Sorting TimeZone

[Wojtek]

How would you sort timezones?

I am trying to sort them according to their offset from UTC. I cannot
use a TreeMap because there are many timezones with the same offset,
which of course over-writes the previously put timezone.

The preferred sort would be offset, then display name.

 

[markspace]

How would you sort timezones?

I am trying to sort them according to their offset from UTC. I cannot
use a TreeMap because there are many timezones with the same offset,
which of course over-writes the previously put timezone.

The preferred sort would be offset, then display name.

I guess put them all in an array, the use Array.sort( Object[],
Comparator<T>) to sort them as you desire.

 

[Wojtek]

markspace wrote :

How would you sort timezones?

I am trying to sort them according to their offset from UTC. I cannot use a
TreeMap because there are many timezones with the same offset, which of
course over-writes the previously put timezone.

The preferred sort would be offset, then display name.

I guess put them all in an array, the use Array.sort( Object[],
Comparator<T>) to sort them as you desire.

Yes, I made my own custom Comparator.

But I just found out what the problem is. You can retrieve a TimeZone
using a variety of names, but that TimeZone only has one display name.
For instance, in Canada:

Canada/Newfoundland Newfoundland Standard Time
Canada/Atlantic Atlantic Standard Time
Canada Eastern Eastern Standard Time
Canada/Central Central Standard Time *
Canada/East-Saskatchewan Central Standard Time *
Canada/Saskatchewan Central Standard Time *
Canada/Mountain Mountain Standard Time
Canada/Pacific Pacific Standard Time *
Canada/Yukon Pacific Standard Time *

I retrieve them using the left names, but the .getDisplayName() returns
the right side. Which throws the sorting out.

Ok, I can make this work. Funny how actually _asking_ a question can
lead to an answer...

 

[markspace]

How would you sort timezones?

I am trying to sort them according to their offset from UTC. I cannot
use a TreeMap because there are many timezones with the same offset,
which of course over-writes the previously put timezone.

The preferred sort would be offset, then display name.

/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/

package sorttimezone;

import java.util.Arrays;
import java.util.Comparator;
import java.util.TimeZone;

/**
*
* @author Brenden
*/
public class Main {

/**
* @param args the command line arguments
*/
public static void main(String[] args) {
// TODO code application logic here
String [] zonesIDs = TimeZone.getAvailableIDs();
TimeZone [] allTZ = new TimeZone[zonesIDs.length];
for( int i=0; i< allTZ.length; i++ ) {
allTZ=TimeZone.getTimeZone( zonesIDs );
}
Arrays.sort( allTZ, new SortByOffsetAndName() );

for( TimeZone zone : allTZ ) {
System.out.println( zone.getDisplayName() + ": "+
zone.getRawOffset()/1000/60/60 );
}
}
}

class SortByOffsetAndName implements Comparator<TimeZone> {

public int compare( TimeZone o1, TimeZone o2 )
{
if( o1.getRawOffset() != o2.getRawOffset() ) {
return o1.getRawOffset() - o2.getRawOffset();
}
return o1.getDisplayName().compareTo( o2.getDisplayName() );
}

}

 

[markspace]

I retrieve them using the left names, but the .getDisplayName() returns
the right side. Which throws the sorting out.

Instead of using getDisplayName() (which was the same thing I did, in my
example I posted), use getID(), that returns the string on the left in
your table.

public int compare( TimeZone o1, TimeZone o2 )
{
if( o1.getRawOffset() != o2.getRawOffset() ) {
return o1.getRawOffset() - o2.getRawOffset();
}
return o1.getID().compareTo( o2.getID() );
}

Ok, I can make this work. Funny how actually _asking_ a question can
lead to an answer...

It works that way for pretty much everyone.

 

[Lew]

How would you sort timezones?

Do you mean 'java.util.TimeZone'?

I am trying to sort them according to their offset from UTC. I cannot
use a TreeMap because there are many timezones with the same offset,
which of course over-writes the previously put timezone.

There's no "of course" about that if you are using your "preferred sort".

Why would you use any kind of 'Map' rather than a 'Collection'?

The preferred sort would be offset, then display name.

'TimeZone' can easily be a map key, yes, even for a 'TreeMap'. I fail to see
the problem. You simply compare based on your "preferred sort".

