instance of a Java class?

If your code shall work under Octave as well as Matlab you should use the function javaObject' to create Java objects. The function java_new is Octave specific and does not exist in the Matlab environment. Using is_octave() to distinguish between environments

if (is_octave)
Passenger = java_new ('package.FirstClass', row, seat);    % works only in Octave
else
Passenger = javaObject ('package.FirstClass', row, seat);  % actually works in both Octave and matlab
end


## How can I handle memory limitations?

In order to execute Java code Octave creates a Java Virtual Machine (JVM). Such a JVM allocates a fixed amount of initial memory and may expand this pool up to a fixed maximum memory limit. The default values depend on the Java version (see help javamem). The memory pool is shared by all Java objects running in the JVM. This strict memory limit is intended mainly to avoid that runaway applications inside web browsers or in enterprise servers can consume all memory and crash the system. When the maximum memory limit is hit, Java code will throw exceptions so that applications will fail or behave unexpectedly.

In Octave as well as in Matlab, you can specify options for the creation of the JVM inside a file named java.opts. This is a text file where you can enter lines containing -X and -D options handed to the JVM during initialization.

In Octave, the Java options file must be located in the directory where javaclasspath.m resides, i.e. the package installation directory, usually something like ...\share\Octave\packages\java-1.2.8. You can find this directory by executing:

pkg list


In Matlab, the options file goes into the MATLABROOT/bin/ARCH directory or in your personal Matlab startup directory (can be determined by a pwd command). MATLABROOT is the Matlab root directory and ARCH is your system architecture, which you find by issuing the commands matlabroot respectively computer('arch').

The -X options allow you to increase the maximum amount of memory available to the JVM to 256 Megabytes by adding the following line to the java.opts file:

-Xmx256m


The maximum possible amount of memory depends on your system. On a Windows system with 2 Gigabytes main memory you should be able to set this maximum to about 1 Gigabyte.

If your application requires a large amount of memory from the beginning, you can also specify the initial amount of memory allocated to the JVM. Adding the following line to the java.opts file starts the JVM with 64 Megabytes of initial memory:

-Xms64m


For more details on the available -X options of your Java Virtual Machine issue the command java -X at the operating system command prompt and consult the Java documentation.

The -D options can be used to define system properties which can then be used by Java classes inside Octave. System properties can be retrieved by using the getProperty() methods of the java.lang.System class. The following example line defines the property MyProperty and assigns it the string 12.34.

-DMyProperty=12.34


The value of this property can then be retrieved as a string by a Java object or in Octave:

octave> javaMethod("java.lang.System", "getProperty", "MyProperty");
ans = 12.34


## How to install the java package in Octave?

### Uninstall the currently installed package java

Check whether the java package is already installed by issuing the pkg list command:

octave> pkg list
Package Name | Version | Installation directory
--------------+---------+-----------------------
java *|   1.2.8 | /home/octavio/octave/java-1.2.8


If the java package appears in the list you must uninstall it first by issuing the command

octave> pkg uninstall java
octave> pkg list


Now the java package should not be listed anymore. If you have used the java package during the current session of Octave, you have to exit and restart Octave before you can uninstall the package. This is because the system keeps certain libraries in memory after they have been loaded once.

### Make sure that the build environment is configured properly

The installation process requires that the environment variable JAVA_HOME points to the Java Development Kit (JDK) on your computer.

• Note that JDK is not equal to JRE (Java Runtime Environment). The JDK home directory contains subdirectories with include, library and executable files which are required to compile the java package. These files are not part of the JRE, so you definitely need the JDK.
• Do not use backslashes but ordinary slashes in the path. Set the environment variable JAVA_HOME according to your local JDK installation. Please adapt the path in the following examples according to the JDK installation on your system. If you are using a Windows system that might be:
octave> setenv ("JAVA_HOME", "C:/Java/jdk1.6.0_21");
If you are using a Linux system this would look probably more like:
octave> setenv ("JAVA_HOME", "/usr/local/jdk1.6.0_21");
Note, that on all systems you must use the forward slash / as the separator, not the backslash \. If on a Windows system the environment variable JAVA_HOME is already defined using the backslash, you can easily change this by issuing the following Octave command before starting the installation:
octave> setenv ("JAVA_HOME", strrep (getenv ("JAVA_HOME"), '\', '/'))

### Test the java package installation

The following code creates a Java string object, which however is automatically converted to an Octave string:

octave> s = javaObject ("java.lang.String", "Hello OctaveString")
s = Hello OctaveString


Note that the java package automatically transforms the Java String object to an Octave string. This means that you cannot apply Java String methods to the result.

This "auto boxing" scheme seems to be implemented for the following Java classes:

• java.lang.Integer
• java.lang.Double
• java.lang.Boolean
• java.lang.String

If you instead create an object for which no "auto-boxing" is implemented, javaObject returns the genuine Java object:

octave> v = javaObject ("java.util.Vector")
v =
<Java object: java.util.Vector>
octave> v.get(0)
ans = 12


If you have created such a Java object, you can apply all methods of the Java class to the returned object. Note also that for some objects you must specify an initializer:

% not:
octave> d = javaObject ("java.lang.Double")
error: [java] java.lang.NoSuchMethodException: java.lang.Double
% but:
octave> d = javaObject ("java.lang.Double", 12.34)
d = 12.340


## Which TEX symbols are implemented in the dialog functions?

The dialog functions contain a translation table for TEX like symbol codes. Thus messages and labels can be tailored to show some common mathematical symbols or Greek characters. No further TEX formatting codes are supported. The characters are translated to their Unicode equivalent. However, not all characters may be displayable on your system. This depends on the font used by the Java system on your computer.

Each TEX symbol code must be terminated by a space character to make it distinguishable from the surrounding text. Therefore the string "\alpha =12.0" will produce the desired result, whereas "\alpha=12.0" would produce the literal text "\alpha=12.0".