SHA2 password hashing in java

I'm trying to hash some passwords with SHA2.

Where can I get a snippet of java code for make that?

I have seen that post but I have something missing: SHA2 password storage with Java

 Mac mac = Mac.getInstance("HmacSha256");
 SecretKeySpec secret = new SecretKeySpec(key.getBytes(), "HmacSha256");
 mac.init(secret);
 byte[] shaDigest = mac.doFinal(phrase.getBytes());
 String hash = "";
 for(byte b:shaDigest) {
     hash += String.format("%02x",b);
 }

The phrase is the String I want encode right? And what is the key (line 2)

Thanks in advance