HashMap

HashMap

• The underlying data structure is Hashtable.
• Hashing insertion order is not preserve, because it based on Hashcode of keys.
• Duplicate keys are not allowed. But values can be duplicated.
• Heterogeneous objects allowed for both keys& value.
• Null key is allowed.
• Null values are allowed, any null of times.

Difference between HashMap& Hashtable:-

HashMap	Hashtable
No method is synchronized. Performance is fast/ high HashMap objects is not thread safe. Null is allowed for both keys& values. It is non legacy, introduced in 1.2 version.	Every method is the synchronized. Hashtable object is thread safe. Performance is low. Null, we can’t use for keys & value violation leads to NullPointerException. it is legacy& introduced in 1.0 version.

Collections class contain synchronized Map method:-

HashMap m = new HashMap ();

Map m2 = collections. synchronizedMap (m);

Constructors of HashMap:-

1. HashMap h = new HashMap ();// creates an empty HashMap object, with default initial capacity& default fill ratio 0.7.
2. HashMap h = new HashMap ( int initial capacity)
3. HashMap h = new HashMap ( int initialcapacity, float fillratio);
4. HashMap h = new HashMap ( Map m )

import java. util. *;
Class HashMapDemo
{
Public static void main ( String[] args)
         {
            HashMap m = new HashMap();
            m. put("narayana", 700);
            m. put("anitha", 800);
            m.put("sunny", 200);
            m. put("vighnesh", 500);
            System.out.println(m);
           System.out.println(m.put("venu",1000));//700
            Set s = m.keySet();
            System.out.println(s);
            collection c = m.values();
            System.out.println(c);// 500, 800, 200, 1000
            Set s1 = m.entrySet();
            iterator itr = s1. iterator();
             while ( its.hasNext())
            {
            Map.Entry m1 =( Map.Entry) itr.next();
            System.out.println(m1.getkey().equals("narayana"))
            {
              m1.setvalue(10000);
             }
            }
            System.out.println(m);
            }
}