http://www.bouncycastle.org/java.html: The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 to JDK 1.8. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.
  • The Legion of the Bouncy Castle Inc. 
package org.bouncycastle.cert.path;

import java.util.HashSet;
import java.util.Set;

import org.bouncycastle.cert.X509CertificateHolder;

class CertPathUtils
{
    static Set getCriticalExtensionsOIDs(X509CertificateHolder[] certificates)
    {
        Set criticalExtensions = new HashSet();

        for (int i = 0; i != certificates.length; i++)
        {
            criticalExtensions.addAll(certificates[i].getCriticalExtensionOIDs());
        }

        return criticalExtensions;
    }
}