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What is the difference between Hashtable and HashMap classes?

Alexey Korneychuk
Greenhorn

Joined: Sep 06, 2005
Posts: 18
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
Ranch Hand

Joined: Sep 28, 2004
Posts: 1228
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 ]

Thanks & regards, Srini
MCP, SCJP-1.4, NCFM (Financial Markets), Oracle 9i - SQL ( 1Z0-007 ), ITIL Certified
Ashwin Kumar
Greenhorn

Joined: Oct 13, 2005
Posts: 27
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.


SCJP 1.4<br />SCBCD 5.0<br />SCWCD 1.4 (Preparing)
Akshay Kiran
Ranch Hand

Joined: Aug 18, 2005
Posts: 220
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.


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