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From: Eric Sosman <esosman@ieee-dot-org.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: exporting a HashMap
Date: Sun, 15 Jan 2012 12:01:55 -0500
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On 1/15/2012 11:30 AM, Roedy Green wrote:
> What is best code to export key-value pairs from a HashMap into some
> structure in key-value order.
>
> One possible way is to extract the keys, sort them, then look up the
> values.  That strikes me as infantile.  Surely there is a better way.
>
> The best I can think of is to invent a Pair dummy class, extract the
> fields from an Entry into the two fields, and sort them in an array.
>
> Part of the problem is generics and arrays don't mix.

     Why invent a Pair when Map.Entry already holds the data?  Just
typed in, unchecked:

	Map<K,V> map = ...;
	List<Map.Entry<K,V>> entries =
	    new ArrayList<Map.Entry<K,V>>(map.entrySet());
	Collections.sort(entries, new Comparator<Map.Entry<K,V>>() {
	    @Override
	    public int compare(Map.Entry<K,V> e1, Map.Entry<K,V> e2) {
	        return e1.getKey().compareTo(e2.getKey());
	    }
	});
	for (Map.Entry<K,V> e : entries) { ... }

There's scary Javadoc about the longevity of Map.Entry, but as long
as the underlying Map isn't changed all should be well.

     Another approach is to use a SortedMap, either ab initio or as
a substitute for the List-and-sort above:

	Map<K,V> map = ...;
	SortedMap<K,V> map2 = new TreeMap<K,V>(map);
	for (Map.Entry<K,V> e : map2.entrySet()) { ... }

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