compareTo sets 10 before 2 when sorting on value

Question

Let me give you an example for what i'm trying to achieve. Look only to the numbers because it's a card game.

I have:

harten5 klaveren4 klaveren7 schoppen5 ruiten5 schoppen2 klaverenheer schoppenheer schoppendame schoppen6 klaverenboer schoppen8 ruitenheer klaveren6...

I want to sort this on the value to:

schoppen2 harten2 ruiten2 klaveren2 ruiten3 harten3 schoppen3 klaveren3 klaveren4 harten4 schoppen4 ruiten4 harten5 schoppen5 ruiten5...

But i get the 10 before the 2 like this:

schoppen10 ruiten10 harten10 klaveren10 schoppen2 harten2 ruiten2 klaveren2 ruiten3 harten3 schoppen3 klaveren3...

This is my basic compareTo Method:

@Override
public int compareTo(Card p) {
    return this.value.compareTo(p.value);
}

Answer 1

Because String's (I guess this.value is String) compareTo compares lexicographically, whilst you want numerically. So you have to reimplement it, take substring (or find the last part which is a digit, convert it to Integer and then user compareTo on that Integer.

In general, I think your class Card could be improved a bit. In particular, I would rewrite it:

       class Card {
         private String value;
         private Integer rank;


        ...

         public int compareTo(Card c) {
          // Additional logic if you need to consider 
// also value (suite) in comparison
          return this.rank.compareTo(c.rank);
        }
       }

But you can also use an enum for this purpose.

String java API compareTo