ObjectOutputStream problem?

I wish to put an ArrayList Object into a File

Can any one tell me what's problem of following code?

public void writeLFile(ArrayList l,File lfFile)

try

{

ObjectOutputStream leafOut=new

ObjectOutputStream(new BufferedOutputStream(new FileOutputStream(lfFile)));

leafOut.writeObject(l);

leafOut.close();

}

catch (Exception ex){

ex.printStackTrace();

}

}

error is:

java.io.NotSerializableException: floatkd.kddata

at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:417)

at floatkd.Dfkdtree.writeLFile(Dfkdtree.java:56)

......

[674 byte] By [pete_he] at [2007-9-26 2:26:18]