Collections Framework

HashMap

• The HashMap class of the Java collections framework provides the functionality of the hash table data structure.
• It stores elements in key/value pairs. Here, keys are unique identifiers used to associate each value on a map.
• The HashMap class implements the Map interface.
• There are 2 ways to create HashMap
• Non-generic
Syntax:
HashMap arrayName=new HashMap();
• Generic
Syntax:
HashMap<key,value> hashmap=new HashMap<key,value>();

• Non-generic HashMap can store Any type of object like Integer, String, Date…etc
• Generic HashMap can store only specify the ClassName.

HashMap Methods

• void put(key,value)
• To add a single element to the hashmap, we use the put method of the HashMap class.
• void putAll( collections )
• The Java HashMap putAll method inserts all the key/value mappings from the specified Map to the HashMap.
• value get( key )
• The get method returns the value corresponding to the specified key in the hashmap.
• collection keySet( )
• The keySet method returns a set view of all the keys present in entries of the hashmap.
• collection values( )
• The values method returns a view of all the values present in entries of the hashmap.

import java.util.*;
class demo
{
  public static void main(String args[])
  {
    HashMap mapA=new HashMap();
    mapA.put(1,"C");
    mapA.put(2,"C++");
    mapA.put(3,"Java");
    mapA.put(4,"MySQL");
    mapA.put(5,"HTML");
    System.out.print(mapA);
  }
}

{1=C, 2=C++, 3=Java, 4=MySQL, 5=HTML}