FULL PRODUCT VERSION :
java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Windows 7 Version 6.1.7601

A DESCRIPTION OF THE PROBLEM :
The method URLDecoder#decode(String, String) method wrongly converts every plus-character to spaces. The plus character only represents a space in the query component of the URL, not the path component.

This topic is discussed on StackOverflow:
http://stackoverflow.com/questions/1005676/urls-and-plus-signs
http://stackoverflow.com/questions/2678551/when-to-encode-space-to-plus-or-20

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
String input = "http://www.example.com/some+thing/";

String output = URLDecoder.decode(input, "UTF-8");

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
the string remains unchanged:

"http://www.example.com/some+thing/"
ACTUAL -
the plus is replaced with a space:

"http://www.example.com/some thing/"

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.io.UnsupportedEncodingException;
import java.net.URLDecoder;

public class TestMain
{

public static void main(String[] args)
{
String input = "http://www.example.com/some+thing/";

String output = null;
try
{
output = URLDecoder.decode(input, "UTF-8");
}
catch(UnsupportedEncodingException e)
{
e.printStackTrace();
}
System.out.println(output);
}

}
---------- END SOURCE ----------