The Java Native Interface (JNI) - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 614

from GetObjectField() is a jobject ,which could be cast into a jstring ,
if appropriate.
For most objects, including all custom objects, there is no built-in “ j „ type to
cast to. The only way to handle such objects is a nesting of the above procedure.
We give a short example here using the MyCustomObject type in the JNIDemo
class. Let's define a very simple MyCustomObject class that contains only one
int field, initialized to the value 13 (the constructor is implied):
class MyCustomObject {
int val = 13;
}
Recall that the my - custom field of JNIDemo is a MyCustomObject type. If
jo is a jobject reference to an instance of JNIDemo , then we first need to find
a jclass for JNIDemo :
// Find the jclass corresponding to the jobject
jclass cls = jenv->GetObjectClass (jo);
Then we need the field ID of the my - custom field of JNIDemo :
// Find the field ID of the 'my - custom' field
jfieldID my - custom - fid = jenv->GetFieldID (
cls, "my - custom", "Ljavatech/jni22/MyCustomObject;"
);
With the field ID in hand, we can get a jobject reference to the my - custom
object:
// Get the jobject reference to 'my - custom'
jobject my - custom - jo = jenv->GetObjectField (jo,
my - custom - fid);
About the only meaningful thing we can do with this jobject is use it to look
into the MyCustomObject class. First, we get the corresponding jclass and
then get the field ID of the val field:
// Find the field ID of the ' val ' field of MyCustomObject.
jclass my - custom - cls = jenv->GetObjectClass (my - custom - jo);
jfieldID val - fid = jenv->GetFieldID (my - custom - cls, "val", "I");
Now we can obtain the value of the val field itself:
// Get the ' val ' field
jint val = jenv- > GetIntField (my - custom - jo, val - fid);
and manipulate it somehow:
// Manipulate it somehow
val * = 2;