Android NDK get ArrayList error

Question

JNIEXPORT jobject JNICALL Java_com_example_androidhellojni_FooFragmentTab_getUserList(JNIEnv *env, jobject obj)
{
    jint i;
    jclass cls_arraylist = (*env)->FindClass(env, "java/util/ArrayList");
    jmethodID init_arraylist = (*env)->GetMethodID(env, cls_arraylist, "<init>", "()V");
    jobject obj_arraylist = (*env)->NewObject(env, cls_arraylist, init_arraylist, "");
    if (obj_arraylist == NULL) LOGD("obj_arrlist fail");
    jmethodID arraylist_add = (*env)->GetMethodID(env, cls_arraylist, "add", "(Ljava/lang/Object;)Z");
    if (arraylist_add == NULL) LOGD("arraylist_add fail");

    jclass cls_int = (*env)->FindClass(env, "java/lang/Integer");
    jmethodID init_int = (*env)->GetMethodID(env, cls_int, "<init>", "(I)V");

    for (i = 0; i < 10; i++) {
        jobject obj_int = (*env)->NewObject(env, cls_int, init_int, i);
        (*env)->CallObjectMethod(env, obj_arraylist, arraylist_add, obj_int);
    }

    return obj_arraylist;
}

This is my sample code to get return ArrayList from C to Java(Android), but compiled and run there exist some error message like:

art/runtime/check_jni.cc:65]     JNI DETECTED ERROR IN APPLICATION: the return type of CallObjectMethod does not match boolean java.util.ArrayList.add(java.lang.Object)
art/runtime/check_jni.cc:65]     in call to CallObjectMethod
art/runtime/check_jni.cc:65]     from java.util.ArrayList com.example.androidhellojni.FooFragmentTab.getUserList()

Answer 1

The error message is pretty clear about what the problem is:

the return type of CallObjectMethod does not match boolean java.util.ArrayList.add(java.lang.Object)

The type in Call<type>Method refers to the type of the method, not the type of the method's argument(s). And the type of the method is boolean, which is not an Object.

Hence you should use CallBooleanMethod to call arraylist_add.