HashMap

HashMap:

• HashMap is collection that stores elements in the form of key-value pairs.

• If key provided later its corresponding value can be easily retrieved from the HashMap .

• Duplicate keys are not allowed but values can be duplicated

• Insertion order is not preserved and it is based on hashcode of the keys.

• Heterogeneous objects are  allowed  for key and value

• Null allowed for the key(only once) and for values(any number of)  

• It is not Synchronized.but we can synchronized by using method of collection class

                   public staic Map synchronizedMap(Map m);

Constructors:

• HashMap m=new HashMap();//creates an empty HashMap object with default  initial capacity 16 and default fill ratio 0.75 .fill ratio is also known as Load Faction

• HashMap m=new HashMap(Map m); 

• HashMap m=new HashMap(int initialcapacity); 

• HashMap m=new HashMap(int initialcapacity float fillratio); 

 HashMapExample

import java.util.Collection;
import java.util.Iterator;
import java.util.HashMap;
import java.util.Set;

public class HashMapExample {

  public static void main(String[] args) {
  
    //create HashMap object
    HashMap hMap = new HashMap();
  
    //add key value pairs to HashMap
    hMap.put(1,"One");
    hMap.put(2,"Two");
    hMap.put(3,"Three");
    hMap.put(4,"four");
    hMap.put(5,"five");
    Set st = hMap.keySet();//get the keys
    System.out.println("get the keys");

   Iterator itr = st.iterator();
   while(itr.hasNext())
   System.out.println(itr.next());

   System.out.println("get the values");
   Collection c = hMap.values();
  
    //obtain an Iterator for Collection
    Iterator itr1 = c.iterator();
  
    //iterate through HashMap values iterator
    while(itr1.hasNext())
      System.out.println(itr1.next());
}
} 

output:

get the keys:

1          

2          

3          

4          

5           

get the values

One         

Two         

Three      

four        

five