Compile the C++ code creating a shared library (or shared object in UNIX)

$ clang++ TestJNI.cpp -o libTestJNI.so -fPIC -shared -std=c++11 -I$HOME/opt/java/include -I$HOME/opt/java/include/linux

Run the application

$ scala -save load.scala
dir = /home/archbox/opengl/jni/libTestJNI.so
Hello world java
i = 0
i = 1
i = 2
i = 3
i = 4
Library version        = version 3.4
vector3DLen(3, 4, 5)   = 7.0710678118654755
Average of 1, 2 ... 8  = 4.5

$ scala -save load.jar
dir = /home/archbox/opengl/jni/libTestJNI.so
Hello world java
i = 0
i = 1
i = 2
i = 3
i = 4
Library version        = version 3.4
vector3DLen(3, 4, 5)   = 7.071

Check compilation output

$ file libTestJNI.so
libTestJNI.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=6a21fdf13b652757f1773db2069095eaa050b759, not stripped

Check symbols

$ nm -D libTestJNI.so | grep " T "
0000000000000f08 T _fini
0000000000000a28 T _init
0000000000000d40 T Java_TestJNI_average
0000000000000cc0 T Java_TestJNI_getVersion
0000000000000c20 T Java_TestJNI_printDisplay
0000000000000cf0 T Java_TestJNI_vector3DLen
0000000000000e00 T main