remove key from hash table

Question

I want to remove key from hash Table without using the remove function. so give me some idea.

Answer 1

You can emulate removed keys by supplementing a Map<K,V> map with a Set<K> removedKeys.

To remove a key, just removedKeys.add(key);.

Whenever a K key is queried, you see if removedKeys.contains(key). If it does, then key has effectively been "removed" from map.

Note that this is a very peculiar way of doing things, and keeping the two structures in sync may cause later headaches. It's more acceptable if removedKeys is localized and short-lived, e.g. when you're iterating over the entries of a Map using for-each and want to remove some keys later, while avoiding ConcurrentModificationException.

So you may have something like this:

static void removeEvenKeys(Map<Integer,String> map) {
    Set<Integer> removedKeys = new HashSet<Integer>();
    for (Map.Entry<Integer,String> entry : map.entrySet()) {
        if (entry.getKey() %2 == 0) {
            removedKeys.add(entry.getKey());
        }
    }
    map.keySet().removeAll(removedKeys);        
}

And then elsewhere:

    Map<Integer,String> map = new HashMap<Integer,String>();
    map.put(1, "One");
    map.put(2, "Two");
    map.put(3, "Three");
    map.put(4, "Four");

    removeEvenKeys(map);
    System.out.println(map);
    // "{1=One, 3=Three}"

See also

• Effective Java 2nd Edition, Item 52: Refer to objects by their interfaces

Related questions

• Java hashmap vs hashtable
• Java: Efficient Equivalent to Removing while Iterating a Collection

Answer 2

You can call remove() on an Iterator instead of on the Hashtable itself:

Hashtable<String, String> map = new Hashtable<String, String>();
map.put("one", "een");
map.put("two", "twee");
map.put("three", "drie");

for (Iterator<Map.Entry<String, String>> i = map.entrySet().iterator(); i.hasNext(); ) {
    Map.Entry<String, String> entry = i.next();

    if ("two".equals(entry.getKey())) {
        // Removes entry from Hashtable; note, this is not the Hashtable.remove() method
        // but the Iterator.remove() method
        i.remove();
    }
}

System.out.println(map);

(NOTE: You should use HashMap instead of the legacy collection class Hashtable).