Hash map key is unique. Add duplicate key, then it will be overwritten.
HashMap hm = new HashMap();
hm.put("1", new Integer(1));
hm.put("2", new Integer(2));
hm.put("3", new Integer(3));
hm.put("4", new Integer(4));
hm.put("1", new Integer(5));// value integer 1 is overwritten by 5
By default Hashmap is not synchronized.