FileDialog fd = new FileDialog(this.frame, "Export");
    fd.setMode(FileDialog.SAVE);
    fd.setLocation(50, 50);
    fd.show();
    String dir = fd.getDirectory();
    String file = fd.getFile();
    
    if( dir != null && file != null) {
	String path = dir+file;
	if (!file.toLowerCase().endsWith(".dat")) {
	  path += ".dat";
	}
	PrintWriter out;
	out = createWriter(path);
	out.println("data");
	out.close();
    }
 

import java.io.*;
public class OnlyExt implements FilenameFilter {
String ext;
public OnlyExt(String ext) {
this.ext = "." + ext;
}
public boolean accept(File dir, String name) {
return name.endsWith(ext);
}
}

FilenameFilter only = new OnlyExt("dat");