例子代碼走起，
MD5 md5sum = new MD5();
md5sum.md5sum("/sdcard/xxx/yy.java");

MD5類文件：
import java.io.FileInputStream;
import java.io.InputStream;
import java.security.MessageDigest;

public class MD5 {
private static final char HEX_DIGITS[] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
'A', 'B', 'C', 'D', 'E', 'F' };

public MD5() {

    // System.out.println(md5sum("/init.rc"));

 }

// public static void main(String[] args)
//{
// System.out.println(md5sum("/init.rc"));
//}

public static String toHexString(byte[] b) {
    StringBuilder sb = new StringBuilder(b.length * 2);
    for (int i = 0; i < b.length; i++) {
        sb.append(HEX_DIGITS[(b[i] & 0xf0) >>> 4]);
        sb.append(HEX_DIGITS[b[i] & 0x0f]);
    }
    return sb.toString();
}

public String md5sum(String filename) {
    InputStream fis;
    byte[] buffer = new byte[1024];
    int numRead = 0;
    MessageDigest md5;
    try{
        fis = new FileInputStream(filename);
        md5 = MessageDigest.getInstance("MD5");
        while((numRead=fis.read(buffer)) > 0) {
            md5.update(buffer,0,numRead);
        }
        fis.close();
        return toHexString(md5.digest());   
    } catch (Exception e) {
        System.out.println("error");
        return null;
    }
}

}