I'd be more concerned about relocation of a key when the offset changes for
DST. But perhaps you're only building this structure for a given moment in
time and that isn't a concern for you.

Regardless, here's uncompiled, off-the-cuff code, using a 'Set' instead of a
'Map':

public class SortedZone
{
private long ref = System.currentTimeMillis();

private final TreeSet <TimeZone> zones =
new TreeSet <TimeZone>
( new Comparator <TimeZone> ()
{
@Override public int compare( TimeZone t0, TimeZone t1 )
{
return
(t0 == null? (t1 == null? 0: -1)
: t1 == null? 1
: t0.getOffset( ref ) > t1.getOffset( ref )? 1
: t0.getOffset( ref ) < t1.getOffset( ref )? -1
: t0.getDisplayName().compareTo( t1.getDisplayName() )
);
}
}
);

// cover methods for the Set ...
}

 

[Roedy Green]

I am trying to sort them according to their offset from UTC. I cannot
use a TreeMap because there are many timezones with the same offset,
which of course over-writes the previously put timezone.

The preferred sort would be offset, then display name.

I would create my own TimeZone Object with the fields I need then cook
up a Comparable/Comparator for it.

see http://mindprod.com/applet/comparatorcutter.html
to generate the code.

 

[Lew]

Wojtek quoted or indirectly quoted someone who said :

I would create my own TimeZone Object with the fields I need then cook
up a Comparable/Comparator for it.

Wojtek was speaking of java.util.TimeZone, judging by the fields he mentioned.

 

[Wojtek]

Lew wrote :

'TimeZone' can easily be a map key, yes, even for a 'TreeMap'.

No it cannot be used as a key. It does not have a 'compareTo' method.

 

[Lew]

Lew wrote :

No it cannot be used as a key. It does not have a 'compareTo' method.

Yes, it can be used as a key!
<http://java.sun.com/javase/6/docs/api/java/util/TreeMap.html#TreeMap
(java.util.Comparator)>

The Javadocs are your friend.

 

[Wojtek]

Lew wrote :

Lew wrote :


Yes, it can be used as a key!
<http://java.sun.com/javase/6/docs/api/java/util/TreeMap.html#TreeMap
(java.util.Comparator)>

The Javadocs are your friend.

Yes with a separate Comparator. However you cannot do:

TreeMap<TimeZone,String>

directly.

 

[Lew]

Lew wrote :

Yes with a separate Comparator. However you cannot do:

TreeMap<TimeZone,String>

directly.

Why doesn't

Map <TimeZone, String> map =
new TreeMap <TimeZone, String> ( new Comparator() { .... } );

work for you?

 

[Lew]

Why doesn't

  Map <TimeZone, String> map =
    new TreeMap <TimeZone, String> ( new Comparator() { .... } );

work for you?

Oops. Naturally I meant to write

Map <TimeZone, String> map =
new TreeMap <TimeZone, String> ( new Comparator <TimeZone> ()
{ .... } );

 

[Wojtek]

Lew wrote :

Lew wrote:

Oops. Naturally I meant to write

Map <TimeZone, String> map =
new TreeMap <TimeZone, String> ( new Comparator <TimeZone> ()
{ .... } );

No arguments that this does work.

However I was replying to your statement "'TimeZone' can easily be a
map key, yes, even for a 'TreeMap'. " which appears to say that you do
not need a custom Comparator:

Map <TimeZone, String> map = new TreeMap <TimeZone, String>();

Which of course fails during runtime.

 

[Lew]

Lew wrote :
Even though I had explicitly mentioned a custom Comparator in the post
to which you were replying? Come on, now. Your explanation is
disingenuous at best.

The natural order and comparator behaviors are equally valid. Throughout
the TreeMap documentation, unqualified statements apply to both cases.
Statements that only apply to the natural order case say so. Given the
way the TreeMap documentation is written, I would not assume a natural
order TreeMap is meant unless the writer says so.

In this case, the writer did explicitly mention use of a custom
Comparator.

I think it is a mistake to treat a Comparable key type as the proper way
to use TreeMap or TreeSet, and a specified Comparator as somehow weird
or "indirect". It can create a mental block to what is often the best
choice for a sorted set or sorted map implementation, when either the
key type does not implement Comparable, or the required order is not its
Comparable order.

