I am trying to write to a file in Google appengine but it giving a error message java.io.FileOutputStream is not supported by Google App Engine's Java runtime environment
even though I imported
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
Well it's not a java compiler error. This class is a restricted API in Google App Engine you are not allowed to use it.
Read about the GAE Java Runtime Environment and restrictions here: http://code.google.com/appengine/docs/java/runtime.html
The closest you will get to file storage on GAE is the Blobstore API: http://code.google.com/appengine/docs/java/blobstore/
If you need to create files in code GAE is not an appropriate platform for you.