How to convert Byte array to PrivateKey or PublicKey type?

If you have a byte[] representing the output of getEncoded() on a key, you can use KeyFactory to turn that back into a PublicKey object or a PrivateKey object.

byte[] privateKeyBytes;
byte[] publicKeyBytes;
KeyFactory kf = KeyFactory.getInstance("RSA"); // or "EC" or whatever
PrivateKey privateKey = kf.generatePrivate(new PKCS8EncodedKeySpec(privateKeyBytes));
PublicKey publicKey = kf.generatePublic(new X509EncodedKeySpec(publicKeyBytes));