There is also the locate command. When combined with grep, they make a fast way to find files and search within them. For example, to find a Java class anywhere on the system:

    sudo updatedb

    for i in $(locate "*.jar");
      do echo $i; jar -tvf $i | grep -Hsi classname;
    done

The best you can do with vanilla Windows is search from the root directory, but it could take a while:

    for /R %G in (*.jar) do @jar -tvf %G | find "ClassName" > NUL && echo %G