Cannot add objects to TreeSet

Question

When adding two Order objects to a TreeSet I see that the first is added, but the second is ignored. The implementation of the Order.equals method is shown below. Both the objects definitely have different orderIds, but more importantly I placed a break point within the equals method and it did not get hit?!

My question is why would the second Order was added to the TreeSet. The only reason I can think of is that it had the same orderId, hence it would have been ignored, but this is definitely not the case.

private final long orderId;

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    Order other = (Order) obj;
    if (orderId != other.orderId)
        return false;
    return true;
}

`TreeSet` doesn't work with `equals` and `hashCode` methods but with a `Comparator`. — Luiggi Mendoza, Nov 26 '13 at 19:16
In this case, the equals and the compare method from the Comparator or compareTo method from Comparable interface should be consistent in order not to break the set contract. — dragon66, Nov 26 '13 at 19:29

score 2 · Answer 1 · answered Nov 26 '13 at 19:18

2

TreeSet uses the object's implemented Comparable<Order> or by a provided Comparator<Order> to determine equivalence.

answered Nov 26 '13 at 19:18

Glenn Lane

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score 0 · Answer 2 · answered Jun 12 '14 at 15:25

0

From the Java documentation, TreeSet can also use the natural ordering, depending which constructor is used. Your way to override the equals method looks fine.

answered Jun 12 '14 at 15:25

Juto

1,246
1
13
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Cannot add objects to TreeSet

2 Answers2