How do I remove an element from a hashtable using its value and not the key?

Question

I'm new to hash table and I'm just figuring out the basic operations on it. I have a hash table created as shown below and inserted values also.

Hashtable<Integer , String> ht = new Hashtable<Integer , String>();
ht.put(1234, "ABCD");
ht.put(2345, "EFGH");
ht.put(4567, "IJKL");

I am able to delete the element needed using the key as shown below

System.out.println("Deleting entry with key 2345");
ht.remove(2345);
System.out.println(ht.toString());

which gives the following output

Deleting entry with key 2345
{4567=IJKL, 1234=ABCD}

I am not able to find any method which helps with locating the element in the hashtable using the value as an index and deleting the element. How do I go about it?

Do you care about how efficient this operation is? If you do, you need a different data structure. — NPE
– NPE, Commented Feb 25, 2014 at 7:24
Uh, you are aware that a value can appear multiple times in a map, right? — fge
– fge, Commented Feb 25, 2014 at 7:28
@fge Yes. I do get that. But like i said, I'm new to using hash tables. Just figuring out heads and tails of it now by using simple operations. — woodhead92
– woodhead92, Commented Feb 25, 2014 at 7:58
Well, the purpose of a Map is to access entries by key, not value. Some libraries (Guava for instance) do have bidirectional maps, but removing a key given a value is not a Map's "simple operation" ;) — fge
– fge, Commented Feb 25, 2014 at 8:00

Evgeniy Dorofeev · Accepted Answer · 2014-02-25 08:22:47Z

7

try this

ht.values().remove("ABCD");

this will remove one entry with the specified value, if there may be multiple entries with the same value you can use this

ht.values().removeAll(Collections.singleton("ABCD"));

edited Feb 25, 2014 at 8:22

answered Feb 25, 2014 at 7:23

Evgeniy Dorofeev

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Luca Basso Ricci · Accepted Answer · 2014-02-25 07:24:18Z

4

Navigate using Map.entrySet() and check for Map.Entry#getValue() equality.
You can have a value multiple times so iterate entrySet using an Iterator and delete elements using Iterator.remove()

void deleteItem(String item) {
  Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator();
  while (it.hasNext()) {
    Map.Entry<Integer, String> entry = it.next();
    if(entry.getValue().equals(item)) {
      it.remove();
    }
  }
}

answered Feb 25, 2014 at 7:24

Luca Basso Ricci

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Elfentech · Accepted Answer · 2014-02-25 07:24:10Z

1

Map<Integer, String> map = ...

Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator();

while (it.hasNext()) {
  Map.Entry<Integer, String> entry = it.next();

  // Remove entry if value equals xxx.
  if (entry.getValue() != null && entry.getValue().equals("X")) {
    // Do something
  }
}

answered Feb 25, 2014 at 7:24

Elfentech

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