end of a thread's execution throws java.lang.NullPointerException


I made an application, call it a.java, that creates 2 thread objects, call them t1 and t2.   t1 and t2 each use a parser object to parse an xml file.  a.java creates the two parser objects, call them p1 and p2, and passes them to t1, and t2, respectively. There is no "communication" between the parser objects and a.java object.
When one of the threads finishes, the other thread dies. The error I get is
Exception in thread "Thread-2" java.lang.NullPointerException
        at iroom.xmlp.process(xmlp.java:153)
        at iroom.xmlp.parse(xmlp.java:87)
        at iroom.ParserRunner.run(ParserRunner.java:29)
        at java.lang.Thread.run(Thread.java:595)
 where iroom is the package, xmlp is the parser class from which p1 and p2 are instantiated, and ParserRunner is the class from which t1 and t2 are instatiated, and it implements Runnable interface.
When I only create one thread the program finishes without any errors.
I am new to threads in Java, and would appreciate any help in solving this problem.

Who is Participating?
Ajay-SinghConnect With a Mentor Commented:
call join()

mirnalimAuthor Commented:
I forgot to add that a.java creates two instances of a third class, Tag, call them tag1 and tag2, and gives tag1 to t1, and tag2 to t2. Since each thread receives its own copy of the xmlp object and Tag object I do not synchronize any of the calls.
can you post process method? (line no 153)
mirnalimAuthor Commented:
I just found out that when one of the theads finishes execution the control is given back to the main function in a.java instance, which then exist and this throws the null pointer exception. So how do I make the main function wait until all of the threads are finished executing?
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