How to access Pair<> of Java in JNI?

Question

I have ArrayList<Pair<Pair<Float, Float>, Pair<Float, Float>>> on Java side and want to use this data in JNI.

What methods and classes to use to convert to std::pair<std::pair<float, float>, std::pair<float, float>>

I tried following

jclass pairClass = env->FindClass("android/util/Pair");

jfieldID pairGetKey = env->GetFieldID(pairClass, "first", "java/util/Objects");

pairGetKey is always null

Answer 1

The type of the field has to be given as a signature; that is, you need to use the int -> I, T[] -> [T, reference.Type -> Lreference/Type; encoding. Also, the type of the field is java.lang.Object, not java.util.Objects.

jfieldID first = env->GetFieldID(pairClass, "first", "Ljava/lang/Object;");
jfieldID second = env->GetFieldID(pairClass, "second", "Ljava/lang/Object;");

The rest is tedious, but not hard:

jfloat extract_float(JNIEnv *env, jobject f) {
    // Note the syntax of signatures: float floatValue() has signature "()F"
    return env->CallFloatMethod(f,
        env->GetMethodID(env->FindClass("java/lang/Float"), "floatValue", "()F"));
}
std::pair<jobject, jobject> extract_pair(JNIEnv *env, jobject p) {
    jclass pairClass = env->FindClass("android/util/Pair");
    jfieldID first = env->GetFieldID(pairClass, "first", "Ljava/lang/Object;");
    jfieldID second = env->GetFieldID(pairClass, "second", "Ljava/lang/Object;");

    return std::pair(env->GetObjectField(p, first), env->GetObjectField(p, second));
}

JNIEnv *env;
jobject pair;
auto [f1, f2] = extract_pair(env, pair);
auto [f11, f12] = extract_pair(env, f1);
auto [f21, f22] = extract_pair(env, f2);
std::pair p(
        std::pair(extract_float(env, f11), extract_float(env, f12)),
        std::pair(extract_float(env, f21), extract_float(env, f22)));

Though, I think I must ask, do you actually need to do this? Can you preprocess the nested pairs to something nicer on the Java side? Doing it on this side is ugly.