what may be the reason for the following message?

D:\program files\Java\jdk1.5.0\bin>java FindNoun
java.lang.NullPointerException
at java.io.Writer.write(Writer.java:126)
at ReplaceText.saveOutput(FindNoun.java:92)
at FindNoun.main(FindNoun.java:109)

In your ReplaceText class, in the saveOutput method found in the file FindNoun.java line 92 you are using a variable that is not defined (or is defined with null). Which is, of course, exactly what the error message says.