Overwrite File with Zeros Android

Question

I am trying to overwrite a file with zeros so that the file cannot be restored..I have searched a lot but there is nothing for android ,all what I found was for windows..So I want to choose w file from internal storage then overwrite it with zeros..any idea how to that will be appreciated.

Answer 1

_{Note: this answer has probably not aged well, and bits of it may be incompatible with Android Q's new file reading/writing model. The general way of writing is still fine, though, but the bits involving file access may require editing before use}

Firstly, add this permission to your manifest:

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

Make sure you request it at runtime if you target API 23+

Then, we want to do the writing:

File file = new File(context.getFilesDir(), filename);
String string = "000000000000000000000000000000000";//this can also be a randomly generated string of 0's. You can mix up numbers as well, or it can just be a static string.
FileOutputStream outputStream;

try {
  outputStream = new FileOutputStream(file);
  // You can also open an InputStream before the OutputStream to read the length of the file if you want a more exact match as opposed to a brute-force override
  for(int i = 0; i < 100; i++)//Loop 100 times to write the 0's many times
      outputStream.write(string.getBytes());//And we write all the 0's to the file 100 times
  outputStream.close();
} catch (Exception e) {
  e.printStackTrace();
}

This should be able to write a lot of 0's to the targeted file. When the file is opened, the new content written to it will overwrite whatever is currently in it. Then it loops 100 times (can be removed if you want to) and writes these 0's 100 times to the file.

If you want the user to be able to pick a specific file, see this answer