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Editorial Team
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Asked: 2026-05-12T11:36:47+00:00

I used a TreeSet with a self written Comparator. Now when I’m adding elements

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I used a TreeSet with a self written Comparator. Now when I’m adding elements to the TreeSet and the Comparator’s compare methods returns 0, it seems like the TreeSet contains only one of the Object with equal ranking.

I didn’t see that this behaviour is documented in the javadocs. Maybe I miss something. Can you confirm this behaviour?

I edited the Comparator. Now it never returns 0 and the TreeSet contains all the Objects with equal ranking.

Is that the way it has to be, if I want to have multiple Objects with equal ranking?

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1 Answer

  1. Editorial Team

    That’s the way it has to be, as a set is defined as including equal objects only once.

    When your Comparator returns 0, two objects are considered equal, therefore only one (probably the first) of all equal objects is included in the set.

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