I need your help guys "HashMap"

Hi,

I'm working in course project to Manage League Games

I'm trying to write a code to generate Matches between Team.

so,
I declare
private List<Team> _data = new ArrayList<Team>(); // this list has all Team in the League
private Map<Matches,MatchesResult> _matches = new HashMap<Matches,MatchesResult>(); // it has all Matches among Teams also the Matches result

Class Matches has attributes Team team1, team2;
Class MatchesResult has attributes Matches match; int team1Result, team2Result;

So I write the following code to generate the Matches;

public void generateMatches(){
Team [] team = new Team[_data.size()];
int k=0;
Iterator t = _data.iterator();
// The following loop used to copy all Team into team array
while (t.hasNext()) {
team[k]= (Team) t.next();
k++;
}
// This code is to generate Matches among Teams
for(int i=0;i<team.length;i++){
for(int j =i+1;j<team.length;j++){
Matches m1 = new Matches(team[i],team[j]);
MatchesResult m2 = new MatchesResult(m1);
_matches.put(m1,m2);
}
}

}

I run the program with the following code;

Team t1 = new Soccer("Brazil");
Team t2 = new Soccer("France");
Team t3 = new Soccer("India");
Team t4 = new Soccer("Italy");

LeagueGames a = new LeagueGames();

System.out.println(a.size());
a.addTeam(t1);
a.addTeam(t2);
a.addTeam(t3);
a.addTeam(t4);
System.out.println(a.size());
a.generateMatches();
System.out.println(a.toString());

System.out.println(a.get(new Matches(t3,t4))); // Here suppose I get the Match between India and Italy BUT I got NULL


Here is the Output
0
4
Leagua Matches:
Brazil vs France : 0 - 0
France vs Italy : 0 - 0
France vs India : 0 - 0
Brazil vs Italy : 0 - 0
India vs Italy : 0 - 0
Brazil vs India : 0 - 0

null
I have no idea why I got NULL.

Could you help, please?

Thank's

There is a lot of stuff missing from what you posted: it's hard to say what a.get() is doing without knowing what a LeagueGames is.

I am guessing that get() obtains what it returns from a map. In that case you might get a null because "new Matches(t3,t4)" is not equal to any of the keys within the map. See get() in the Map API docs.

To be more pointed about it, get() involves comparing keys using their equals() method: does your Matches class override equals()? Because it should.

Yes, you are right

Here's implementation of get() method in LeagueGames

public MatchesResult get(Matches m) {

if ( _matches.containsKey(m))
return _matches.get(m);
else
return null;
}


But when I print out the map, "new Matches(t3,t4)" is already there in the map

I have no doubt that there is some match between India and Italy as a key in the map. But how is the _matches map supposed to know that "new Matches(t3,t4)" match between India and Italy is equal() to the match it has as a key?

And, again, does Matches override equals()?

It's not just maps, but a lot of the collection classes use equals():

Code:

---

import java.util.ArrayList;
import java.util.List;

class MyInt {
        private int val;
       
        MyInt(int val) {
                this.val = val;
        }
       
        @Override
        public String toString() {
                return String.valueOf(val);
        }
}

public class EqualityCounts {
        public static void main(String[] args) {
                List<MyInt> list = new ArrayList<MyInt>();
                MyInt devil = new MyInt(666);
               
                list.add(devil);
                list.add(new MyInt(0));
                list.add(new MyInt(1));
               
                System.out.print("The list is ");
                for(MyInt i :list) {
                        System.out.print(i);
                        System.out.print(" ");
                }
                System.out.println();
                System.out.println("list contains 666: " + list.contains(new MyInt(666)));
        }
}

---

Try replacing MyInt with Integer inside the main() method. The difference is that Integer overrides equals(), but MyInt does not.

hah,

Code:

---

       

public boolean equal(Matches that){
                if ( this._team1.equals(that._team1) && this._team2.equals(that._team2)){
                        return true;
                }
                else
                        return false;
        }

---

So, I have to override the equal() in all class
Team
Matches
MatchesResult

Yes, if Matches is going to function as a key in a map which depends only on its two team attributes then it will need an equals() that expresses that fact. (I don't know about the other classes; it depends on how they're used.)

Have a read of Roedy Green's equals() page. Note the comment about hashCode() that he makes near the end.

what about compareTo() still I have to override it too,

I really appreciate your efforts,

Thank you,

Do any of your classes have a compareTo()?

hashCode() and equals() are something every class has. But compareTo() is only for classes that implement the Comparable interface.

No, I don't have classes implement the Comparable interface.

Quote:

---

No, I don't have classes implement the Comparable interface.

---

You should be good then. Comparable is used for collections which are sorted (SortedSet etc) and with them care has to be taken to make equals() and compareTo() consistent.