Why ConcurrentSkipListSet.contains requires comparator and not equals

Question

I am using ConcurrentSkipListSet and using contains method.

As per JAVA doc for contains method

Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that o.equals(e).

But as per my test , it appears that equals method is not used but rather Comparator is mandatory. Please help me understand this anomaly between JAVA spec and implementation

ConcurrentSkipListSet

/** * If using comparator, return a ComparableUsingComparator, else * cast key as Comparable, which may cause ClassCastException, * which is propagated back to caller. */ private Comparable comparable(Object key)

at java.util.concurrent.ConcurrentSkipListMap.comparable(ConcurrentSkipListMap.java:663) at java.util.concurrent.ConcurrentSkipListMap.doGet(ConcurrentSkipListMap.java:821) at java.util.concurrent.ConcurrentSkipListMap.containsKey(ConcurrentSkipListMap.java:1608)

I am using Oracle JDK 7

Answer 1

I think there are two questions/concerns, (1) Why does the contains require a Comparator or Comparable. (2) The Javadoc says it will use the equals method.

1. A ConcurrentSkipListSet is a navigable ordered collection so all elements either must maintain a natural order, or you must specify a comparator.
2. I think the Javadoc is stated incorrectly, or at the very least it is misleading. Under the hood, the CSLS will delegate to a ConcurrentSkipListMap.containsKey so it, right now, does not control the contains implementation. That said, I think there can be an argument to clarify the javadocs.

EDIT: There is also a throws doc for the fact these objects aren't comparable

ClassCastException - if the specified element cannot be compared with the elements currently in this set