Loading new level OpenGL - opengl-es

Question

I am working on a small 2d game engine for android-ndk using opengl.
I am facing difficulty on how to change levels, eg. from menu to game screen.
Because the texture ids are not working when loading new textures for game screen using glGenTextures, glGenTextures keeps returning duplicate ids.
// class to bind ndk with java
Renderer.java
void onSurfaceChanged(gl, width, height){
nativeSurfaceChanged();
}
void onDrawFrame(){
nativeDrawFrame();
}
// c++ code
Game.cpp
Screen *screen;
void SetScreen(Screen *scrn){
screen = scrn;
// load textures and create openGL objects (mesh and textures)
screen->Initialize();
}
void Update(){
screen->Update();
screen->Render();
}
NDKActivity.cpp
Game *game;
void nativeSurfaceChanged(){
// initlizes stuff like audio engine, rendering engine
game->Initialize();
// set current screen to main menu
game->SetScreen(new MainMenu());
}
void nativeDrawFrame(){
game->Update();
}
MainMenu.cpp
void Update(){
// if some button is clicked
game->SetScreen(new GameScreen());
}
Now when menu is initialized, everything works fine. But on loading GameScreen textue ids get all mixed up
I basically ported it from windows app where I was using GLUT for creating OpenGL context and there it was working fine.
And please let me know if more info is needed

Answer 1

glGenTextures keeps returning duplicate ids
That should not happen, unless you're deleting the texture with that ID somewhere (else) or end up in a different GL context.
I am facing difficulty on how to change levels, eg. from menu to game screen.
It's as easy as drawing something different. Have two functions draw_menu and draw_level; maybe a draw_loadingscreen and while you're in the loading screen or menu you can replace all the stuff the level uses.
Or you could use a modern streaming approach, where you don't discriminate between menu, loading screen and level and instead upon drawing each frame collect the resources required to draw it, load what's not yet loaded into the GL context and then draw.
Deleting GL resources can be mostly offloaded into a background garbage collection routine. I.e. every frame for every resource of your game you increment a counter; in the frame setup, when you actually are about to draw it you reset the counter to zero. Once the counter hits a certain threshold you delete it. If when loading resources into GL you hit an out-of-memory condition you can work yourself through the resources with the highest counter value downward, deleting them from the GL context.

Answer 2

so as it turns out I was listening for change screen method in OnTouch() method which apparently runs on different thread than opengl and thus every thing was falling apart. So I had to add a callback which would change screen in onDrawFrame and it fixed the issue.
thanks for helping :)