HowTo Calculate an MD5 hash in Java

If you're dealing with networking, or just want to make sure your files don't get corrupted/modified between times your app runs.

It is critical to use the algorithm.reset() command before using a MessageDigest instance, as if there are bytes still remaining from a prior digest, it will completely ruin your output.

Code

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

// calculates an MD5sum for a given bit of text
public class MD5 
{
    public static void main(String[] args)
    {
        try 
        {
            MessageDigest algorithm = MessageDigest.getInstance("MD5");
            algorithm.reset(); // clear the digest back to starting state.

            // Get selected digest of the given text.
            byte[] digest = algorithm.digest( "Hello, world!".getBytes() );

            System.out.println(byteArrToHex(digest));
        } 
        catch (NoSuchAlgorithmException e) 
        {
            // no such algorithm, MD5
        }
    }

    // converts bytes to text.
    private static String byteArrToHex(byte[] bytes)
    {
        StringBuilder sb = new StringBuilder();
        for(byte b : bytes) 
            sb.append(String.format("%02X", b)); 
        return sb.toString().toLowerCase();
    }
}

Other Notes

• To calculate SHA1 hashes instead, use SHA1 as the instance type.
• The NoSuchAlgorithmException likely won't be thrown on desktop platforms, but it is possible that certain algorithms won't be implemented in certain areas due to them being considered military grade encryption; SHA1 and MD5 shouldn't have this problem because they are just simple hashes.