// fetch a certificate from a PKCS12 certificate
KeyStore ks = KeyStore.getInstance ( "PKCS12" );

// for security, KeyStore wants certificate password as char[]
char[] password = "Sesame".toCharArray();

ks.load( new FileInputStream( "yourcert.p12" ), password );
Certificate c = ks.getCertificate( "thecert" );
PublicKey p = c.getPublicKey();

// To fetch a certificate from .keystore use
KeyStore ks = Keystore.getInstance ( "JKS"  );
// instead instead of .getInstance ( "PKCS12" )