When downloading a file over internet, the uploader usually provides an MD5 checksum for the file to ensure the integrity of the file. MD5 checksum (message digest) is something like a fingerprint or digital signature for  a file. The checksum is unique for a file and  there is very small possibility of getting two identical checksums for two different files. In Android, using java security package we can compute the md5 message digest for a file. Here is a sample class to get MD5 checksum from an input stream:

package net.londatiga.android.util;

import java.security.MessageDigest;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class Util {
	private static char[] hexDigits = "0123456789abcdef".toCharArray();

	public static String md5(InputStream is) throws IOException {
		String md5 = "";

		try {
		    byte[] bytes = new byte[4096];
		    int read = 0;
		    MessageDigest digest = MessageDigest.getInstance("MD5");

		    while ((read = is.read(bytes)) != -1) {
		        digest.update(bytes, 0, read);
		    }

		    byte[] messageDigest = digest.digest();

		    StringBuilder sb = new StringBuilder(32);

		    for (byte b : messageDigest) {
		        sb.append(hexDigits[(b >> 4) & 0x0f]);
		        sb.append(hexDigits[b & 0x0f]);
		    }

		    md5 = sb.toString();
		} catch (Exception e) {
			e.printStackTrace();
		}

	    return md5;
	}
}

To get md5 checksum from a file and compare it with original md5:

try {
      String md5Origin	= "";//original file's md5 checksum
	  String filePath   = ""; //fill with the real file path name

	  FileInputStream fis   = new FileInputStream(filePath);
	  String md5Checksum	= Util.md5(fis);

	  if (md5Checksum.equals(md5Origin)) {
		  //file is valid
	  }
} catch (Exception e) {
}

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