HashMap Class in Java

Introduction

In Java, a map interface is an interface that consists of only unique elements. The map interface in Java consists of values that are based on key/value pairs. We can look at any value in Java using the specific key. A HashMap is a part of the map interface in Java.

HashMap Class in Java

• HashMap consists of the values that are based on the key.
   
• It consists of only unique elements.
   
• Map interface is implemented and AbstractMap class is extended by HashMap.
   
• A HashMap has no order.
   
• It may consist of multiple null values but only one null key.

Ranking of HashMap in Java

 

Example Using the NetBeans IDE

Step 1

Open the NetBeans IDE.

 

Step 2

Create a new project.

 

Step 3

Choose "Java" --> "Java Application".

 

Step 4

Provide the project  a name.

 

Step 5

Write the following code.

package demo;

 

import java.util.*;

 

public class Demo

{ 

    public static void main(String args[])

    {

        HashMap hs_mp=new HashMap(); 

       

        hs_mp.put(99,"Raj"); 

        hs_mp.put(199,"Raja"); 

        hs_mp.put(299,"Rajan");

      

        Set set=hs_mp.entrySet(); 

        Iterator itr=set.iterator(); 

 

        while(itr.hasNext())

        { 

            Map.Entry mp=(Map.Entry)itr.next(); 

            System.out.println(mp.getKey()+" "+mp.getValue()); 

        } 

    } 

}

Step 6

Run the code.

(This process of working with NetBeans will be used in all the following examples.)

 

Output

 

Methods of HashMap Class in Java

• clear( )
  
  All the mappings from the map are removed by this method.
  
  Syntax
   

  publicvoid clear()
  
  Example
   

  package demo;

   

  import java.util.*;

   

  publicclass Demo

  {

     public staticvoid main(String args[])

      {

          HashMap map =new HashMap();

         

          map.put(99,"Raj");

          map.put(199,"Raja");

          map.put(299,"Rajan");

       

          System.out.println("INITIALLY: " + map);

         

          map.clear();

         

          System.out.println("\n");

       

          System.out.println("ULTIMATELY: " + map);

      }   

  }
  
  Output

   

  
   

• clone( )
  
  A shallow copy of the map instance is returned by this method.
  
  Syntax

  public Object clone()
  
  Example
   

  package demo;

   

  import java.util.*;

   

  publicclass Demo

  {

     public staticvoid main(String args[])

      {

          HashMap map1 =new HashMap();

          HashMap map2 =new HashMap();

         

          map1.put(99,"Raj");

          map1.put(199,"Raja");

          map1.put(299,"Rajan");

       

          map2=(HashMap)map1.clone();

       

          System.out.println("MAP ONE: " + map1);

          System.out.println("\n");

          System.out.println("MAP TWO(CLONED): " + map2);  

      }   

  }
  
  Output

   

  
   

• containsKey(Object key)
  
  In the identity HashMap, it is used to check that the given object reference is a key.
  
  Syntax

  public boolean containsKey(Object key)
  
  It will return true if the given condition is satisfied.
  
  Example
   

  package demo;

   

  import java.util.*;

   

  publicclass Demo

  {

     public staticvoid main(String args[])

      {

          HashMap map =new HashMap();

         

          map.put(99,"Raj");

          map.put(199,"Raja");

          map.put(299,"Rajan");

         

          System.out.println("CHECK FOR KEY NINETY NINE :  " + map.containsKey(99));

      }   

  }
  
  Output

   

  
   

• containsValue(Object value)
  
  The identity HashMap is used to check that the given object is a value.
  
  Syntax
   

  public boolean containsValue(Object value)
  
  Will return true if the map consists of one or more keys.
  
  Example
   

  package demo;

   

  import java.util.*;

   

  publicclass Demo

  {

     public staticvoid main(String args[])

      {

          HashMap map =new HashMap();

       

          map.put(99,"Raj");

          map.put(199,"Raja");

          map.put(299,"Rajan");

         

          System.out.println("CHECK FOR VALUE RAJAN : " +

          map.containsValue("Rajan"));

     }   

  }
  
  Output

   

  
   

• get(Object key)
  
  A value is returned to which the given key is mapped. A null is returned if there is no mapping.
  
  Syntax
   

  public A get(Object key)
  
  Example
   

  package demo;

   

  import java.util.*;

   

  publicclass Demo

  {

     public staticvoid main(String args[])

      {

          HashMap map =new HashMap();

       

          map.put(99,"Raj");

          map.put(199,"Raja");

          map.put(299,"Rajan");

         

          String val=(String)map.get(99);

         

          System.out.println("VALUE FOR KEY NINETY NINE : " + val);

     }   

  }
  
  Output
  
  
   

• isEmpty( )
  
  It is checked by this method that the map consists of no key-value mappings.
  
  Syntax
   

  public boolean isEmpty()
  
  It will return true if the given condition is satisfied.
  
  Example

   

  package demo;

   

  import java.util.*;

   

  publicclass Demo

  {

     public staticvoid main(String args[])

      {

          HashMap map =new HashMap();

       

          map.put(7,"John");

          map.put(8,"Johny");

          map.put(9,"Jonty");

         

          boolean val=map.isEmpty();

         

          System.out.println("EMPTY: " + val);

      }   

  }
  
  Output
  
  
   

• putAll( )
  
  It is used to copy mappings from one map to the other.
  
  Syntax
   

  publicvoid putAll(Map m)
  
  Example

   

  package demo;

   

  import java.util.*;

   

  publicclass Demo

  {

     public staticvoid main(String args[])

      {

          HashMap map1 =new HashMap();

          HashMap map2 =new HashMap();

         

          map1.put(7,"John");

          map1.put(8,"Johny");

          map1.put(9,"Jonty");

       

          System.out.println("MAP 1 : "+ map1);

         

          map2.putAll(map1);

         

          System.out.println("\n");

   

          System.out.println("MAP 2 : "+ map2);

     }   

  }
  
  Output
  
  
   

• remove( )
  
  Mapping is removed for the specific key in the given map.
  
  Syntax

   

  public A remove(Object key)
  
  Example
   

  package demo;

   

  import java.util.*;

   

  publicclass Demo

  {

     public staticvoid main(String args[])

      {

          HashMap map =new HashMap();     

         

          map.put(7,"John");

          map.put(8,"Johny");

          map.put(9,"Jonty");

       

          System.out.println("BEFORE: "+ map);

         

          System.out.println("\n");

         

          map.remove(8);

   

          System.out.println("AFTER: "+ map);

      }   

  }
  
  Output
  
  
   

• values( )
  
  It will return all the values of the map.
  
  Syntax
   

  public Collection values()
  
  Example
   

  package demo;

   

  import java.util.*;

   

  publicclass Demo

  {

     public staticvoid main(String args[])

      {

          HashMap map =new HashMap();     

       

          map.put(7,"John");

          map.put(8,"Johny");

          map.put(9,"Jonty");

         

          System.out.println("VALUES: "+ map.values());

     }   

  }
  
  Output
  
   

Constructors Supported by HasMap

• HashMap( )
  
  A default HashMap is created by this constructor.
   
• HashMap(m)
  
  HashMap is initialized by this constructor.
   
• HashMap( int capacity)
  
  Capacity is initialized by this constructor.
   
• HashMap(int capacity, float fillRatio)
  
  Capacity and the fill ratio are initialized by this constructor.

Summary

This article explains the HashMap class and its various methods in Java.