How to convert Java Map to cocos2d ValueMap?

Question

Short Question:

How Can I convert jobject to cocos2d::ValueMap?

Detailed Question:

I have Java part:

public class MyCallback implements MyListener {

    public native void callback(Object data);

    public MyCallback(){}

    @Override
    public void onResponse(Map<String, String> data) {
        callback(data);
    }
}

I want to return Map data to Cocos2d Class. So I wrote:

JNIEXPORT void JNICALL Java_com_comp_ MyCallback_ callback
    (JNIEnv *env, jobject obj, jobject data) {

and I get jobject data

So now how to get ValueMap from jobject?

cocos2d::ValueMap myData = ... ???

Answer 1

HashMaps are object that have to be accessed to get the content. So, lots of code ahead of you ;)

Take a look below:

#include <stdio.h>
#include "jni.h"
#include "recipeNo037_PassHashMap.h"

JNIEXPORT int JNICALL Java_recipeNo037_PassHashMap_displayHashMap
  (JNIEnv *env, jclass obj, jobject objarg) {

  /* Get objarg's class - objarg is the one we pass from
     Java */
  jclass clsHashMap = (*env)->GetObjectClass(env, objarg);

  /* Remember that you can alway get method signature using javap tool
     > javap -s -p java.util.HashMap | grep -A 1 key
         public java.util.Set<K> keySet();
           descriptor: ()Ljava/util/Set;
  */

  jmethodID midKeySet =
    (*env)->GetMethodID(env, clsHashMap, "keySet", "()Ljava/util/Set;");

  /* We have to make sure that method exists */
  if (midKeySet == NULL) {
    return -1; /* method not found */
  }

  /* Now, it's time for getting Set of keys */
  jobject objKeySet = (*env)->CallObjectMethod(env, objarg, midKeySet);

  /* Then, we can proceed to accessing keys */
  jclass clsSet = (*env)->GetObjectClass(env, objKeySet);

  /* The same story goes here - use javap to get propper descriptor
     > javap -s -p java.util.Set | grep -A 1 toArray
         public abstract java.lang.Object[] toArray();
           descriptor: ()[Ljava/lang/Object;
  */

  jmethodID midToArray =
    (*env)->GetMethodID(env, clsSet, "toArray", "()[Ljava/lang/Object;");

  if (midKeySet == NULL) {
    return -2; /* method not found */
  }

  jobjectArray arrayOfKeys = (*env)->CallObjectMethod(env, objKeySet, midToArray);

  int arraySize = (*env)->GetArrayLength(env, arrayOfKeys);

  for (int i=0; i < arraySize; i++)
  {
    jstring objKey = (*env)->GetObjectArrayElement(env, arrayOfKeys, i);
    const char* c_string_key = (*env)->GetStringUTFChars(env, objKey, 0);

    /* Once we have key, we can retrieve value for that key */
    jmethodID midGet =
      (*env)->GetMethodID(env, clsHashMap, "get", "(Ljava/lang/Object;)Ljava/lang/Object;");

    /* It's time to get Value for Key */
    jobject objValue = (*env)->CallObjectMethod(env, objarg, midGet, objKey);
    const char* c_string_value = (*env)->GetStringUTFChars(env, objValue, 0);

    printf("[key, value] = [%s, %s]\n", c_string_key, c_string_value);

    (*env)->ReleaseStringUTFChars(env, objKey, c_string_key);
    (*env)->DeleteLocalRef(env, objKey);

    (*env)->ReleaseStringUTFChars(env, objValue, c_string_value);
    (*env)->DeleteLocalRef(env, objValue);
  }

  return 0;

}

For a full sample code, take a look here: recipeNo037

I, personally, would have HashMap replaced by two Array objects. This way, lots of code can be removed.

Take a look here (recipeNo038) for alternative approach:

This time, we are passing two Arrays of Strings. They are aligned such way, that corresponding indexes contain key/value from HashMap. This way, we can heavily reduce the code in C while overhead in Java is not that big.

JNIEXPORT int JNICALL Java_recipeNo038_PassHashMap_displayHashMap
  (JNIEnv *env, jclass obj, jobjectArray keys, jobjectArray values) {

  /* We need to get array size. There is strong assumption that
     keys and values have the same length
  */
  int arraySize = (*env)->GetArrayLength(env, keys);

  /* For all elements in array, we will convert them to C based strings
  */
  for (int i=0; i < arraySize; i++)
  {
    /* First, we take key */
    jstring objKey = (*env)->GetObjectArrayElement(env, keys, i);
    const char* c_string_key = (*env)->GetStringUTFChars(env, objKey, 0);

    /* Then, we take the value value  */
    jobject objValue = (*env)->GetObjectArrayElement(env, values, i);
    const char* c_string_value = (*env)->GetStringUTFChars(env, objValue, 0);

    /* And we print some info for user */
    printf("[key, value] = [%s, %s]\n", c_string_key, c_string_value);

    /* Make sure to release stuff */
    (*env)->ReleaseStringUTFChars(env, objKey, c_string_key);
    (*env)->DeleteLocalRef(env, objKey);

    (*env)->ReleaseStringUTFChars(env, objValue, c_string_value);
    (*env)->DeleteLocalRef(env, objValue);
  }

  return 0;

}