Server returned HTTP response code: 400

Thank you everybody. This is a weird problem but at last I solved it.

The URL I am requesting is

http://api.themoviedb.org/2.1/Movie.search/en/json/api_key/a nightmare on elm street 

Now browser replaces the spaces between "a nightmare on elm street" by "%20" internally and parses. That is why the requested server can response by that request. But From Java I didn't replaced that spaces by "%20", so it turns into Bad Request, source.

Now it is working.

BufferedReader reader = new BufferedReader(new InputStreamReader(((HttpURLConnection) (new URL(urlString)).openConnection()).getInputStream(), Charset.forName("UTF-8")));

I had a similar issue and my url was:

http://www.itmat.upenn.edu/assets/user-content/documents/ITMAT17. October 10 2017_.pdf

which obviously contained spaces.

These caused java.io.IOException Server returned HTTP response code: 400 in the following code:

java.net.URL url = new URL(urlString);  
java.io.InputStream in = url.openStream();

If you copy the above url and paste in browser, you will realize that browser adds '%20' for the spaces. So I did it manually with the following code and the problem is solved.

if(urlString.contains(" "))
    urlString = urlString.replace(" ", "%20");

Complete code/answer should be:

if(urlString.contains(" "))
    urlString = urlString.replace(" ", "%20");
java.net.URL url = new URL(urlString);  
java.io.InputStream in = url.openStream();