Trying to write to a HashMap from C++

Question

I've no idea if this is possible, never mind sensible, but what I want to do is pass the C++ method a HashMap, on which it calls the put(key, value) method thereby adding data to it.
My code looks a bit like this:
JNIEXPORT jboolean JNICALL Java_...snip...(JNIEnv *env, jobject obj, jobject hashmap)
     jclass cls = env->GetObjectClass(hashmap);
jmethodID mid = env->GetMethodID(cls, "put", "(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;");
     if (mid==0) //doesn't happen
          return false;
     env->CallObjectMethod(hashmap, mid, "somekey", "somevalue"); //cause of problem
     return true;
The jobject hashmap parameter sent to it is, naturally, a HashMap. When I run this code, or my alternative attempts with it, I encounter various problems, the main being that I'm given a NullPointerException presumably on the callback to the 'put' method.
I wondered if this is because I should really be sending a java.lang.Object to it rather than my strings, but I'm neither convinced nor do I have any idea how. Any suggestions to work directly on the HashMap, i.e. without adding my own indirect 'put' method within the calling Java class?

Answer 1

The hashmap wants object parameters, not C strings.
You have to create the String - jstring - objects.