Collections(Java)-Map Interface example program

 import java.util.HashMap;  
 import java.util.TreeMap;  
 import java.util.Map;  
 import java.util.Iterator;  
 import java.util.Map.Entry;  
 import java.util.Hashtable;  
 public class CollectionsMapDemo {  
      public static void main(String [] args){  
           HashMap<Object,Object> map=new HashMap<Object,Object>();  
           map.put(1, "filter");  
           map.put("a", "claim");  
           map.put("3", "smart");  
           map.put(null,"filter");  
           map.put(null,"filter");  
           map.put(4, "sentinal");  
           map.put(4, "sentinal");  
           if(!map.isEmpty()){  
                Iterator<Entry<Object, Object>> it=map.entrySet().iterator();  
                System.out.println("HashMap can have one null key with multiple values \n");  
                while(it.hasNext()){  
                     Map.Entry<Object, Object> obj=(Map.Entry<Object, Object>)it.next();  
                     System.out.println(obj.getValue());  
                }  
           }  
           boolean isExist=map.containsKey("3");  
           boolean valueExist=map.containsValue("sentinal");  
           if(valueExist==true){  
                System.out.println("If it is true, then show me the key \n"+map.get(null));  
           }  
           System.out.println("Check whether hashMap contains filter as a value? "+valueExist);  
           System.out.println("Does key 3 exist in HashMap? "+isExist);  
           if(isExist==true){  
                System.out.println("If it is true,then show me the value \n"+map.get("3"));  
           }  
           System.out.println("\n HashTable can't have any null key or any null values");  
           Hashtable<Object,Object> table=new Hashtable<Object,Object>();  
           table.put(1, "filter");  
           table.put(2, "claim");  
           table.put("3", "smart");  
           table.put(4, "sentinal");  
           table.put(4, "sentinal");  
           if(!table.isEmpty()){  
                Iterator<Entry<Object,Object>> its=table.entrySet().iterator();  
                while(its.hasNext()){  
                     Map.Entry<Object, Object> obj1=(Map.Entry<Object, Object>)its.next();  
                     System.out.println(obj1.getValue());  
                }  
           }  
           boolean findValue=table.containsKey(3);  
           System.out.println("Does HashTable contains 2 as key? "+findValue);  
           if(findValue==true){  
                System.out.println("If it is true,then show me the value \n"+findValue);  
           }  
           System.out.println("\n TreeMap");  
           TreeMap<Object,Object> tree=new TreeMap<Object,Object>();  
           tree.put(1, "one");  
           tree.put(2, "two");  
           tree.put(3, "Three");  
           tree.put(6, null);  
           tree.put(4, "four");  
           tree.put(5, "five");  
           if(!tree.isEmpty()){  
                Iterator<Entry<Object,Object>> itrs=tree.entrySet().iterator();  
                while(itrs.hasNext()){  
                     Map.Entry<Object,Object> obj2=(Map.Entry<Object, Object>)itrs.next();  
                     System.out.println(obj2.getValue());  
                }  
           }  
      }  
 }  

Output:

HashMap can have one null key with multiple values

filter
filter
claim
smart
sentinal
If it is true, then show me the key
filter
Check whether hashMap contains filter as a value? true
Does key 3 exist in HashMap? true
If it is true,then show me the value
smart

 HashTable can't have any null key or any null values
smart
sentinal
claim
filter
Does HashTable contains 2 as key? false

 TreeMap
one
two
Three
four
five
null