How to change the background of another div when hovering over a div?

Question

Java

Tried like this

.div:hover .main-div{
background: url('img.jpg');
}

doesn't work because you can't change the background of a parent div when hovering over a child div

Answer 1

Do the correct host, port come to the socket?
Print their values ​​to the console to be sure. Let's try to localize the problem.
By the way, Yandex does not have ready-made examples of how to work with them from java?
Well, I would use some convenient library for access via Http. You can, of course, on sockets, but it's too low-level.
For example Apache Http Client
hc.apache.org/httpclient-3.x/userguide.html
or similar.
If you do not want to pull additional libraries, then you can use the built-in java docs.oracle.com/javase/7/docs/api/java/net/HttpURL...
To write-read from a file, you can also look at FileWriter/Reader
https http://docs.oracle.com/javase/8/docs/api/java/io/...

Answer 2

codepen.io/anon/pen/PzWpZX

Answer 3

only on CSS in any way.
Implement it in JavaScript.