How get value from LinkedHashMap based on index not on key?

Question

I have

LinkedHashMap<String, List<String>> hMap;

I want to get List<String> by position not on key.

I don't want to use iterate.

Is there any other way to get Value based on index ?

If you choose this data structure by yourself then probably it is not the best one for this task. — Nikolay Kuznetsov, Nov 27 '12 at 10:25
@NikolayKuznetsov: can you suggest me structure how it should be? — MAC, Nov 27 '12 at 10:32
okey thanks but now I am using key to get value. Index is not a good idea... thanks again... — MAC, Nov 27 '12 at 10:59
Why is this question closed without a reference to the supposedly good answer. All answers here are no good. — mjs, Jul 31 '15 at 15:22
Here is an answer I posted to this anyhow, I can't post it here, this thread is locked: http://stackoverflow.com/a/31749969/961018 — mjs, Jul 31 '15 at 15:49

score 61 · Accepted Answer · edited Apr 02 '15 at 13:19

61

You can't get the value of the Map based on index, Maps just don't work that way. A workaround would be to create a new list from your values and get the value based on index.

LinkedHashMap<String, List<String>> hMap;
List<List<String>> l = new ArrayList<List<String>>(hMap.values());
l.get(0);

edited Apr 02 '15 at 13:19

Intrications

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answered Nov 27 '12 at 10:23

PermGenError

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If i want to insert value based on index? how to achieve that ? – Kanagavelu Sugumar Jun 20 '14 at 06:56
1

Here is an answer I posted to this anyhow, I can't post it here, this thread is locked: http://stackoverflow.com/a/31749969/961018 – mjs Jul 31 '15 at 15:49

score 23 · Answer 2 · answered Nov 27 '12 at 10:23

23

public List<String> getByIndex(LinkedHashMap<String, List<String>> hMap, int index){
   return (List<String>) hMap.values().toArray()[index];
}

answered Nov 27 '12 at 10:23

Kevin Bowersox

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this can be expensive if its called often with many entries in map – Binkan Salaryman Sep 04 '18 at 10:32

score 16 · Answer 3 · answered Nov 27 '12 at 10:32

16

you may want to consider either using another class to store your data, or write an extension to the linkedHashMap. something like

//this is pseudo code
public class IndexedLinkedHashMap<K,V> extends LinkedHashMap{

HashMap<int,K> index;
int curr = 0;

    @Override
    public void add(K key,V val){
        super.add(key,val);
        index.add(curr++, key);
    }

    public V getindexed(int i){
        return super.get(index.get(i));
    }

}

answered Nov 27 '12 at 10:32

schippi

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This seems to be by far the best method, but I don't see the purpose of the variable `curr`. – Hasen Sep 06 '22 at 00:32

makrom · Answer 4 · 2014-03-28T15:53:54.337

9

As Kevin Bowersox stated, it's as simple as

List<String> result = (List<String>) hMap.values().toArray()[position];

But it should be noted that this will still iterate by using .toArray(). It's a simple statement and I'm not sure if there is one with better performance, but be aware that complexity is not log(n) (like indexed access in case of B*), but just n. Since LinkedHashMap is based on LinkedList, there is no way to randomly access elements, only in sequential order.

The cast to List is an unavoidable evil, since .toArray() follows the archaic concept of returning Object instead of a generic data type.

While this might not be the main concept of a map, LinkedHashMap isn't just a map. it extends HashMap, and as an extending class, it's perfectly fine to bring additional methods that support the idiosyncracies of that class.

edited Mar 28 '14 at 15:53

answered Mar 28 '14 at 14:32

makrom

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create a new array .. not very pretty – mjs Jul 31 '15 at 15:02
1

Here is an answer I posted to this anyhow, I can't post it here, this thread is locked: http://stackoverflow.com/a/31749969/961018 – mjs Jul 31 '15 at 15:49
@momo My post was simply meant to explain that what Kevin Bowersox proposed doesn't avoid iteration and furthermore that there's no such way. The creation of an array can easily be avoided by iterating the values manually, but that's not the ideal answer when the question is how to do it without iterating. Obviously you can avoid that by creating your own implementations with a different indexing concept, but that seems unrelated to the question. – makrom Oct 26 '15 at 17:12

score 5 · Answer 5 · edited Nov 09 '16 at 15:53

There is no direct DS in the standard Java Collections API to provide a indexed map. However, the following should let you achieve the result:

// An ordered map
Map<K, V> map = new LinkedHashMap<K, V>();
// To create indexed list, copy the references into an ArrayList (backed by an array)
List<Entry<K, V>> indexedList = new ArrayList<Map.Entry<K, V>>(map.entrySet());
// Get the i'th term
<Map.Entry<K,V>> entry = indexedList.get(index);
K key = entry.getKey();
V value = entry.getValue();

You might still want to retain the concerns of data persistence in the map separate from the retrieval.

Update: Or use LinkedMap from Apache Commons.

How get value from LinkedHashMap based on index not on key?

5 Answers5

Linked