How can I safely encode a string in Java to use as a filename?

Question

I\'m receiving a string from an external process. I want to use that String to make a filename, and then write to that file. Here\'s my code snippet to do this:

Answer 1

It depends on whether the encoding should be reversible or not.

Reversible

Use URL encoding (java.net.URLEncoder) to replace special characters with %xx. Note that you take care of the special cases where the string equals ., equals .. or is empty!¹ Many programs use URL encoding to create file names, so this is a standard technique which everybody understands.

Irreversible

Use a hash (e.g. SHA-1) of the given string. Modern hash algorithms (not MD5) can be considered collision-free. In fact, you'll have a break-through in cryptography if you find a collision.

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_{¹ You can handle all 3 special cases elegantly by using a prefix such as "myApp-". If you put the file directly into $HOME, you'll have to do that anyway to avoid conflicts with existing files such as ".bashrc".}

public static String encodeFilename(String s)
{
    try
    {
        return "myApp-" + java.net.URLEncoder.encode(s, "UTF-8");
    }
    catch (java.io.UnsupportedEncodingException e)
    {
        throw new RuntimeException("UTF-8 is an unknown encoding!?");
    }
}