What is the difference between Hashtable and HashMap classes?

[Alexey Korneychuk]

Hello, ranches!

Why does run-time exception occur?
What is the difference between Hashtable and HashMap classes?

public class J {

public static void main(String[] args) throws Exception {
HashMap hm = new HashMap();
hm.put(null, null);
System.out.println(hm);

Thread.sleep(1000);

Hashtable ht = new Hashtable();
ht.put(null, null);
System.out.println(ht);


}
}

Result:
{null=null}
java.lang.NullPointerException
at java.util.Hashtable.put(Hashtable.java:397)
at fgff.Jhsdhdjshdjhsdjhjhjsdhjs.main(J.java:28)
Exception in thread "main"

Alexey

[Srinivasa Raghavan]

These are the main differences between HashMap & Hashtable:

Hashtable does not accept null key / value. Hashtable is synchronized but hash map is not.
[ October 14, 2005: Message edited by: Srinivasa Raghavan ]

[Ashwin Kumar]

That is correct.

Also note that only one NULL value is allowed as a key in HashMap. HashMap does not allow multiple keys to be NULL. Nevertheless, it can have multiple NULL values.

[Akshay Kiran]

Originally posted by AshwinC Kumar:
That is correct.

Also note that only one NULL value is allowed as a key in HashMap. HashMap does not allow multiple keys to be NULL. Nevertheless, it can have multiple NULL values.

that is however obvious because the keys are implemented in a "Set" and a Set doesn't allow duplicates.