code: public void addCertPathChecker(PKIXCertPathChecker checker) {
          if (checker != null) {
              certPathCheckers.add((PKIXCertPathChecker)checker.clone());
          }
      }
docs: Adds a {@code PKIXCertPathChecker} to the list of certification
  path checkers. See the {@link #setCertPathCheckers setCertPathCheckers}
  method for more details.
  <p>
  Note that the {@code PKIXCertPathChecker} is cloned to protect
  against subsequent modifications.
  
  @param checker a {@code PKIXCertPathChecker} to add to the list of
  checks. If {@code null}, the checker is ignored (not added to list).