JNI and interface classes

[Oyvind Idland]

I am trying to get an object-handle from a Java interface,
without luck. It works well for "normal" classes, but now
I seem to be out of luck.

The Java code implementation of the interface looks like
this:


package myClasses;

public interface someInterface{
public void someMethod(String str1, String str2);
}



The following lines works well (C++):

cls = env->FindClass("myClasses/someInterface");
mid = env->GetMethodID(cls, "someMethod", "(Ljava/lang/String;Ljava/lang/StringV");

Since there is no constructor here, I try to create the object-handle
like this:

obj = env->AllocObject(cls);

...which fails. I guess I my thinking may be wrong here.


Any suggestions is welcome


Oyvind

 

[Chris Uppal]

I am trying to get an object-handle from a Java interface,
without luck. It works well for "normal" classes, but now
I seem to be out of luck. [...]
..which fails. I guess I my thinking may be wrong here.

It is. You /can't/ create instances of interfaces -- the concept has no
meaning.

I'm not trying to be rude, but I really do think you need to go back to basics
and find out about Java interfaces, what they mean, and what they are used for.

-- chris

 

[Thomas Fritsch]

I am trying to get an object-handle from a Java interface,
without luck. It works well for "normal" classes, but now
I seem to be out of luck.

The Java code implementation of the interface looks like
this:

package myClasses;

public interface someInterface{
public void someMethod(String str1, String str2);
}

The following lines works well (C++):

cls = env->FindClass("myClasses/someInterface");
mid = env->GetMethodID(cls, "someMethod",
"(Ljava/lang/String;Ljava/lang/StringV");

Since there is no constructor here, I try to create the object-handle
like this:
obj = env->AllocObject(cls);

..which fails. I guess I my thinking may be wrong here.

It fails for the same reason why the Java code
new someInterface()
would fail: because an interface has no contructor.

Any suggestions is welcome

You definitely need a concrete class implementing the interface.
public class SomeClass implements someInterface {
public SomeClass() {
// initialize something
}
public void someMethod(String str1, String str2) {
// do something
}
}
Only then you are able to create a new object of this class, both via
Java or via JNI.

 

[Oyvind Idland]

You definitely need a concrete class implementing the interface.
public class SomeClass implements someInterface {
public SomeClass() {
// initialize something
}
public void someMethod(String str1, String str2) {
// do something
}
}
Only then you are able to create a new object of this class, both via
Java or via JNI.

Thanks alot, this is the answer I was looking for.

Oyvind

 

[Roedy Green]

obj = env->AllocObject(cls);

..which fails. I guess I my thinking may be wrong here.

For a moment imagine there were no C++ involved. You can't instantiate
an Interface. You need a factory or a constructor to create an
instance of some concrete class that implements the interface.

The same is true when you add the complication of JNI.