OkhttpPeerCertificateExtractor.java

//In order to extract a peer certificate:

File certificate = new File(certificateUri);
String peerCertificate = PeerCertificateExtractor.extract(certificate);

//Retrieved peer certificates format:
sha256/u4QJrwx7aSejc080BBQKGyTaoJovFBg4SbQ9nhoohb8=

//Enabling okhttp certificate pinning is now made the following way:

client = new OkHttpClient.Builder()
    .certificatePinner(new CertificatePinner.Builder()
      .add("publicobject.com", peerCertificate)
      .build())
    .build();

https://github.com/fabiomsr/okhttp-peer-certificate-extractor