如果要想在sd卡中的一个子目录下创造文件FileOutputStream fos = new FileOutputStream("/sdcard/Wallpaper/"+fileName);
// create a File object for the parent directory
File wallpaperDirectory = new File("/sdcard/Wallpaper/");
// have the object build the directory structure, if needed.
wallpaperDirectory.mkdirs();
// create a File object for the output file
File outputFile = new File(wallpaperDirectory, filename);
// now attach the OutputStream to the file object, instead of a String representation
FileOutputStream fos = new FileOutputStream(outputFile);
当然别忘了检查sd卡存在不Environment.getExternalStorageDirectory()