Question

So I’ve been struggling with a problem for a while now, figured I might as well ask for help here.

I’m adding Ticket objects to a TreeSet, Ticket implements Comparable and has overridden equals(), hashCode() and CompareTo() methods. I need to check if an object is already in the TreeSet using contains(). Now after adding 2 elements to the set it all checks out fine, yet after adding a third it gets messed up.

running this little piece of code after adding a third element to the TreeSet, Ticket temp2 is the object I’m checking for(verkoopLijst).

    Ticket temp2 = new Ticket(boeking, TicketType.STANDAARD, 1,1);
    System.out.println(verkoop.getVerkoopLijst().first().hashCode());
    System.out.println(temp2.hashCode());

    System.out.println(verkoop.getVerkoopLijst().first().equals(temp2));
    System.out.println(verkoop.getVerkoopLijst().first().compareTo(temp2));
    System.out.println(verkoop.getVerkoopLijst().contains(temp2));

returns this:

22106622
22106622
true
0
false

Now my question would be how this is even possible?

Edit:

public class Ticket implements Comparable{

    private int rijNr, stoelNr;
    private TicketType ticketType;
    private Boeking boeking;


    public Ticket(Boeking boeking, TicketType ticketType, int rijNr, int stoelNr){    
        //setters
    }

    @Override
    public int hashCode(){
        return boeking.getBoekingDatum().hashCode();     
    }

    @Override
    @SuppressWarnings("EqualsWhichDoesntCheckParameterClass")    
    public boolean equals(Object o){
       Ticket t = (Ticket) o;

       if(this.boeking.equals(t.getBoeking())
               &&
          this.rijNr == t.getRijNr() &&  this.stoelNr == t.getStoelNr()
               &&
          this.ticketType.equals(t.getTicketType()))
       {
           return true;
       }

       else return false;

    }

    /*I adjusted compareTo this way because I need to make sure there are no duplicate Tickets in my treeset. Treeset seems to call CompareTo() to check for equality before adding an object to the set, instead of equals().


     */
    @Override
    public int compareTo(Object o) {
        int output = 0;
        if (boeking.compareTo(((Ticket) o).getBoeking())==0)
        {
            if(this.equals(o))
            {
                return output;
            }
            else return 1;
        }
        else output = boeking.compareTo(((Ticket) o).getBoeking());
        return output;
    }

    //Getters & Setters

Answer 1

On compareTo contract

The problem is in your compareTo. Here’s an excerpt from the documentation:

Implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y.

Your original code is reproduced here for reference:

// original compareTo implementation with bug marked

@Override
public int compareTo(Object o) {
    int output = 0;
    if (boeking.compareTo(((Ticket) o).getBoeking())==0)
    {
        if(this.equals(o))
        {
            return output;
        }
        else return 1; // BUG!!!! See explanation below!
    }
    else output = boeking.compareTo(((Ticket) o).getBoeking());
    return output;
}

Why is the return 1; a bug? Consider the following scenario:

• Given Ticket t1, t2
• Given t1.boeking.compareTo(t2.boeking) == 0
• Given t1.equals(t2) return false
• Now we have both of the following:
  • t1.compareTo(t2) returns 1
  • t2.compareTo(t1) returns 1

That last consequence is a violation of the compareTo contract.

---

Fixing the problem

First and foremost, you should have taken advantage of the fact that Comparable<T> is a parameterizable generic type. That is, instead of:

// original declaration; uses raw type!
public class Ticket implements Comparable

it’d be much more appropriate to instead declare something like this:

// improved declaration! uses parameterized Comparable<T>
public class Ticket implements Comparable<Ticket>

Now we can write our compareTo(Ticket) (no longer compareTo(Object)). There are many ways to rewrite this, but here’s a rather simplistic one that works:

@Override public int compareTo(Ticket t) {
   int v;

   v = this.boeking.compareTo(t.boeking);
   if (v != 0) return v;

   v = compareInt(this.rijNr, t.rijNr);
   if (v != 0) return v;

   v = compareInt(this.stoelNr, t.stoelNr);
   if (v != 0) return v;

   v = compareInt(this.ticketType, t.ticketType);
   if (v != 0) return v;

   return 0;
}
private static int compareInt(int i1, int i2) {
   if (i1 < i2) {
     return -1;
   } else if (i1 > i2) {
     return +1;
   } else {
     return 0;
   }
}

Now we can also define equals(Object) in terms of compareTo(Ticket) instead of the other way around:

@Override public boolean equals(Object o) {
   return (o instanceof Ticket) && (this.compareTo((Ticket) o) == 0);
}

Note the structure of the compareTo: it has multiple return statements, but in fact, the flow of logic is quite readable. Note also how the priority of the sorting criteria is explicit, and easily reorderable should you have different priorities in mind.

Related questions

• What is a raw type and why shouldn’t we use it?
• How to sort an array or ArrayList ASC first by x and then by y?
• Should a function have only one return statement?