(octave.info)How to use Java from within Octave

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A.4.2 How to use Java from within Octave
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The function Note: javaObject. creates Java objects.  In
fact it invokes the public constructor of the class with the given name
and with the given parameters.

   The following example shows how to invoke the constructors
‘BigDecimal(double)’ and ‘BigDecimal(String)’ of the builtin Java class
‘java.math.BigDecimal’.

     javaObject ("java.math.BigDecimal",  1.001 );
     javaObject ("java.math.BigDecimal", "1.001");

   Note that parameters of the Octave type ‘double’ are implicitly
converted into the Java type ‘double’ and the Octave type (array of)
‘char’ is converted into the java type ‘String’.  A Java object created
by Note: javaObject. is never automatically converted
into an Octave type but remains a Java object.  It can be assigned to an
Octave variable.

     a = 1.001;
     b = javaObject ("java.math.BigDecimal", a);

   Using Note: isjava, it is possible to check whether a
variable is a Java object and its class can be determined as well.  In
addition to the previous example:

     isjava (a)
     ⇒ ans = 0
     class (a)
     ⇒ ans = double
     isjava (b)
     ⇒ ans = 1
     class (b)
     ⇒ ans = java.math.BigDecimal

   The example above can be carried out using only Java objects:

     a = javaObject ("java.lang.Double", 1.001);
     b = javaObject ("java.math.BigDecimal", a);

     isjava (a)
     ⇒ ans = 1
     class (a)
     ⇒ ans = java.lang.Double
     isjava (b)
     ⇒ ans = 1
     class (b)
     ⇒ ans = java.math.BigDecimal

   One can see, that even a ‘java.lang.Double’ is not converted to an
Octave ‘double’, when created by Note: javaObject.  But
ambiguities might arise, if the Java classes ‘java.lang.Double’ or
‘double’ are parameters of a method (or a constructor).  In this case
they can be converted into one another, depending on the context.

   Via Note: javaObject. one may create all kinds of
Java objects but arrays.  The latter are created through Note:
javaArray.

   It is possible to invoke public member methods on Java objects in
Java syntax:

     a.toString
     ⇒ ans = 1.001
     b.toString
     ⇒ ans = 1.000999999999999889865...

   The second result may be surprising, but simply comes from the fact,
that ‘1.001’ cannot exactly be represented as ‘double’, due to rounding.
Note that unlike in Java, in Octave methods without arguments can be
invoked with and without parentheses ‘()’.

   Currently it is not possible to invoke static methods with a Java
like syntax from within Octave.  Instead, one has to use the function
Note: javaMethod. as in the following example:

     java.math.BigDecimal.valueOf(1.001);                    # does not work
     javaMethod ("valueOf", "java.math.BigDecimal", 1.001);  # workaround

   As mentioned before, method and constructor parameters are converted
automatically between Octave and Java types, if appropriate.  For
functions this is also true with return values, whereas for constructors
this is not.

   It is also possible to access public fields of Java objects from
within Octave using Java syntax, with the limitation of static fields:

     java.math.BigDecimal.ONE;                  # does not work
     java_get ("java.math.BigDecimal", "ONE");  # workaround

   Accordingly, with Note: java_set. the value of a field
can be set.  Note that only public Java fields are accessible from
within Octave.

   The following example indicates that in Octave empty brackets ‘[]’
represent Java’s ‘null’ value and how Java exceptions are represented.

     javaObject ("java.math.BigDecimal", []);
     ⇒ error: [java] java.lang.NullPointerException

   It is not recommended to represent Java’s ‘null’ value by empty
brackets ‘[]’, because ‘null’ has no type whereas ‘[]’ has type
‘double’.

   In Octave it is possible to provide limited Java reflection by
listing the public fields and methods of a Java object, both static or
not.

     fieldnames (<Java object>)
     methods (<Java object>)

   Finally, an examples is shown how to access the stack trace from
within Octave, where the function Note: debug_java. is
used to set and to get the current debug state.  In debug mode, the Java
error and the stack trace are displayed.

     debug_java (true)  # use "false" to omit display of stack trace
     debug_java ()
     ⇒ ans = 1
     javaObject ("java.math.BigDecimal", "1") ...
       .divide (javaObject ("java.math.BigDecimal", "0"))