Simple Java HashMap example

This simple Java Example shows how to use Java HashMap. It also describes how to add something to HashMap and how to retrieve the value added from HashMap.

import java.util.HashMap;
import java.util.Set;

public class JavaHashMapExample {

 public static void main(String[] args) {

  // create object of HashMap
  HashMap phoneBook = new HashMap();

  /**
   * Add key value pair to HashMap using Object put(Object key, Object
   * value) method of Java HashMap class, where key and value both are
   * objects put method returns Object which is either the value
   * previously tied to the key or null if no value mapped to the key.
   **/

  phoneBook.put("John", "245745");
  phoneBook.put("Joy", "245786");
  phoneBook.put("Roy", "233783");

  // retrieve value using Object get(Object key) method of Java HashMap
  // class
  Object phoneNo = phoneBook.get("Joy");
  System.out.println("Phone number of Joy: " + phoneNo);

  System.out.println("The Map contains...");

  /**
   * Set keySet() - Returns a set view of the keys contained in this map.
   * The set is backed by the map, so changes to the map are reflected in
   * the set, and vice-versa
   **/

  Set nameSet = phoneBook.keySet();
  for (Object name : nameSet) {
   System.out.println("Name: " + name + " Phone: "
     + phoneBook.get(name));
  }

  /**
   * put - If the map previously contained a mapping for this key, the old
   * value is replaced and the put method returns the old value.
   **/

  Object oldPhone = phoneBook.put("Roy", "222222");

  System.out.println("Roy's old phone number:" + oldPhone);
  System.out.println("Roy's new phone number:" + phoneBook.get("Roy"));

 }
}

The output is:


Phone number of Joy: 245786
The Map contains...
Name: Roy Phone: 233783
Name: Joy Phone: 245786
Name: John Phone: 245745
Roy's old phone number:233783
Roy's new phone number:222222