The next step is a quick one. Let's make the cube spin. This is achieved by rotating the cubeTransform matrix a little bit for each frame. We can define a TIME_DELTA value for this. Add the static variable, as follows:

    // Viewing variables
    private static final float TIME_DELTA = 0.3f;

Then, modify cubeTransform for each frame, and add the following line of code to the onNewFrame method:

Matrix.rotateM(cubeTransform, 0, TIME_DELTA, 0.5f, 0.5f, 1.0f);

The Matrix.rotateM function applies a rotation to a transformation matrix based on an angle and an axis. In this case, we are rotating by an angle of TIME_DELTA around the axis vector (0.5, 0.5, 1). Strictly speaking, you should provide a normalized axis, but all that matters is the direction of the vector and not the magnitude.

Build and run it. Now the cube is spinning. Animazing!