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Hashtable(Collection)

Hashtable:-Hashtable is a legacy class where object store in the form of key value pairs but key cannot be null as well as values cannot be null. All the methods of Hashtable class are synchronized.
Example1:- TestHashtable.java
import java.util.*;
class TestHashtable
{
public static void main(String[]args)
{
Hashtable<Object,Object> m=new Hashtable<Object,Object>();
m.put(101,"Sumit");
m.put(104,"Sunita");
m.put(102,"Suman");
m.put(107,"Sonu");
Set s=m.entrySet();
Iterator itr=s.iterator();
System.out.println("Key   :::::::::Value");
while(itr.hasNext())
{
Map.Entry me=(Map.Entry)itr.next();
System.out.println(me.getKey()+"    :::: "+me.getValue());
}
}
}
Output:-
Key   :::::::::Value
107    :::: Sonu
104    :::: Sunita
102    :::: Suman
101    :::: Sumit

Example2:- TestHashtable.java
import java.util.*;
class TestHashtable
{
public static void main(String[]args)
{
Hashtable<Object,Object> m=new Hashtable<Object,Object>();
m.put(101,"Sumit");
m.put(104,"Sunita");
m.put(102,"Suman");
m.put(107,"Sonu");
Set s=m.entrySet();
Iterator itr=s.iterator();
System.out.println("Key   :::::::::Value");
while(itr.hasNext())
{
Map.Entry me=(Map.Entry)itr.next();
System.out.println(me.getKey()+"    :::: "+me.getValue());
}
}
}
Output:-
Key :::::::::Value
107    :::: Sonu
104    :::: Sunita
102    :::: Suman
101    :::: Sumit
Example3:- TestHashtable.java
import java.util.*;
class TestHashtable
{
public static void main(String[]args)
{
Map<Object,Object> m=new Hashtable<Object,Object>();
m.put(101,"Sumit");
m.put(104,"Sunita");
m.put(102,"Suman");
m.put(107,null);
Set s=m.entrySet();
Iterator itr=s.iterator();
System.out.println("Key   :::::::::Value");
while(itr.hasNext())
{
Map.Entry me=(Map.Entry)itr.next();
System.out.println(me.getKey()+"    :::: "+me.getValue());
}
}
}
Output:- Exception in thread "main" java.lang.NullPointerException
Example4:- TestHashtable.java
import java.util.*;
class TestHashtable
{
public static void main(String[]args)
{
Map<Object,Object> m=new Hashtable<Object,Object>();
m.put(null,"Sumit");
Set s=m.entrySet();
Iterator itr=s.iterator();
System.out.println("Key   :::::::::Value");
while(itr.hasNext())
{
Map.Entry me=(Map.Entry)itr.next();
System.out.println(me.getKey()+"    :::: "+me.getValue());
}
}
}

Output: - Exception in thread "main" java.lang.NullPointerException
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