CODEWITHSUNDEEP
javahashmap
user2081119
user2081119

Reputation: 321

Access to HashMap's elements

Can I access HashMap's first and second elements separately? I mean if I have a HashMap

Map<Integer, Integer> testMap = new HashMap<Integer, Integer>();
int f = testMap.first();

I know that there's no such function, but how can I access that element?

Upvotes: 0

Views: 2414

Answers (6)

c.pramod
c.pramod

Reputation: 606

 Iterator it = testMap.entrySet().iterator();

                    while (it.hasNext()) {
                            Map.Entry pairs = (Map.Entry) it.next();
                           System.out.println("Key"+pairse.getKey());
                          System.out.println("Value"+pairs.getValue());
}

Upvotes: 0

Subhrajyoti Majumder
Subhrajyoti Majumder

Reputation: 41230

HashMap does not maintain order.

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.

Reference

You can use LinkedHashMap which maintain insertion order or a TreeMap.

LinkedHashMap

This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map. (A key k is reinserted into a map m if m.put(k, v) is invoked when m.containsKey(k) would return true immediately prior to the invocation.)

TreeMap

The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used.

Upvotes: 2

Mikhail Vladimirov
Mikhail Vladimirov

Reputation: 13900

HashMap <Integer, Integer> testMap = new HashMap <Integer, Integer> ();
testMap.put (1, 2);
testMap.put (2, 3);
Iterator <Integer> i = testMap.values ().iterator ();
System.out.println ("First element: " + i.next ());
System.out.println ("Second element: " + i.next ());

Note, that because HashMap does not remember which order elements were added in, "first element" does not necessary mean "first added".

Upvotes: 0

MayurB
MayurB

Reputation: 3649

You can use LinkedHashMap which has methods like firstKey and lastKey.But for 2nd key you have to iterate and you get the element in order in which keys entered in the map

Upvotes: 0

Java Spring Coder
Java Spring Coder

Reputation: 1027

no. you can't as HapMap/HashTable store the data in Entry for (http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Map.Entry.html) which holds key and value together.

you can alway get Iterator of entrySet() and access the entries. Or try to get keySet() which returns set of keys only.

hope this helps.

Upvotes: 0

Gabriele Mariotti
Gabriele Mariotti

Reputation: 364988

Maps are not ordered, so there is no such thing as 'the first entry', and that's also why there is no get-by-index method on Map (or HashMap).

You could do this:

Map<Integer, Integer> map = ...;  // wherever you get this from

// Get the first entry that the iterator returns
Map.Entry<Integer, Integer> entry = map.entrySet().iterator().next();
(Note: Checking for an empty map omitted).

Your code doesn't get all the entries in the map, it returns immediately (and breaks out of the loop) with the first entry that's found.

Note: Calling iterator() does not mean that you are iterating over the whole map.

Upvotes: 0

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