[java] What does %5B and %5D in POST requests stand for?

I'm trying to write a Java class to log in to a certain website. The data sent in the POST request to log in is

user%5Blogin%5D=username&user%5Bpassword%5D=123456

I'm curious what the %5B and %5D means in the key user login.

How do I encode these data?

They represent [ and ]. The encoding is called "URL encoding".

Not least important is why these symbols occur in url. See https://www.php.net/manual/en/function.parse-str.php#76792, specifically:

parse_str('foo[]=1&foo[]=2&foo[]=3', $bar);

the above produces:

$bar = ['foo' => ['1', '2', '3'] ];

and what is THE method to separate query vars in arrays (in php, at least).

To take a quick look, you can percent-en/decode using this online tool.

[] is replaced by %5B%5D at URL encoding time.

Well it's the usual url encoding

So they stand for [, respectively ]

The data would probably have been posted originally from a web form looking a bit like this (but probably much more complicated):

<form action="http://example.com" method="post">

  User login    <input name="user[login]"    /><br />
  User password <input name="user[password]" /><br />

  <input type="submit" />
</form>

If the method were "get" instead of "post", clicking the submit button would take you to a URL looking a bit like this:

http://example.com/?user%5Blogin%5D=username&user%5Bpassword%5D=123456

or:

http://example.com/?user[login]=username&user[password]=123456

The web server on the other end will likely take the user[login] and user[password] parameters, and make them into a user object with login and password fields containing those values.