Re: Need clarification on Object.equals.

Patricia Shanahan <>
Wed, 19 Dec 2012 09:39:06 -0800
On 12/19/2012 9:06 AM, Arne Vajh?j wrote:

On 12/19/2012 11:43 AM, Lars Enderin wrote:

2012-12-19 17:22, Arne Vajh?j skrev:

On 12/19/2012 11:13 AM, FredK wrote:

On Tuesday, December 18, 2012 10:56:19 AM UTC-8, Peter Duniho wrote:

On Tue, 18 Dec 2012 04:48:16 -0800, Roedy Green wrote:

Node a = new Gate();
Monitor b = new Monitor();
System.out.println(a.equals(b)); // --> prints 'true'

Gate or one of its superclasses is implementing equals.

And doing it incorrectly, I'd say.

Why would you think it was done incorrectly?
For most classes, a.equals(b) is not the same as a==b.
Think about string:
string a = "abc");
string b = new string(a);
Clearly a==b is false, but a.equals(b) is true.

Usually the equals() method returns true if the internal state of both
objects is the same.

It is rather unusual to have objects of different classes considered

Your example is different.

Both Gate and Monitor were extensions of AbstractSet, and empty, which
eventually led to Set#containsAll, and thus equals, returning true.

Yes. We know that now. And the problem was solved by not
extending that.

But if the class should continue to extend from
that it should probably have overridden equals again
with an implementation that did use instanceof to test
for type (it could have called super.equals after that
if appropriate).

AbstractSet implements Set, so all its subclasses also implement Set.

That means they are bound by the Set contract for .equals, which treats
all classes that implement Set the same way. There might be a
performance advantage to overriding AbstractSet's .equals in some cases,
but its result would have to be preserved.

The right solution is, of course, the action the OP is already taking -
don't extend AbstractSet unless you really are implementing a Set, and
intend to follow the Set contract.


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