** Hash Map & hash Table **

public static void main(String[] args) { new Thread() { @Override public void run() { HashMap<String, Integer> hm = new HashMap<String, Integer>(); 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<Entry<String, Integer>> it = hm.entrySet().iterator(); while (it.hasNext()) { Map.Entry<String, Integer> pair = (Map.Entry<String, Integer>)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<String, Integer> ht = new Hashtable<String, Integer>(); 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(); } 

• Both belongs to Map Interface. Keys – cannot be duplicate.
• Hash Map  not synchronized but we can convert it to synchronizedMap. It allows ony one null for key and value, if supply more it overrides.
• Hash Table  synchronized as it is legacy class. It not allow null eighter in key/value.
• Hash – will not ensure the order of insertion

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

public static void main(String[] args) { new Thread() { @Override public void run() { HashMap<String, Integer> hm = new HashMap<String, Integer>(); 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<Entry<String, Integer>> it = hm.entrySet().iterator(); while (it.hasNext()) { Map.Entry<String, Integer> pair = (Map.Entry<String, Integer>)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<String, Integer> ht = new Hashtable<String, Integer>(); 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(); }