import java.util.HashMap;
import java.util.Iterator;

public class HashMapExample {
    
    public static void main(String args[]){
        
        // constructs a new empty HashMap with default initial capacity
        HashMap hashMap = new HashMap();
        
        /*
         
         To specify initial capacity, use following constructor
         HashMap HashMap = new HashMap(100);
         
         To create HashMap from map use following constructor
         HashMap HashMap = new HashMap(Map myMap);
         
         */
        
        hashMap.put("One", new Integer(1)); // adding value into HashMap
        
        hashMap.put("Two", new Integer(2));
        
        hashMap.put("Three", new Integer(3));
        
        
        //get number of keys present in the HashMap
        System.out.println("HashMap contains " + hashMap.size() + " key value pairs.");
        
        /*
         
         To check whether HashMap is empty or not, use isEmpty() method.
         isEmpty() returns true is HashMap is empty, otherwise false.
         
         /*
         
         Finding particular value from the HashMap:
         
         HashMap's containsValue method returns boolean depending upon
         the presence of the value in given HashMap
         
         Signature of the containsValue method is,
         boolean containsValue(Object value)
         
         */
        
        if(hashMap.containsValue(new Integer(1))){
            System.out.println("HashMap contains 1 as value");
        } else {
            System.out.println("HashMap does not contain 1 as value");
        }
        
        /*
         
         Finding particular Key from the HashMap:
         HashMap's containsKey method returns boolean depending upon the
         Presence of the key in given HashMap
         
         Signature of the method is,
         boolean containsKey(Object key)
         
         */
        
        if( hashMap.containsKey("One") ){
            System.out.println("HashMap contains One as key");
        } else {
            System.out.println("HashMap does not contain One as value");
        }
        
        /*
         Use get method of HashMap to get value mapped to particular key.
         Signature of the get method is,
         Object get(Object key)
         */
        
        Integer one = (Integer) hashMap.get("One");
        System.out.println("Value mapped with key \"One\" is " + one);
        
        /*
         IMPORTANT:  get method returns Object, so we need to downcast it.
         */
        
        /*
         To get all keys stored in HashMap use keySet method
         Signature of the keysSet method is,
         Set keySet()
         */
        
        System.out.println("Retrieving all keys from the HashMap");
        Iterator iterator = hashMap.keySet().iterator();
        
        while(iterator. hasNext()){
            System.out.println(iterator.next());
        }
        
        /*
         To get all values stored in HashMap use entrySet() method.
         Signature of the entrySet() method is,
         Set entrySet()
         */
        
        System.out.println("Retrieving all values from the HashMap");
        iterator = hashMap.entrySet().iterator();
        
        while(iterator. hasNext()){
            System.out.println(iterator.next());
        }
        
        /*
         To remove particular key - value pair from the HashMap use remove method.
         Signature of remove method is,
         Object remove(Object key)
         This method returns value that was mapped to the given key,
         otherwise null if mapping not found.
         
         */
        
        System.out.println( hashMap.remove("One") + " is removed from the HashMap.");
        
    }
    
}

/*
 OUTPUT of the above given Java HashMap Example would be :
 HashMap contains 3 key value pair.
 HashMap contains 1 as value
 HashMap contains One as key
 Value mapped with key "One" is 1
 Retrieving all keys from the HashMap
 Three
 Two
 One
 Retrieving all values from the HashMap
 Three=3
 Two=2
 One=1
 1 is removed from the HashMap.
 */