CODEWITHSUNDEEP
javahashtable
Andrius
Andrius

Reputation: 21168

Java - hashtable return any value or any key?

Let say I have such Hashtable:

Hashtable<Object, Object> test = new Hashtable<>();

test.put("t", 1);
test.put(2, "t123");
test.put(3, true);

How could I just return any value from it or get any key from this hash table without knowing any key or value? Should I just iterate it and get first value or maybe there is some simpler better way?

Upvotes: 1

Views: 2153

Answers (4)

Marko Topolnik
Marko Topolnik

Reputation: 200206

You can't get a random element from a hashtable in O(1), but the most efficient way to do it in O(n) would be something like this:

int chosenIndex = (int) Math.random()*map.size();
    i = 0;
for (Object v : map.values())
  if (i++ == chosenIndex) 
    return v;

BTW never use the Hashtable class, which is still around only for backwards compatibility. Use HashMap.

Upvotes: 1

Ruslan Ostafiichuk
Ruslan Ostafiichuk

Reputation: 4712

If you meant "any":

test.keys().nextElement();

or

test.element().nextElement();

Upvotes: 1

Trying
Trying

Reputation: 14278

HashMap<Integer, String> map = new   HashMap<Integer, String>();
        map.put(10, "ddsadas");
        map.put(23, "sdss");
        map.put(24, "sdss");
        map.put(90, "sdss");

now random key generation O(N):

        int size = map.keySet().size();
        int index=(int)(Math.random() * size)+1;
        System.out.println(index);
        for(Integer i : map.keySet()){
            if(index==1){
                System.out.println(i);
                break;
            }
            else{
                index--;
            }
        }

You can achieve this in O(1) by putting there set in to an array and generate a random number among the array index and just return the value in the index of array. This is o(1).

Upvotes: 1

noone
noone

Reputation: 19776

You can put all entries into a List and then use a random index to retrieve a random entry. 

Hashtable<Object, Object> test = new Hashtable<Object, Object>();

test.put("t", 1);
test.put(2, "t123");
test.put(3, true);

List<Entry<Object, Object>> entries = new ArrayList<Entry<Object, Object>>(test.entrySet());
Random random = new Random();
Entry<Object, Object> randomEntry = entries.get(random.nextInt(entries.size()));

Object randomValue = randomEntry.getValue();
Object randomKey = randomEntry.getKey();

Upvotes: 1

Related Questions