How to sort Map

[Nishan Patel]

Hello every one,


I am using one HasMap which contains Integer key and List type value. Like.

private Map<Integer ,Items> itemMap = new HashMap<Integer, Items>();

items is POJO DTO hibernate calass Objects.

So how can i sort using values contains item name.

Items contains item Title Name.

So, I want to know how to sort using item name.

[Nitesh Kant]

I am not sure whether i understood what is the content of your list but if you want to sort the Items then you have to get the values using values() and then sort it as you would sort any other list. (Colections.sort())

Moving to Beginners forum.

[Ulf Dittmer]

It is important to realize that a Map has no sort order. You can either sort the list of keys or sort the list of values (as Nitesh said). Neither is going to impact the Map in any way.

[Rob Spoor]

Ulf Dittmer wrote:It is important to realize that a Map has no sort order.

The SortedMap interface, and its implementation TreeMap, disagree with you

However, these only impose an order on the keys, not on the values. There is no known implementation that sorts on the values, and the reason is clear: it basically turns your Map into a list of Map.Entry objects as far as performance goes. get() looks up values based on a key, and if the Map is sorted on the value the Map will have to check every entry until a match is found.

Of course, if you really need this, you can implement it yourself:

public class ValuesSortedMap<K,V> extends AbstractMap<K,V>
{
    private List<Map.Entry<K,V>> entries = new ArrayList<Map.Entry<K,V>>();

    private Comparator<Map.Entry<K,V>> entryComparator;

    public ValuesSortedMap(final Comparator<? super V> comparator)
    {
        entryComparator = new Comparator<Map.Entry<K,V>>()
        {
            public int compare(Map.Entry<K,V> entry1, Map.Entry<K,V> entry2)
            {
                return comparator.compare(entry1.getValue(), entry2.getValue());
            }
        };
    }

    // implement the necessary methods; I did a basic setup of put for you

    public V put(K key, V value)
    {
        V old = null;
        // get the entry for the key
        Map.Entry<K,V> entry = getEntryForKey(key);
        if (entry == null)
        {
            // use AbstractMap.SimpleEntry, public since Java 6
            entry = new SimpleEntry(key, value);
            entries.add(entry);
        }
        else
        {
            old = entry.setValue(value);
        }
        Collections.sort(entries, entryComparator);
        return old;
    }
}

[Ulf Dittmer]

Rob Prime wrote:The SortedMap interface, and its implementation TreeMap, disagree with you

Not really - a Map is not a SortedMap.

[Mike Simmons]

But would you agree that a SortedMap is a Map? And thus, some Maps do have sort order, even while others do not?

I think when you say "a Map has no sort order", it sounds to Rob and me as if you're saying no Map has sort order, when you probably mean that a Map, in general, cannot be assumed to have sort order. Unless of course one has more specific knowledge about the type of Map they're dealing with.

[Harshit Rastogi]

Nishan Patel wrote:
Hello every one,


I am using one HasMap which contains Integer key and List type value. Like.

private Map<Integer ,Items> itemMap = new HashMap<Integer, Items>();

items is POJO DTO hibernate calass Objects.

So how can i sort using values contains item name.

Items contains item Title Name.

So, I want to know how to sort using item name.

If you want to sort the Map using the keys then use Treemap so
SortedMap map = new TreeMap();

[Rob Spoor]

Harshit Rastogi wrote:If you want to sort the Map using the keys then use Treemap so
SortedMap map = new TreeMap();

Correct, but Nishan does not want to sort on the keys but on a part of the values.