Build errors while compiling Java TD client - c++

Question

Trying to build TD:
https://github.com/tdlib/td
https://github.com/tdlib/td/tree/master/example/java
The main build works fine, but the Java examples do not.
My setup :
Ubuntu 18.04.2 LTS
cmake 3.10.2
gcc (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
OpenSSL 1.1.1 11 Sep 2018
1:1.2.11.dfsg-0ubuntu2
gperf 3.1-1
After 30 minutes or so CMake spits this out.. any ideas what it means and where I can go from here?
Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0xc8): undefined reference to vtable for td::td_api::formattedText' Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0xeb): undefined reference to vtable for td::td_api::textEntity'
Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x15d): undefined reference to vtable for td::td_api::photo' Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x190): undefined reference to vtable for td::td_api::photoSize'
Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x1ab): undefined reference to vtable for td::td_api::file' Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x1c3): undefined reference to vtable for td::td_api::remoteFile'
Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x201): undefined reference to vtable for td::td_api::localFile' Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x29a): undefined reference to vtable for td::td_api::animation'
Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x2b5): undefined reference to vtable for td::td_api::file' Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x2ce): undefined reference to vtable for td::td_api::remoteFile'
Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x305): undefined reference to vtable for td::td_api::localFile' Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x34c): undefined reference to vtable for td::td_api::photoSize'
Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x369): undefined reference to vtable for td::td_api::file' Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x380): undefined reference to vtable for td::td_api::remoteFile'
Game.cpp:(.text._ZNK2td4Game15get_game_objectEPNS_2TdE+0x3b6): undefined reference to vtable for td::td_api::localFile' libtdcore.a(Game.cpp.o): In function td::process_input_message_game(td::ContactsManager const*, td::tl::unique_ptrtd::td_api::InputMessageContent&&)':
Game.cpp:(.text._ZN2td26process_input_message_gameEPKNS_15ContactsManagerEONS_2tl10unique_ptrINS_6td_api19InputMessageContentEEE+0xbc): undefined reference to `vtable for td::td_api::inputMessageGame'
collect2: error: ld returned 1 exit status
make[2]: *** [tg_cli] Error 1
make[1]: *** [CMakeFiles/tg_cli.dir/all] Error 2
make: *** [all] Error 2
What does undefined reference to vtable mean? Did I forget to compile something else first? or in a wrong folder?

Answer 1

I must have messed up a path somewhere . I deleted the folder, cloned it again and follows the steps in this tool:
https://tdlib.github.io/td/build.html?language=Java