How to get element position from Java Map

Question

I have this Java Map:

Can you tell me how I can get the 6-th element of the Map?

private static final Map cache = new HashMap<         


        

Answer 1

Correct!! you will have to use other collection for getting values on index(position). You can use ArrayList

Answer 2

The HashMap has no defined ordering of keys.It's Unordered.

You can use LinkedHashMap which will store your keys in order of insertion.You can retrieve them by calling keySet().

Answer 3

According to documentation, HashMap is a Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

That's why it is not wise to use this kind of Collection.

UPDATE:

Based on @Prateek implementation of LinkedHashMap I would suggest something like:

LinkedHashMap<String,User> linkedHashMap = new LinkedHashMap<String,User>();
// or LinkedHashMap<String,User> linkedHashMap = new LinkedHashMap<>(); //for java 7+

linkedHashMap.put("1",userObj1);
linkedHashMap.put("2",userObj2);
linkedHashMap.put("3",userObj3);

/* Get by position */
int pos = 1; // Your position
User tmp= (new ArrayList<User>(linkedHashMap.values())).get(pos);
System.out.println(tmp.getName());

Answer 4

HashMap doesn't grantee the order. If you concern about order you should use LinkedHashMap

Map<String, Users> orderedMap=new LinkedHashMap<>();

Now when you put an element it will keep the order what you put.

If you want to get 6th element, now you can do it since you have your elements in order.

 orderedMap.values().toArray()[5]// will give you 6th value in the map. 

Example

 Map<String, String> orderedMap=new LinkedHashMap<>();
 orderedMap.put("a","a");
 orderedMap.put("b","b");
 System.out.println(orderedMap.values().toArray()[1]);  // you will get b(value)
 System.out.println(orderedMap.keySet().toArray()[1]);  // you will get b(key)
    }

Answer 5

If the ordering is to mean anything significant, you could consider using a SortedMap.

Concrete implementation: TreeMap

Answer 6

Though a bit late to answer. But the option is to use LinkedHashMap: this map preserves the order according to insertion of elements, as everyone has suggested. However, As a warning, it has a constructor LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) which will create a linked hash map whose order of iteration is the order in which its entries were last accessed. Don't use this constructor for this case.

However, if I needed such functionality, i would extend it and implement my necessary function to re-use them in OOP way.

class MyLinkedMap<K, V> extends LinkedHashMap<K, V>
{

    public V getValue(int i)
    {

       Map.Entry<K, V>entry = this.getEntry(i);
       if(entry == null) return null;

       return entry.getValue();
    }

    public Map.Entry<K, V> getEntry(int i)
    {
        // check if negetive index provided
        Set<Map.Entry<K,V>>entries = entrySet();
        int j = 0;

        for(Map.Entry<K, V>entry : entries)
            if(j++ == i)return entry;

        return null;

    }

}

Now i can instantiate it and can get a entry and value either way i want:

MyLinkedMap<String, Integer>map = new MyLinkedMap<>();
map.put("a first", 1);
map.put("a second", 2);
map.put("a third", 3);

System.out.println(map.getValue(2));
System.out.println(map.getEntry(1)); 

Output:

3
a second=2