This sounds like file manipulation, not String manipulation. To create a file and write to it, you should use either FileOutputStream or FileWriter. See the link to the Java tutorial in my signature for some information about these and other input and output stream classes.
to write something on file is called serialization (your topic could have been "writing a string on file" rather than "string manipulation") to read/write from files, you need to first import the java.io package and then use the classes to open the desired file and write the string at hand onto it. I think you are new to I/O and I suggest you do a bit of reading before asking for a solution directly. Try searching the I/O forum and make a beginning with the java tutorial.
"It's not enough that we do our best; sometimes we have to do<br />what's required."<br /> <br />-- Sir Winston Churchill