Question

It might not be related, but this is under Android 2.3.3 and I work with openjdk7.

I’m trying to compute the velocity and displacement of an object using its acceleration values, given coordinates in 3 spaces plus time. For that, I created a class named Coord4D, and Velocity, Displacement and Acceleration are child of this class. Like this:

Coord4D:

class Coord4D {

    // Statics //
    private static final long ONE_TO_NANO_FACTOR = 1000000000;

    // Members //
    private long key;
    private float xValue;
    private float yValue;
    private float zValue;

  // ... more ...
}

and an example of inheriting class Velocity:

public class Velocity extends Coord4D{

  // ... members and methods and stuff ...

}

And now the problem. In Coord4D, I generalize the method to obtain an antiderivative from derivatives. For instance, I want to retrieve a Velocity instance given two Acceleration instances (past and current) and the Velocity instance from a previous frame.

Everything was working fine when the methods to do this were in Displacement and Velocity, but I found it was redundant since both were computing the exact same things, and I hate writing two times the same lines of code. So I decided to move these methods into Coord4D, and to use generics like this:

protected static <Derivative extends Coord4D, 
AntiDerivative extends Coord4D> AntiDerivative getIntegrationStep(
        Derivative previous, 
        Derivative current, 
        AntiDerivative previousStep){

    // Time is in nanoseconds, must change it in seconds
    float dT = ( current.getTime() - previous.getTime() ) 
            / ONE_TO_NANO_FACTOR;

    // Parameters have seconds for unit : ..., ?*s, ?, ?/s, ?/s², ... 
    float dX = current.getX() - previous.getX();
    float dY = current.getY() - previous.getY();
    float dZ = current.getZ() - previous.getZ();


    // do the integration step and add the previous step value
    float x = dX * dT + previousStep.getX();
    float y = dY * dT + previousStep.getY();
    float z = dZ * dT + previousStep.getZ();

    return (AntiDerivative) new Coord4D(current.getTime(), x, y, z);
}

and the Velocity and Displacement objects have this method (example from Velocity).

public static Velocity getVelocity(Acceleration previousAccel, 
        Acceleration currentAccel, 
        Velocity initialVelo) {

    return getIntegrationStep(previousAccel, 
            currentAccel, 
            initialVelo); 

}

Now, for some reason, this line

return getIntegrationStep(previousAccel, currentAccel, initialVelo);

result in a ClassCastException. I don’t understand why, as I think my logic with the generics is fine. Can someone help me finding where is the flaw?

The stack trace (from LogCat):

W/dalvikvm(8891): threadid=1: thread exiting with uncaught exception (group=0x4001d648)
E/AndroidRuntime(8891): FATAL EXCEPTION: main
E/AndroidRuntime(8891): java.lang.ClassCastException: me.aybabt.android.prototypes.physics.Coord4D
E/AndroidRuntime(8891):     at me.aybabt.android.prototypes.physics.Velocity.getVelocity(Velocity.java:40)
E/AndroidRuntime(8891):     at me.aybabt.android.prototypes.AcceleratorActivity.onSensorChanged(AcceleratorActivity.java:199)
E/AndroidRuntime(8891):     at android.hardware.SensorManager$ListenerDelegate$1.handleMessage(SensorManager.java:539)
E/AndroidRuntime(8891):     at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime(8891):     at android.os.Looper.loop(Looper.java:150)
E/AndroidRuntime(8891):     at android.app.ActivityThread.main(ActivityThread.java:4293)
E/AndroidRuntime(8891):     at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(8891):     at java.lang.reflect.Method.invoke(Method.java:507)
E/AndroidRuntime(8891):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:849)
E/AndroidRuntime(8891):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
E/AndroidRuntime(8891):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

This line

return (AntiDerivative) new Coord4D(current.getTime(), x, y, z);

can’t work.

AntiDerivate is a subclass of Coord4D. You can’t cast from a class to its subclass. (Only the other direction works.)

What you can do: Instead of using new Coord4D you get the class of previousStep, which is an instance of Antiderivate, and with Class.newInstance() you create the new instance which you want to return.