i tried to implement the same session counter.java in WSAD

i found error like:
1) import javax.servlet.http.HttpSessionListener

error:( for import)
javax.servlet.http.HttpSessionListener cannot be resolved

2)public class SessionCounter implements
HttpSessionListener

error: HttpSessionListner cannot be resovled or not a valid suuper interface

plz help to solve the above problem

in turn which interface must i use to execute the sessioncounter.java