What may be the easiest way to append multiple jstrings in JNI?

Question

I'm working on an android app where I need to append 5-6 jstrings and want to return a single jstring in native code. Strings are like...

jstring1= "Hello, "
jstring2= "How"
jstring3= "are"
jstring4= "you"
jstring5= "today?"

and I want to return a jstring like, "Hello, how are you today?" I'm Just wandering to know the easiest way to do it.

Answer 1

First, you make a StringBuilder:

jclass cls_StringBuilder = env->FindClass("java/lang/StringBuilder");
jmethodID ctr_StringBuilder = env->GetMethodID(cls_StringBuilder, "<init>", "(I)V");
jobject stringBuilder = env->NewObject(cls_StringBuilder, ctr_StringBuilder, 100);

And then you append string by string:

jmethodID mid_StringBuilder_append = env->GetMethodID(cls_StringBuilder, "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;");
for (auto str: { jstring1, jstring2, jstring3, jstring4, jstring5 }) {
  env->CallObjectMethod(stringBuilder, mid_StringBuilder_append, str);
}

Finally grab the contents of the StringBuilder:

jmethodID mid_StringBuilder_toString = env->GetMethodID(cls_StringBuilder, "toString", "()Ljava/lang/String;");
jstring final = (jstring) env->CallObjectMethod(stringBuilder, mid_StringBuilder_toString);
return final;