The Android Native Development Kit (NDK) - Beginning Android 3D Game Development - page 408

Modifying the Physics Class
Next, the Physics class has to be modified to call the native class.
The ApplyGravityToObjectNative() function has to be declared as a native function in the
Physics class.
native float ApplyGravityToObjectNative(float YAccel);
The ApplyGravityToObject() function calculates the new y component of the acceleration by calling
the native function ApplyGravityToObjectNative() with the current y acceleration of the object.
(See Listing 11-10.)
Listing 11-10. Calling the Native Gravity Calculation Function
void ApplyGravityToObject()
{
// Do Native Apply Gravity
float YAccel = m_Acceleration.y;
m_Acceleration.y = ApplyGravityToObjectNative(YAccel);
}
Rotating Objects from Native Code
Demonstrating the rotation of objects from native C code using Java functions requires modifications
to the hello-jni.c file, the Physics class, and the MyGLRenderer class.
Modifying the hello-jni.c File
The hello-jni.c file has to be modified to add the native class AddRotationNative() .
The AddRotationNative() function is almost identical to the function we discussed in Listing
11-5. The difference is that the full function name involves a different package and class. What
AddRotationNative() does is add RotationAngle degrees to the rotation of the object whose
orientation is represented by the input Orient parameter. This is done by calling the actual Java
language method "AddRotation()" . (See Listing 11-11.)
Listing 11-11. Adding a Rotation
Java_ com_robsexample_glhelloworld_Physics _AddRotationNative(JNIEnv* env,
jobject thiz,
jobject Orient,
jfloat RotationAngle)
{
/*
GetObjectClass
jclass GetObjectClass(JNIEnv *env, jobject obj);
*/
jclass OrientationClass = (*env)->GetObjectClass(env, Orient);