XML : SAMLResponse getting object not initialized for signature or verification

I was given a certificate to generate a SAML. This is what I did so far but I get org.apache.xml.security.signature.XMLSignatureException: object not initialized for signature or verification error at Signer.signObject(singature).
If I comment out this line and try I get the SAMLResponse however without SignatureValue and DigestValue. But of course I need to have them.
Any ideas?

(Also I only have the publicKey() from the cert as this was not generated by me. Please let me know if this is even right in the first place. I didn't find any relevant answered questions in SO so posted here.)

          doSmthMethod() {        signature = getSignature(); // See below       assertion.setSignature(signature);        try       {        Configuration.getMarshallerFactory().getMarshaller(assertion).marshall(assertion);      Signer.signObject(signature);  //**Errors out Here**      } catch (MarshallingException e) {          logger.log(Level.SEVERE,"Marshalling Exception:" + e);                } catch (SignatureException e) {          logger.log(Level.SEVERE,"Signature Exception:"+e);          }      }      ...      ...      ...      ...      ...        public static Signature getSignature() {        Credential signingCredential = getCredential(); // See below        Signature signature = null;      try {          DefaultBootstrap.bootstrap();      } catch (ConfigurationException e) {          e.printStackTrace();      }               signature = (Signature) Configuration.getBuilderFactory()              .getBuilder(Signature.DEFAULT_ELEMENT_NAME)              .buildObject(Signature.DEFAULT_ELEMENT_NAME);      signature.setSignatureAlgorithm(SignatureConstants.ALGO_ID_SIGNATURE_RSA_SHA1);      signature.setSigningCredential(signingCredential);        SecurityConfiguration secConfig = Configuration.getGlobalSecurityConfiguration();         //Added below for Keyinfo              KeyInfo keyInfo = null;/*   (KeyInfo)Configuration.getBuilderFactory()              .getBuilder(Signature.DEFAULT_ELEMENT_NAME)              .buildObject(Signature.DEFAULT_ELEMENT_NAME);*/              NamedKeyInfoGeneratorManager namedKeyInfoGeneratorManager = secConfig.getKeyInfoGeneratorManager();               KeyInfoGeneratorManager keyInfoGeneratorManager = namedKeyInfoGeneratorManager.getDefaultManager();               KeyInfoGeneratorFactory keyInfoGeneratorFactory = keyInfoGeneratorManager.getFactory(signingCredential);               KeyInfoGenerator keyInfoGenerator = keyInfoGeneratorFactory.newInstance();               try{              try {                  keyInfo = keyInfoGenerator.generate(signingCredential);              } catch (org.opensaml.xml.security.SecurityException e) {                  logger.log(Level.SEVERE,"Exception while creating keyInfo:: "+ e);              }              }catch (SecurityException e) {              logger.log(Level.SEVERE,"Exception occured: "+e);              }               signature.setKeyInfo(keyInfo);      try {          logger.log(Level.INFO,"Prepating signature params...");          System.out.println("In getSignature.....before preparingSignatureParams");          SecurityHelper.prepareSignatureParams(signature, signingCredential, secConfig, null);          System.out.println(" After preparing params");      } catch (SecurityException e) {          logger.log(Level.SEVERE,"Failed to prepareSignature:: "+ e);          System.out.println(" *******Got securirytExcption");          e.printStackTrace();      } catch (org.opensaml.xml.security.SecurityException e) {          logger.log(Level.SEVERE,"Failed to prepareSignature:: "+ e);          System.out.println(" *******Got openSaml securirytExcption");          e.printStackTrace();      }      catch(Exception e)      {          System.out.println(" *******Got Exception in SelfSignassertion");          e.printStackTrace();      }      return signature;  }    private static Credential getCredential()  {            InputStream inStream = new FileInputStream("C:\\samplecert.cer");          CertificateFactory cf = CertificateFactory.getInstance("X.509");            X509Certificate cert=    (X509Certificate)cf.generateCertificate(inStream);          BasicX509Credential credential = new BasicX509Credential();            credential.setEntityCertificate(cert);          credential.setPublicKey(cert.getPublicKey());          return credential;  }