It's time to throw in a little more excitement. Let's animate the cube so that it rotates. This'll help demonstrate the shading as well.

For this, we need a Time class. This is a singleton utility class that ticks off frames and makes that information available to the application, for example, via getDeltaTime. Note that this is a final class, which explicitly means that it cannot be extended. There is no such thing as a static class in Java, but if we make the constructor private, we can ensure that nothing will ever instantiate it.

Create a new Time class in the renderbox/ folder. It won't be getting extended, so we can declare it final. Here's the code:

public final class Time {
    private Time(){}
    static long startTime;
    static long lastFrame;
    static long deltaTime;
    static int frameCount;

    protected static void start(){
        frameCount = 0;
        startTime = System.currentTimeMillis();
        lastFrame = startTime;
    }
    protected static void update(){
        long current =System.currentTimeMillis();
        frameCount++;
        deltaTime = current - lastFrame;
        lastFrame = current;
    }

    public static int getFrameCount(){return frameCount;}

    public static float getTime(){
        return (float)(System.currentTimeMillis() - startTime) / 1000;
    }

    public static float getDeltaTime(){
        return deltaTime * 0.001f;
    }
}

Start the timer in the RenderBox setup:

    public RenderBox(Activity mainActivity, IRenderBox callbacks){
        ...
        Time.start();
    }

Then, in the onNewFrame method of RenderBox, call Time.update():

    public void onNewFrame(HeadTransform headTransform) {
        Time.update();
        ...
}

Now, we can use it to modify the cube's transform each frame, via the preDraw() interface hook. In MainActivity, make the cube rotate 5 degrees per second about the X axis, 10 degrees on the Y axis, and 7.5 degrees on the Z axis:

    public void preDraw() {
        float dt = Time.getDeltaTime();
        cube.rotate(dt * 5, dt * 10, dt * 7.5f);
    }

The getDeltaTime() method returns the fraction of a second since the previous frame. So, if we want it to rotate 5 degrees around the X axis each second, we multiply deltaTime by 5 to get the fraction of a degree to turn this particular frame.

Run the app. Rock and roll!!!