The function 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(String) of the builtin Java
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)
converted into the java type
String. A Java object created by
javaObject is never automatically converted
into an Octave type but remains a Java object. It can be assigned to an
a = 1.001; b = javaObject ("java.math.BigDecimal", a);
Using 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 javaObject.
But ambiguities might arise, if the Java classes
double are parameters of a method (or a constructor). In this case
they can be converted into one another, depending on the context.
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 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 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
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
null has no type whereas
 has type
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 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"))