What she said, especially since I had actually referred to a custom
Comparator in my post, and others had also chimed in with that same
suggestion, including Wojtek himself! Pretending that that was not
evident smacks of deliberate obtuseness.

 

[Wojtek]

Lew wrote :

Lew wrote :

Even though I had explicitly mentioned a custom Comparator in the post
to which you were replying? Come on, now. Your explanation is
disingenuous at best.

To me a 'key' is the part which goes into the 'K' part of Map<K,V>.

For a Comparator, the signature is Comparator<T> where the 'T' refers
to a 'Type':
http://www.j2ee.me/javase/6/docs/api/java/util/Comparator.html

 

[Wojtek]

Eric Sosman wrote :

Yes, that's right.


... which means only that the K in Map<K,V> is the same as
the T in Comparator<T>, or is a subclass/subinterface of T. You
shouldn't read too much into the particular letters chosen to name
type parameters; the names are as arbitrary as those of method
parameters, and carry no deeper meaning.

Does Map<String,String> bother you, because String == String
but K != V? If not, K != T shouldn't bother you, either, even if
you have a Map<Book,Date> and a Comparator<Book> with Book == Book.

Semantics!

We work in a precise field. An extra semi-colon can make a world of
difference (brought down a phone system a few years ago).

So yes, the labelling DOES make a difference. Whereas a Book is a Book,
where it is used does change its meaning. So in Map<Book,Date>, Book is
a key and in Comparator<Book>, Book is a Type.

Otherwise the Javadoc author would not have made that distinction.

It is used differently. As a key, it must provide its own comparison
methodology. As a type, the Comparator makes the comparison, possibly
using an external conversion such as I18N.

 

[Wojtek]

Wojtek wrote :

We work in a precise field. An extra semi-colon can make a world of
difference (brought down a phone system a few years ago).

Ah sorry, it was a break statement:
http://users.csc.calpoly.edu/~jdalbey/SWE/Papers/att_collapse.html

 

[Lew]

Semantics!

The word "semantics" means "meaning", so it isn't trivial.

We work in a precise field. An extra semi-colon can make a world of
difference (brought down a phone system a few years ago).

So yes, the labelling DOES make a difference. Whereas a Book is a Book,
where it is used does change its meaning. So in Map<Book,Date>, Book is
a key and in Comparator<Book>, Book is a Type [sic].

Now I see where your difficulty lies and I'm much more sympathetic to your cause.

In both contexts the type parameter signifies a type. In 'Map <K, V>' there
are two type parameters, indicating the types to which the Map is bound. The
first is the type of the key, and the second is the type of the value.

There is no semantic difference between expressing that as 'Map <K, V>',

Otherwise the Javadoc author would not have made that distinction.

There is no distinction. The choice of letters in the Javadocs is arbitrary
and designed to help us understand the usage of the two types managed by the
'Map', but regardless a type parameter is a type parameter is a type parameter.

It is used differently. As a key, it must provide its own comparison
methodology. As a type, the Comparator makes the comparison, possibly
using an external conversion such as I18N.

That is not true. Take a look again at the Javadocs for 'TreeMap' to which I
pointed you earlier:

<http://java.sun.com/javase/6/docs/api/java/util/TreeMap.html#TreeMap(java.util.Comparator)>

Note that they describe the comparator type parameter as '<? super K>' - that
is the exact same 'K' as in the 'TreeMap <K, V>' type parameter.

For further understanding, check out the rules for type parameters in the JLS.

 

[Lew]

That is not true. Take a look again at the Javadocs for 'TreeMap' to
which I pointed you earlier:

<http://java.sun.com/javase/6/docs/api/java/util/TreeMap.html#TreeMap(java.util.Comparator)>

Note that they describe the comparator type parameter as '<? super K>' -
that is the exact same 'K' as in the 'TreeMap <K, V>' type parameter.

I was unclear, in part because your antecedents are hazy. It is true that a
'TreeMap' that uses "natural" order does not need a 'Comparator' and that one
that needs a 'Comparator' does need a 'Comparator'. It is not true that the
type parameter 'K' in 'TreeMap <K, V>' is different from the type parameter
'K' in the 'TreeMap' constructor argument 'Comparator <? super K>'.