HashSet.contains and HashSet.remove specifications

Question

In the API for java.util.HashSet (this is probably also true for some other parts of the Collections framework), the contains method specification reads "returns true if and only if this set contains an element e such that (o==null ? e==null : o.equals(e))." However, this isn't what I'm experiencing -- it looks like contains returns true iff the set contains an element e such that (o==null ? e==null : o.hashCode()==e.hashCode()). What am I doing wrong here? I have a simplified version of my code below... Thanks.
public class MyInt {
  int number;
  public MyInt(int number) { this.number = number; }
  public boolean equals(Object other) { return other instanceof MyInt ? this.number == (MyInt)other.number ? false; }
/* later in the code */
HashSet<MyInt> set = new HashSet<MyInt>();
set.add(new MyInt(17));
set.contains(new MyInt(17));                      // returns false, should return true, right?
set.size();                                                 // returns 1
/* modifying MyInt class... */
public class MyInt {
  public int hashCode() { return this.number; }
/* then... */
HashSet<MyInt> set = new HashSet<MyInt>();
set.add(new MyInt(17));
set.contains(new MyInt(17));                      // returns true
set.size();                                                 // returns 0

Answer 1

kempshall wrote:
Fair enough, but in case whoever writes the API documentation reads this, I think the wording still needs to be changed: contains does not return true iff there exists at least one element e in the collection such that o == null ? e == null : o.equals(e). Same for remove. Thanks.The API is correct. The fault lies with you not honoring the hashCode() method.
if o.hashCode() = e.hashCode() then o dose not necessarily equal e.
if o.hashCode() != e.hashCode() then o necessarily must not equal e
Hence the exact language would be something like
o == null? e == null : o.hashCode() == e.hashCode() ? moreThanOneObjectWithSameHashCode? o.equals(e) : true:false;Obviously this is a little more wordy, and strickly speaking it's just implementation detail. Just like the API says, HashSet is a set. And mathematically, the set will contain object o if there is at least one element e such that o.equals(e). You need only to honor the equals() method which states in its documentation.
it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codesBasically the API is allowed to assume you've correctly overriden both.