In order to test the default initialization of AlgorithmParameterGenerator class, I use test code as follows:

  AlgorithmParameterGenerator paramGen =
AlgorithmParameterGenerator.getInstance("DH");
 
  AlgorithmParameters params = paramGen.generateParameters();
  DHParameterSpec spec = (DHParameterSpec)
  params.getParameterSpec(DHParameterSpec.class);
  BigInteger bigG=spec.getG();
  BigInteger bigP=spec.getP();
  int randomExponent=spec.getL();

see the attachment for complete code.
after I run the code, I got the 0 value of randomExponent. Actually, this value is supposed to be the private key, so it can not be a fixed value.