I would say that the fastest way is not using Eclipse, but external tools like bash scripting. For example, in Linux's shell or Windows Cygwin terminal you can use:
find src_folder -type f | egrep "\.java$" |
while read -r file_name
do
cat "$file_name" >> myoutputfile.txt
done
So what it actually does is find all the files which have the .java
extension, print them and redirect and append stdout
to myoutputfile.txt
.