Why does Set.contains() not seem to be using o.equals()?

Question

I have a TreeSet containing wrappers which store a Foo object at a certain position, defined like so:

class Wrapper implements Comparable<Wrapper> {
  private final Foo foo;
  private final Double position;

  ...

  @Override boolean equals(Object o) {

    ... 

    if(o instanceof Wrapper)
        return o.getFoo().equals(this.foo);

    if(o instanceof Foo)
        return o.equals(this.foo);
  }

  @Override public int compareTo(MarkerWithPosition o) {
      return position.compareTo(o.getPosition());
  }
}

NavigableSet<Wrapper> fooWrappers = new TreeSet<Wrapper>();

because I want my TreeSet to be ordered by position but searchable by foo. But when I perform these operations:

Foo foo = new Foo(bar);
Wrapper fooWrapper = new Wrapper(foo, 1.0);
fooWrappers.add(fooWrapper);

fooWrapper.equals(new Wrapper(new Foo(bar), 1.0));
fooWrapper.equals(new Foo(bar));
fooWrappers.contains(fooWrapper);
fooWrappers.contains(new Wrapper(foo, 1.0));
fooWrappers.contains(new Wrapper(new Foo(bar), 1.0));
fooWrappers.contains(new Wrapper(foo, 2.0));
fooWrappers.contains(foo);

I get:

true
true
true
true
true
false
Exception in thread "main" java.lang.ClassCastException: org.gridqtl.Marker cannot be cast to java.lang.Comparable
    at java.util.TreeMap.getEntry(TreeMap.java:325)
    at java.util.TreeMap.containsKey(TreeMap.java:209)
    at java.util.TreeSet.contains(TreeSet.java:217)

when I expecting them all to return true, so it seems like TreeSet.contains is not using my equals method as the API suggests. Is there another method I need to overwrite?

Answer 1

TreeSet is a Set implementation that does indeed use compareTo, as explained in the javadoc - emphasis mine:

Note that the ordering maintained by a set (whether or not an explicit comparator is provided) must be consistent with equals if it is to correctly implement the Set interface. (See Comparable or Comparator for a precise definition of consistent with equals.) This is so because the Set interface is defined in terms of the equals operation, but a TreeSet instance performs all element comparisons using its compareTo (or compare) method, so two elements that are deemed equal by this method are, from the standpoint of the set, equal. The behavior of a set is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the Set interface.