Getting Map Value Pairs Using Index

Question

Is there a way of iterating a LinkedHashMap (which has a defined order) by using an index instead of a foreach loop? I need to access elements using index.

The following code prints the entire map:

public void print(Map<String, Integer> map)
{
    for (Map.Entry<String, Integer> entryMap : map.entrySet())
    {
        System.out.println(entryMap.getValue() + " = " + entryMap.getKey());
    }
}

How can I do the same but access the elements using index instead?

public void print(Map<String, Integer> map)
{
    for (int i = 0; i< map.size(); i++)
    {
        // getValue() and getKey() are undefined
        System.out.println(map.get(i).getValue() + " = " + map.get(i).getKey());
    }
}

The following only returns the keys, but I also need the values:

public String getByIndex(Map<String, Integer> map, int index)
{
    List<String> keys = new ArrayList<>(map.keySet());

    return (String) keys.get(index);
}

duplicate of the duplicate at http://stackoverflow.com/questions/13581997/how-get-value-from-linkedhashmap-based-on-index-not-on-key — austin wernli, Nov 05 '14 at 20:01
`LinkedHashMap` retains the insert order, so iterating over its `values()` using enhanced for loop should give you the elements in the same order you have put them. If you need a counter just define it ouside the for loop in increment it inside. — A4L, Nov 05 '14 at 20:02
Do you just need the indices while you're processing the entries, or do you need random access? If the former, you could create a Pair class and a stream of `Pair>` values, and then you don't need to build a complete parallel copy. If the latter, iterate the `entrySet()` and put the values into an `ArrayList>`. — David Conrad, Nov 05 '14 at 20:10

score 3 · Answer 1 · answered Nov 05 '14 at 20:03

3

Try this :

List<String> keys = new ArrayList(map.keySet());
for (int i = 0; i < keys.size(); i++)
{
    String key = keys.get(i);
    Integer value = map.get(key);
    //...
}

answered Nov 05 '14 at 20:03

ToYonos

16,469
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Radiodef · Accepted Answer · 2014-11-05T20:18:30.673

Well you can write a method to do this.

public static <K, V> Map.Entry<K, V> getEntryByIndex(Map<K, V> map, int i) {
    if(i >= map.size()) {
        throw new IndexOutOfBoundsException(String.valueOf(i));
    }

    // use Iterator
    Iterator<Map.Entry<K, V>> it = map.entrySet().iterator();

    // skip to i
    for(; i > 0; --i) {
        it.next();
    }

    return it.next();
}

This pretty much treats it like a linked list. If you are finding you do this a lot you may want to permanently keep an ArrayList along with the Map.

score 0 · Answer 3 · answered Nov 05 '14 at 20:00

0

One of the possible workarounds would be to put all keys also into ArrayList where they can be accessed using index. Direct access seems not possible because keySet returns a set that does not support the indexed access.

answered Nov 05 '14 at 20:00

Audrius Meškauskas

20,936
12
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I tried that before but it only returned the `keys` from the map but I also need the `values`. – BullyWiiPlaza Nov 05 '14 at 20:02
Put keys, not values into ArrayList. You can get the value, when needed, directly from the map, using the key. – Audrius Meškauskas Nov 05 '14 at 20:02

Getting Map Value Pairs Using Index

3 Answers3