What are the differences between Hashmap vs Hashtable in theory?

Question

Are there are differences between hashmap and hashtable in theory?

I don\'t mean in the concrete definitions given in Java (or the implementation), but in theory. I

Answer 1

Hash Map & Hashtable

• Both belongs to Map Interface. Keys – cannot be duplicate.
• Hash Map  not synchronized but we can convert it to synchronizedMap. It allows maximum one null key and any number of null values.
• Hash Table  synchronized as it is legacy class. It not allow null either in key/value.

• The default initial capacity of Hashtable is 11, HashMap is 16.

• Hash – will not ensure the order of insertion

• For More information see my stack post

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public static void main(String[] args) {
    new Thread() {
        @Override public void run() {
            HashMap hm = new HashMap();
            hm.put("key0", 10); // Compiler Widens.
            hm.put("key1", null);
            hm.put("key0", new Integer(16)); // Overridden.
            hm.put(null, 20);
            hm.put(null, 70);
            hm.put(null, null);
            System.out.println("HashMap : "+hm); // hm.toString()

            Iterator> it = hm.entrySet().iterator();
            while (it.hasNext()) {                  
                Map.Entry pair = (Map.Entry)it.next();
                System.out.println(pair.getKey() + " = " + pair.getValue());
                it.remove(); // avoids a ConcurrentModificationException
            }
            // we can conver HashMap to synchronizedMap
            Collections.synchronizedMap(hm);

        }
    }.start();
    new Thread() {
        @Override public void run() {
            Hashtable ht = new Hashtable();
            try {
                ht.put("product1", 12);
                ht.put("product2", 13);
                ht.put("product2", 14);
            //  ht.put(null, 20);
            //  ht.put("product2", null);
                System.out.println("hash Table : "+ht);

            } catch (NullPointerException nul) {
                System.out.println("HashTable will not accept null's eighter in key/value");
                IllegalArgumentException iae = new IllegalArgumentException("nulls not accept");
                iae.initCause(nul);
                throw iae;
            }               
        }
    }.start();
}

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