Basic OOP Poker - Deck, Cards and Hands
Welcome to Code Review, and welcome to Java! Overall your code is clean and easy to understand, so thank you for that.

Enums

Storing these values in a string array is not very extensible:

public final String[] SUITS = { "H", "D", "C", "S" };
public final String[] RANKS = { "A", "K", "Q", "J", "10", "9", "8", "7", "6", "5", "4", "3", "2" }


The first thing to note is that a List provides more options and flexibility than a raw array.  However, I think it would be best to store these values as an Enum:

public enum SUITS {
    H, D, C, S
}


This will allow you to use these values in a switch statement, and provides other benefits.  Most of all, it is more readable and more extensible.

Variable Names

I would strongly recommend against single letter variable names outside of simple constructs such as for (int i = 0; i &lt; something; i++);.  Instead of H, D, C, and S, I would just spell them out as HEARTS, DIAMONDS, etc.  There is no disadvantage to using the extra characters to make the code more readable. In the rest of your code you do not make this mistake, so that is good.

Improper OOP

I do not think that Hand should inherit from Deck:

class Hands extends Deck {


I would advise that you think about inheritance as an "is a" relationship.  A hand is not a deck, and should not be treated like one. For example, it is unlikely that you will need to "shuffle" a hand.  To me, this is the same as having Hand or Deck inherit from a Card.  It is not going to end up being helpful to the organization or understanding of the code.

I don't understand why Cards is a class:

public class Cards {


A singular Card should be the class here.  Then, the Hand and Deck can have a "has a" relationship with cards, and use Card objects.  The way you have this set up here:

    List&lt;String&gt; cards = new ArrayList&lt;&gt;();
    Deck deck = new Deck(cards);
    deck.shuffle(cards);

    Hands hands = new Hands();
    hands.getHand(cards);
    hands.showHand();


really does not make a whole lot of sense.  Why would a Cards object have Decks or Hands as objects?  It should definitely be the other way around.

I look forward to seeing a revised version of your code in another question!