[Java Core] - hashCode method

hashCode()
There are 4 points when override hashMap().
-  When equals() was override, hashCode must be override too.
-  The result of hashCode must be not change. This mean that you shoudn't include the variable usually change or not unique
-  If equals() return true when called with two objects, Java program call hashCode() on each of those object must return the same result.
- If equals() return false when called with two objects, calling hashCode() on each of those object doesn't have return a different result.

Example: Not Override hashCode()

public class Card {
    private String rank;
    private String suit;
    public Card(String r, String s) {
        if (r == null || s == null) {
            throw new IllegalArgumentException("arg1,arg2 are null.");
        }
       
        rank = r;
        suit = s;
    }

    @Override
    public String toString() {
        return "rank:" + this.rank + " suit: " + this.suit;
    }

    @Override
    public boolean equals(Object obj) {
        if ( !(obj instanceof Card)) return false;
        Card c = (Card) obj;
        return rank.equals(c.rank) && suit.equals(c.suit);
    }
   
}

public class HashCodeTest {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Card card1 = new Card("1", "1");
        Card card2 = new Card("1", "1");
       
        Set<Card> cardList = new HashSet<>();
        cardList.add(card1);
        cardList.add(card2);
       
        System.out.println(cardList);
       
    }
   
}

Result: 
run:
[rank:1 suit: 1, rank:1 suit: 1]
BUILD SUCCESSFUL (total time: 0 seconds)

Example: Override hashCode()
Add block code to Card class
    @Override
    public int hashCode() {
        return rank.hashCode();
    }

Result:
run:
[rank:1 suit: 1]
BUILD SUCCESSFUL (total time: 0 seconds)