Get the last value in map

Question

How can i get the last and first key/value of a Map? For example:

Map<String,Integer> ret = new HashMap<String,Integer>();

And this is ret's value:

{33=1, 12=2, 21=2, 93=2, 48=9, 68=10}

A `HashMap` isn't ordered, so 'first' and 'last' don't really mean anything. — tzaman, Jan 18 '12 at 07:30
possible duplicate of [Accessing the last entry in a Map](http://stackoverflow.com/questions/3527216/accessing-the-last-entry-in-a-map) — Sean Patrick Floyd, Jan 18 '12 at 07:34
@T.J.Crowder thanks :) I was just writing up an answer with a bit more detail. — tzaman, Jan 18 '12 at 07:35

Mairbek Khadikov · Answer 1 · 2012-01-18T07:39:31.397

4

You cannot do it with HashMap because keys are not ordered. Consider using LinkedHashMap

You can get last element using this method:

public <K,V> Map.Entry<K,V> getLast(LinkedHashMap<K,V> map) {
    Iterator<Map.Entry<K,V>> iterator = map.entrySet().iterator();
    Map.Entry<K, V> result = null;
    while (iterator.hasNext()) {
        result = iterator.next();
    }
    return result;
}

edited Jan 18 '12 at 07:39

answered Jan 18 '12 at 07:32

Mairbek Khadikov

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score 4 · Answer 2 · edited Jan 18 '12 at 11:58

4

A HashMap is an unordered map, so it doesn't have any concept of 'first' or 'last.
If you want a map that retains things in insertion order, you can use a LinkedHashMap, and then iterate its entrySet() method to pick the first and last values.
You could also use the SortedMap interface (TreeMap impl.) which orders inserted entries by the natural ordering of keys (or a provided Comparator).

edited Jan 18 '12 at 11:58

Mairbek Khadikov

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answered Jan 18 '12 at 07:33

tzaman

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*which orders inserted entries by key size* by key size??? I'd rephrase that. +1 for the rest – Sean Patrick Floyd Jan 18 '12 at 07:35
from the TreeMap javadocs: "The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time". There's nothing about the key size there – Sean Patrick Floyd Jan 18 '12 at 07:39
K, fixed. size is the natural ordering of ints, which were the keys in the OP's example. – tzaman Jan 18 '12 at 07:43

score 2 · Answer 3 · answered Jan 18 '12 at 07:33

It isn't possible (or more like doesn't make sense) to differentiate between "first" and "last" when it comes to HashMap. If you want to preserve insertion order, you might want to use LinkedHashMap<K,V>. But then again, you should elaborate your question to let us know the exact use case demanding this.

score 1 · Answer 4 · answered Jan 18 '12 at 07:32

1

since there is no defined order you will basically get a random element if you are unlucky.

else you could go the way HashMap -> getEntrySet -> toArray -> get(size-1)

answered Jan 18 '12 at 07:32

light_303

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score 1 · Answer 5 · answered Jan 18 '12 at 07:45

Technically, you can get the first object from the map via:

Map.Entry<Integer,Integer> entry = map.entrySet().iterator().next();

And the last via:

Iterator<Map.Entry<Integer,Integer>> iter = map.entrySet().iterator();
Map.Entry<Integer,Integer> entry = null;
while(iter.hasNext()) {
    entry = iter.next();
}
// now you have the "last" item in the map stored in 'entry'

But, as has been stated in other answers, this doesn't mean anything with a HashMap. Substitute it with a LinkedHashMap, however, and you can get the first and last inserted pairs out using the above code.

score 0 · Answer 6 · answered Jan 18 '12 at 07:33

0

Unless you use a http://docs.oracle.com/javase/6/docs/api/java/util/SortedMap.html, in which case you could get the first and last key..

answered Jan 18 '12 at 07:33

quaylar

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Get the last value in map

6 Answers6