Re: problem with method overloading

From:

Eric Sosman <esosman@ieee-dot-org.invalid>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 14 Jun 2010 14:25:21 -0400

Message-ID:

<hv5s83$8oj$1@news.eternal-september.org>

On 6/14/2010 1:46 PM, Simon Brooke wrote:

On Mon, 14 Jun 2010 10:05:08 -0700, Lew wrote:

Your method overload question is quite important, actually. It's a
common mistake to overload 'equals()' when the intent is to override,
e.g.,

  public class Foo
  {
   ...
    public boolean equals( Foo other )
    {
     ...
    }
  }

This method will appear to work if passed a 'Foo' argument because the
'Foo' argument matches more specifically than the 'Object' argument to
the other 'equals()' method in the class, but it will fail in general
when clients pass an 'Object' argument and bypass the specific overload,
getting the parent-class version instead.

Sorry, why is this a mistake? If a 'Bar' argument is passed, equals must
return false anyway. Does it matter which implementation of equals is
invoked?

     The second sentence is often true, even "usually" true, but
not universally true. It sometimes makes sense for the equals()
of one class to return true when handed an instance of a different
class. For example, consider the equals() contract of Set:

    Set<String> hs = new HashSet<String>();
    hs.add("hello");
    hs.add("world");

    Set<String> ts = new TreeSet<String>();
    ts.addAll(hs);

    assert hs.getClass() != ts.getClass();
    assert hs.equals(ts);

--
Eric Sosman
esosman@ieee-dot-org.invalid