Java Security - RSA Key generation

I have created the following class RSAM that generates a RSA public key and writes it to exportedRSAKey

Then another class ImportRSA reads exportedRSAKey and generates the key, however the key is not generated and "key" contains null values , could some body find what's wrong

I have done the same thing for the public key and its working fine. The purpose is well known - to do asymettric encryption

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import java.security.*;
import java.security.spec.*;
import java.io.*;

public class RSAM {
    public static void main(String args[]) {
        try {
            KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
            kpg.initialize(512, new SecureRandom(  ));
            KeyPair kp = kpg.generateKeyPair(  );
            Class spec = Class.forName(
                            "java.security.spec.RSAPrivateKeySpec");
            KeyFactory kf = KeyFactory.getInstance("RSA");
            RSAPrivateKeySpec ks = (RSAPrivateKeySpec)
                                  kf.getKeySpec(kp.getPrivate(  ), spec);
            FileOutputStream fos = new             FileOutputStream("exportedRSAKey");
            ObjectOutputStream oos = new ObjectOutputStream(fos);

            oos.writeObject(ks.getModulus());
            oos.writeObject(ks.getPrivateExponent(  ));        



        } catch (Exception e) {
            e.printStackTrace(  );
        }
    }
}

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import java.security.*;
import java.security.spec.*;
import java.io.*;
import java.math.*;

public class ImportRSA {
    public static void main(String args[]) {
        try {
            FileInputStream fis = new FileInputStream("exportedRSAKey");
            ObjectInputStream ois = new ObjectInputStream(fis);
            RSAPrivateKeySpec ks = new RSAPrivateKeySpec(
                        (BigInteger) ois.readObject(  ),
                        (BigInteger) ois.readObject(  )                      
                        );
            KeyFactory kf = KeyFactory.getInstance("RSA");
            PrivateKey pk = kf.generatePrivate(ks);
            System.out.println("Got private key");
         System.out.println("private Key encoded :"+pk.getEncoded());

        } catch (Exception e) {
            e.printStackTrace(  );
        }
    }
}